Originally posted by Sandman
You guys are going to have to explain how an aircraft can get airborne if it's not moving forward.
Originally posted by Maverick
Sandy,
The wheels do not make the plane go forward. The wheels only allow the plane to roll accross the ground and are free wheeling. No motive power is applied through the wheels. The motive power is applied by the airplane engines and are used to move air.
Originally posted by rabbidrabbit
A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.
The question is:
Will the plane take off or not?
Originally posted by Debonair
no pilot would taxi onto that contraption
Originally posted by rpm
An electric train is moving North at 50MPH.
The wind is blowing from the South at 50MPH.
Which way does the smoke go?
Originally posted by sluggish
Sure it will. Thrust has nothing (or very little) to do with wheel RPM.
Originally posted by JB88
however.
were you to create a fan big enough and an anchor strong enough...
Originally posted by Kurt
...The plane makes enough lift to lift the plane.
...
Originally posted by APDrone
So you're saying that every airplane generates enough air to lift itself off the ground.
If that is the case...
why do we need runways?
THIS would be a good one for mythbusters.
Originally posted by JB88
oh.
so, how does a wind tunnel work again?
you know, within the realm of physics and all.
tap tap tap...is this thing on?
:confused:
Originally posted by Kurt
Because in the absence of a convyer belt, the plane will move forward for a while before attaining adequate airspeed. The key here is that the speed is not attained by moving the freakin wheels, its attained by moving the freakin AIR... why doesn't anyone get it? IN your world, as soon as the airplane leaves the ground it loses its motive force and comes back down... Yet you know this is not the case - Because propellers and jets act against AIR, not concrete - and most especially not against conveyors.
Add a conveyor to prevent it moving forward and it will fly after covering that same amount of distance on the conveyor.
Come on Drone, you're smarter than this. I know it.
Besides, everything the Mythbusters have looked at in the last 12 months is WAY below their level of knowledge... Its become a mainstream circle-jerk...
Originally posted by eagl
You're forgetting that airplane wheels don't push the plane forward. In fact, airplane wheels push straight down (weight) and actually "push" forward as they resist the rearward drag between the wheels and the ground. The only force from the wheels that the engines must overcome on takeoff is that drag. The wheels are simply not responsible for accelerating the plane forward so their only contribution to the whole thing is how much drag they're producing because the engines must push against that drag.
Originally posted by Kurt
BINGO!
Originally posted by FBplmmr
wow, if the navy knew about this they wouldnt have spent all that money on catapualts! lol
they would just park the jet on the edge of the carrier and lock down the wheels , spool up the engine and release.
the reason you tie down a plane at an airport is becuse wind creates airflow over the wing that will flip the little bastage up into the air.
if the plane has no forward moentum there is little airflow over the wing (probably more from a prop plane than a jet becuse of engine placement)
with little to no airflow ....
i hear they have alot of trouble with nextel cup cars getting airborne on the dyno lol:lol
Originally posted by FBplmmr
i dont need a catapult we have majic no airflow thrust :lol
That Professor Goddard, with his 'chair' in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react - to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools.
Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error.
Originally posted by Donzo
Did you even think about the question I posed?
Originally posted by Donzo
Put one of these fancy conveyors under a plane on the catapult. What would happen at launch?
Originally posted by FBplmmr
yes briefly, my stance is that without air passing over the wings the plane can't achieve flight.
if the plane is in effect staionary(either by a belt or lockdowns) and the thrust being produced is parralell to the ground the plane will remain stationary. when released from eitherfrom the belt or the lock downs the plane will begin forward and will not acheive flight untill sufficient airflow goes over the wings to allow it.
to be honest trying to visulize a conveyour under a jet on a catapault made my head bleed =)
Originally posted by APDrone
The catapult is an external force in relation to the conveyor belt.
Originally posted by Donzo
Yes, without air passing over the wings the plane will not fly.
What is stopping the air fom passing over the wings?
The conveyor under the plane on the catapult is the same as a plane sitting on a conveyor runway. Think of the "thrust" that will propell the plane forward as being the catapult itself.
Once the button is pressed and the catapult is fired, what will stop the plane from going forward....the fact that it's wheels are on a conveyor that matches the wheel speed in the oppostite direction? Think about that.
Originally posted by eagl
the bearings keep the spinny parts spinning and the engine casing (and airframe) not spinning.
Originally posted by Kurt
The funniest thing here... Is that 5 posts into this thread, a link was posted that explained exactly why the airplane does fly... But so many didn't read it... Here it is again for those of you who are just here to argue..
http://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/
I will try one more time, and then I'm jumping out of this discussion because the utter lack of understanding is really depressing. I suppose some of you believe if a tree falls in the forest and no one is there, then it really doesn't make a sound?
Ok... 3rd Law of Motion... For every action there is an equal an opposite reaction..
So you use a fan to act on the air. We agree? Good.
The fan does not act on the ground at all... This is why planes keep flying even when the ground stops touching the plane.
Since your motive force is acting in the medium of air, that is where the reaction will occur, this moves the plane. The conveyor simply isn't part of the math, well, perhaps to a negligible amount due to the tiny friction, but these are wheels on ball bearings, the brakes are not applied, they are simply rolling at whatever speed they roll at, they have no relationship to the thrust being created by the propeller.
Above, and also in the linked article, the analogy of a string tied to the plane are used, this is a great example... You put a toy plane on a piece of paper, you pull the string one way (thrust) you move the paper the other way (conveyor) and the distance and speed that the plane travels is the distance and speed that the string pulled it, it has nothing to do with the movement of the piece of paper. Just like in the full size experiment, it has nothing to do with the conveyor.
By contrast, the action of moving the conveyor has the opposite reaction of moving the wheels of the plane, but nothing more. So maybe it adds 40 pounds of friction.. Thats not going to prevent the accelleration of the airplane.
Originally posted by hitech
Boy I'm glad some of you were not involved in the conservation of energy conversations we had concerning the torque of jet engines a few years back.
Originally posted by eskimo2
I think that the “It won’t move or fly” believers have either chimed out or now get it…
Originally posted by APDrone
If you try to push more thrust than the wheels can keep up, you get to where the plane is going faster than the landing gear, and then this happens:
(http://www.airmageddon.com/Airmageddon/CM/Nose_Down.jpg)
If the wheels cannot spin fast enough to stay under the center of gravity, the plane will attempt to leave the wheels behind. Remember, if this happens before you have attained stall speed, you have wreckage.
Originally posted by Kurt
And suppose its not a tail dragger APDrone... Suppose its got tricycle gear like pretty much everything does these days. Its certainly not going to nose over like that.
Originally posted by icemawThat's not the problem though. The problem is with generating the 1.21 gigawatts of electricity nessecary to activate the flux capacitor. I'm sure that in the future plutonium is available at every corner store but right now it's a little hard to come by!
everybody know once you reach 88mph the flux capacitor kicks in and you get transported back to the future.
Originally posted by eagl
I think that makes sandman the loser, for immediately posting the wrong answer.
Drone gets the loser cheerleader award for trying to disprove Newton in support of sandman's answer :)
Originally posted by rabbidrabbit
...Even if it was a super duper treadmill going super duper fast, do you think that you would not be able to pull yourself forward?
Originally posted by rabbidrabbit
A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.
The question is:
Will the plane take off or not?
Originally posted by hitech
APDrone: Your argument is missing one large piece.
No mater how fast you run the conveyor under the wheels, it will not transmit any more force to the plane. The force will be the same at 1 MPH or 200 MPH. The rolling friction force of the wheel will be the same as long as the same weight stays on the wheel.
Hence there is no way to stop the acceleration of the plane do to thrust.
2nd The question quite frankly sucks, and is totally none solvilble from the wheel speed statment. The problem is the equation can not be written to be solved based on the wheel speed. (unless you define wheel speed not in rpm, but axle moving accross the ground)
The only solution to the equation is 0 rpm on the wheel. At any other speed, it will instantaneously solve to infinity.
HiTech
Originally posted by hitech
APDrone: Your argument is missing one large piece.
No mater how fast you run the conveyor under the wheels, it will not transmit any more force to the plane. The force will be the same at 1 MPH or 200 MPH. The rolling friction force of the wheel will be the same as long as the same weight stays on the wheel.
HiTech
Originally posted by hitech
Boy I'm glad some of you were not involved in the conservation of energy conversations we had concerning the torque of jet engines a few years back.
HiTech
Originally posted by eskimo2
The plane will take off, but what you described is impossible. The conveyer can’t match the wheel speed in the opposite direction; the plane will move forward because the engines are pushing and pulling air, not the conveyer. The conveyer can speed up all it wants to try and stop the plane but the wheels will just spin as fast as the conveyer PLUS the speed of the accelerating plane.
Originally posted by APDrone
Dagnabbit! I want to go take a nap..
Ok Boss..
Please explain how this acceleration can be translated into forward motion without causing the wheels to turn at a different rpm than they were before the acceleration was applied.
Originally posted by eagl
Drone,
Your argument is valid if you take Hitech's observation that the instant an external force is applied to the aircraft, the wheel speed would instantaneously jump from zero to infinity, and that the all energy used to further accelerate the aircraft would simply be pumped into the theoretical infinite-speed treadmill.
Originally posted by eagl
It can't be predictive because if the belt starts moving before the engine thrust starts, then the plane would shoot backwards.
Originally posted by eagl
The entire question is based around the idea that any force on the aircraft would be translated without loss into wheel spin. That's why the wheel speed goes to infinity the moment a force is applied that would otherwise accelerate the plane.
Originally posted by eagl
The point is that the second ANY force is applied to the plane, the wheel speed jumps to infinity. If there is any delay in the spin-up of the treadmill and wheels, the plane will move which violates the premise that the treadmill will accelerate to match the wheelspin. If the plane is moving, then the treadmill must be moving slower than the wheels are spinning, which violates the whole premise behind the question. And it still results in the wheels and treadmill moving at an infinite speed. Which isn't possible.
The situation described in the question is impossible. There is no way the treadmill could ever instantaneously match the wheel speed because the instant the plane moved forward through the air, the wheels would be turning faster than the treadmill was going, which violates the premise behind the question.
So everyone but HT was wrong
Originally posted by eagl
It's like saying "if you put the color blue and red on a scale, which would weigh more?"
It's not a valid question to begin with :)
Originally posted by Kurt
A very important feature in modern jets... In the old ones where the plane spun and the fans stood still I used to get awfully sick...
Its like in the old days when they used to spin the radio station and the record stood still. And everyone wondered why DJ's were weird.
Does anyone remember a few years ago when the Earth blew up, and we moved here... And the government decided not to tell the stupider people because they thought that it might effect....
Oh, I've said too much...
Originally posted by Wes14
just get me a darn Harrier jet and i can prove a get couls take off that "treadmill";)
Originally posted by Wes14
thats one bad pilot
waste of a perfectly good jet:furious
Originally posted by Wes14
if their was a problem wouldnt the pilot be able to compensate (im always loosing an aleron or flap or other things on AH and i dont crash):confused:
Originally posted by Kurt
You see, JB88... Flight is not magic as you might believe. Its physics... Just like the physics that say if you drop an 85 pound rock on your bare foot, its going to hurt... Even if you have a conveyor belt.
Originally posted by DREDIOCK
Sorry I dont buy it.
I dont beleive that if JB88 drops an 85 pound rock on his bare foot that it will hurt.
I wanna see the proof! :D
Originally posted by Mini D
sweet thread!
It makes me wonder why a plane taking off against the rotation of the earth wouldn't have a tougher time than a plane taking off with the rotation of the earth. I'm sure airspeed has nothing to do with it.
I do have to take exception to Hitech's explanation. If there were a belt capable of rapid acceleration to counter the tire's rotation, it would rely on some sort of friction that would inevitably react with the air. This would create a mass of air turbulence that would actually launch the plan faster and before reaching anywhere near infinity. Of course, if the tire were only exposed on the bottom to this magical wonder belt and nowhere else, this effect would be reduced, though the effect of spinning this magical belt at near infinite speeds would most certainly cause it to expand in diameter effectively launching the plane straight up anyways.
So many effects... so little consideration. Tsk tsk tsk.
Originally posted by Mark Luper
Kurt,
I commented to him it may have been a hover flight problem. He responded apologeticaly showing me that he hadn't thought of that aspect of it.
Originally posted by Wes14
yep:aok
acually im thinking of being a bomber pilot when i grow up:D
any suggestions?
Originally posted by Mini D
sweet thread!
It makes me wonder why a plane taking off against the rotation of the earth wouldn't have a tougher time than a plane taking off with the rotation of the earth. I'm sure airspeed has nothing to do with it.
Originally posted by Holden McGroin
Launching payloads into orbit takes the rotation of the earth into consideration. Thats why all but a few payloads are launched east. They get a 800 mph boost to get to 17,000 mph
Originally posted by Holden McGroin
Several thousaand people living in Coco Beach, Fr. Giuana, Kazakhstan, and a few other places.
Originally posted by Holden McGroin
Launching payloads into orbit takes the rotation of the earth into consideration. Thats why all but a few payloads are launched east. They get a 800 mph boost to get to 17,000 mph
Originally posted by Kurt
Lucky for you, right now, I've got nothing else to spend that energy on.
A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.
Originally posted by eaglActually, the warp theory Star Trek uses is based on the actual bending or warping of space. The bubble around the ship keeps the space constant so the ship will not disintegrate. It is a tricky balancing act.
Snip. This is why all theoretical faster than light drives use some sort of field to either reduce the effective mass of the ship to zero, or create some sort of bubble around the ship so that the ship is surrounded by sub-light velocity space while the bubble, which is effectively massless, moves through the rest of the universe. Star Trek uses the second trick, creating a "sub-space" field around the ship so the ship never gets close to the speed of light. You are postulating the first trick, where your wheels and treadmill are capable of moving at an infinite speed. To be able to do so, they would need to have zero mass or infinite strength (or possibly a diameter of zero... hmmm)
Originally posted by Skuzzy
Actually, the warp theory Star Trek uses is based on the actual bending or warping of space. The bubble around the ship keeps the space constant so the ship will not disintegrate. It is a tricky balancing act.
It works like a magnet does. Take a magnet, place it on paper and drop some metal particles on it. At each end of the poles of the magnet will be a greater density of metal particles. And at the very center of the magnet everything is balanced. All the fields counter each other. Warp drive theory is based on the same principles.
If you can generate a force which bends/warps/compresses space in front of the ship, then use a normal drive syste, to push the ship through the compressed/warped area of space, and then release the space after the ship passes though the compressed area, it will have the effect of moving the ship great distances at sub-light speeds.
The warp factor used in Star Trek is an exponential amount of power used to bend/compress/warp space X amount. By the time you reach Warp 10, it is therotically impossbile to bend space any further without causing space to collapse on itself and the ship.
One of the many fallacies of how Star Trek used the theories is what happens when a ship, traveling at Warp speeds, explodes. The ships remains would be spread over billions of miles of space (depending on the warp factor) and not be localized at all. The particles would indeed form a long cylindrical pattern though.
The overall impact of what happens to space which was warped then immedaitely returned to a non-warped state is unknown. However, the final episodes of TNG attempted to address the potential issues with the constant warping and releasing of compressed space. The probable theories are still being defined.
One of the things they did get right (and they actually got more right than they did wrong) was the visual effect of entering warped space. The object would indeed appear to elongate until it disappeared from view in a matter of micro-seconds.
---
Uh, did my geek level just increase?
Originally posted by Skuzzy
Actually, the warp theory Star Trek uses is based on the actual bending or warping of space. The bubble around the ship keeps the space constant so the ship will not disintegrate. It is a tricky balancing act.
It works like a magnet does. Take a magnet, place it on paper and drop some metal particles on it. At each end of the poles of the magnet will be a greater density of metal particles. And at the very center of the magnet everything is balanced. All the fields counter each other. Warp drive theory is based on the same principles.
If you can generate a force which bends/warps/compresses space in front of the ship, then use a normal drive syste, to push the ship through the compressed/warped area of space, and then release the space after the ship passes though the compressed area, it will have the effect of moving the ship great distances at sub-light speeds.
The warp factor used in Star Trek is an exponential amount of power used to bend/compress/warp space X amount. By the time you reach Warp 10, it is therotically impossbile to bend space any further without causing space to collapse on itself and the ship.
One of the many fallacies of how Star Trek used the theories is what happens when a ship, traveling at Warp speeds, explodes. The ships remains would be spread over billions of miles of space (depending on the warp factor) and not be localized at all. The particles would indeed form a long cylindrical pattern though.
The overall impact of what happens to space which was warped then immedaitely returned to a non-warped state is unknown. However, the final episodes of TNG attempted to address the potential issues with the constant warping and releasing of compressed space. The probable theories are still being defined.
One of the things they did get right (and they actually got more right than they did wrong) was the visual effect of entering warped space. The object would indeed appear to elongate until it disappeared from view in a matter of micro-seconds.
---
Uh, did my geek level just increase?
Originally posted by Holden McGroin
Better grammer would be to say, "nothing else on which to spend that energy."
Originally posted by Kurt
Well, if you are 14 as everyone is guessing, its likely there won't be any active A-10's when you are flying. The Air Force has been trying to deactivate them for 15 years.
I'll never understand why though, it is the perfect design for the role.
Originally posted by Skuzzy
Uh, did my geek level just increase?
Originally posted by hitech
Eskimo2: Even in your theoretical case, the conveyor and wheel speed do not equal a change in force.
Visualize yourself on a pair of roller skates standing on a moving side walk like in an air port. You are motionless relative to the building holding yourself motionless by holding a rope. On the end of the rope is a scale measuring how hard you are pulling, lets say 10 lb of force.
Speed up the side walk , the scale will read 10 lb's.
Slow down the side walk, the scale will still read 10 lb's.
Take you theoretical wheels and conveyor, scale still reads 10 lb's.
Hi Francisco,
If I properly understand your travelator, the travelator moves at
exactly the speed of the airplane, but in the opposite
direction. This means the wheels rotate twice as fast as they would
on a normal runway and nothing else is different. Right?
In that case, I claim the plane would take off normally except that
the wheels would be rotating twice as fast as normally. Since the
frictional force is, as you say, f=uR, the frictional force will be
exactly the same in the two cases since v does not appear in the
equation for the frictional force. In other words, the frictional
force is independent of the speed. In that case the forces on the
plane are exactly the same whether the travelator is operating or
not and so the plane takes off the same way in the two cases.
Did I understand your question correctly?
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Originally posted by Wes14
How aboutwe tweak this question:D
if u put a fan/jet engine/thrust creating device infront of this said airplane while its on the treadmill,will it take off?:D
Originally posted by hitech
Eskimo2: Even in your theoretical case, the conveyor and wheel speed do not equal a change in force.
Visualize yourself on a pair of roller skates standing on a moving side walk like in an air port. You are motionless relative to the building holding yourself motionless by holding a rope. On the end of the rope is a scale measuring how hard you are pulling, lets say 10 lb of force.
Speed up the side walk , the scale will read 10 lb's.
Slow down the side walk, the scale will still read 10 lb's.
Take you theoretical wheels and conveyor, scale still reads 10 lb's.
Originally posted by john9001
if your conveyor belt can move at infinite speed, then my airplane wheels have no drag, no matter how fast your belt moves my wheels will keep up , but the thrust from my engines will still push the airplane forward.
the wheels will have no effect. Freewheeling.
what if your belt was covered with ice?
Originally posted by lukster
What about the black hole created by the infinite speed belt? It'll suck the universe into it and we'll all die, very slowly. ;)
Originally posted by eskimo2No.. it wouldn't work like that at all. It couldn't work like that. How would you back the plane up by moving at the same speed as the wheel? Fundamentally, this perfect system would have to know exactly how fast the wheel is turning and react to it instantly (0 seconds..). This perfect system would have to function with an imperfect wheel to generate the friction.
For such a system to work, it would have to function like an automobile cruise control. If the plane moves forward an inch the conveyor speeds up faster and faster until the plane moves back to its original position. If the plane falls back an inch, the system backs off a bit.
If this system could accelerate the rotation of the wheels beyond anything realistic (they hold together and don’t overheat, etc.) then the system could prevent the plane from moving forward purely by the energy being loaded into the wheels.The system would not accelerate the wheels. It cannot. The motion of the plane is the only thing that can accelerate the wheels. That is the premise of it "reacting".
Try this: Clamp your belt sander onto a table upside down, perfectly level, and turn it on. Now drop a ball or wheel onto the belt. Watch what happens. Read my post above; hitech gets it. I wouldn’t be surprised if no one else does.That is not the same scenario at all. You are generating the engergy and propulsion with the belt. In the scenario described, the belt is reacting to motion not causing it. Once again, the reactionary state would make it incapable of stoping the forward momentum of the aircraft in a frictionaless wheelbearing scenario.
Originally posted by eskimo2Fundamentally, anything that is capable of preventing a plane from moving foward will cause it not to fly. It's just that a conveyer that reacts to the motion of the tire will not do that.
I’d like to be the first to apologize to APDrone. Although he didn’t explain how the conveyor could keep the plane from moving forward, he was right. If the conveyor prevents the plane from moving forward, it won’t fly.
Originally posted by Mini D
Fundamentally, anything that is capable of preventing a plane from moving foward will cause it not to fly. It's just that a conveyer that reacts to the motion of the tire will not do that.
Originally posted by Mini D
The inertia on the tire is generated by the motion of the aircraft, not by the belt.
Originally posted by Mini D
Put the same belt sander on a frictionless wheel and turn it on... it will stay stationary. Put it on a frictionless wheel and push it with your hand... it will move forward. Fundamentally, this is the exact same scenario being described.
Originally posted by Mini D
PS.. you have to assume the absence of friction. Given that scenario, the belt and the wheel would both instantly go to infinite speed and the fuselage of the plane would start moving.
Originally posted by eskimo2
I’d like to be the first to apologize to APDrone. Although he didn’t explain how the conveyor could keep the plane from moving forward, he was right. If the conveyor prevents the plane from moving forward, it won’t fly.
If such a system were real, it would literally explode in a second or two. If it had infinite strength and power and the aircraft wheels and bearings didn’t create heat and also had infinite strength, however, it would work.
Originally posted by eskimo2You are assuming the absolute absence of friction.
The belt and wheel would not go to an infinite speed instantly; they would gain rpms by the thousands or millions per second. By the time the plane ran out of gas, the wheel would have the same energy (stored as rotational energy) as all of the fuel consumed by the plane’s motors.
Originally posted by APDrone
Thank you, Eskimo. I gave up trying to explain things. I can see the whole scenario very clearly, but trying to explain it in words has been very difficult, so I quit trying.
<.S>
Originally posted by Mini D
You are assuming the absolute absence of friction.
If this were the case, the belt and the wheel would not react at all. The wheel would slide across never turning.
You are eliminating too much to make this scenario possible. Way too much. You are holding onto one constant and completely eliminating several others.
There really is one correct answer: The scenario described is impossible. Come up with a real question.
Originally posted by hitech
I agree on the moment argument. You could theoretically create a large enough force to hold the plane.
HiTech
Originally posted by eskimo2Now, you're making things up.
I’ve stated repeatedly:
It would require unlimited strength for the conveyor, wheel and bearing.
Now, you're making more things up.
The conveyor would also need unlimited power.
The bearing would have to be immune to heat.If it is turning, there is friction and it is impossible to be immune to heat.
The tire would obviously require friction/traction on the conveyor.Once again... if there is friction, there is heat. There is no other solution.
In real life it would explode in a second or two; there is no question that this is theoretical only.It is not theoretical, it is situational. A situation where you have eliminated the impacts that don't support your outcome in order to generate the desired effect.
Originally posted by Terror
From a Physics Forum on this exact topic:
459 Pages, 6876 replies!!
Physics Forum Thread (http://forum.physorg.com/index.php?showtopic=2417&st=0)
Hitech, this thread may just fill your HD space !!
Terror
Originally posted by eskimo2Of course I did. At the end of the experiement, we'd also figure out the % error.
You took high school and/or college physics? Right?
Don’t you remember solving theoretical problems with frictionless surfaces, pulleys and wheels?
Originally posted by Golfer
oh for the love of god...
Where's my icon of the smiley smashing his head against a brick wall?
Originally posted by lukster
Spin your treadmill up to 2 -3 hundred miles per hour and put the plane on it. Go ahead and stand to the rear if you think it'll take off.
Originally posted by lukster
Spin your treadmill up to 2 -3 hundred miles per hour and put the plane on it. Go ahead and stand to the rear if you think it'll take off.
Originally posted by eskimo2
Actually, that would only nudge it back, once the wheels get to 2 – 300 MPH the plane would start moving forward (provided the tires don’t blow, bearings overheat, etc. What must happen it the treadmill is going 300 MPH at 1 second, 600 MPH at 2 seconds, 900 MPH at 3 seconds, etc. The treadmill and aircraft wheels must be accelerating at a fantastic rate for this to work.
Want another twist? If we change the wheels to be mass-less, the conveyor and wheels would go to an infinite speed instantly and the plane would take off normally!
Originally posted by Golfer
I'd be glad to. I'd happily sit in the airplane as well provided the tires have no speed limitation on them.
Originally posted by lukster
We're talking about a model airplane with wheels designed to rotate at maybe 40 mph. Golfer said the speed of the conveyor didn't matter but it does. If 200-300 isn't enough try 2-3 thousand. Massless wheels and belt must by definition have no ability to influence the motion of the other.
Originally posted by lukster
It's your model plane, contruction of the tires is up to you. You want to try a real plane? Try spinning the belt to 10,000 mph and see if the planes wheels can keep up. It's obvious we're talking friction here and the wheel's ability to convert the movement from the belt into a pure rotational speed. If the engine is powerful enough to overcome the wheel friction then yes the plane will accelerate and takeoff. Let's not mix theoretical and practical or at least if we do let's not confuse them.
Originally posted by Golfer
You used both of those underlined sections in the same paragraph for your agrument?
Originally posted by hitechI don't have a problem understanding this. No need to explain.
Mini D: Picture a 10000 LB wheel suported by a bearing and none of it sides are touching. Picture your slef trying to turn the wheel. Obviously you would have to push fairly hard to get it turning. Now stop pushing, the wheel will very gradually slow to a stop. Where all your force went is into the rotational acceleration of the wheel. It is derived from F = m * A. (Force = Mass * Acceleration). Sorry can't pull the rotational version out of my head at the moment. (no pun intended)
In the airplanes case, this force would be transmitted to the airplane, and if the force of the conveyor equaled the thrust of the plane. The plane would not move.This is where we drastically depart. The friction would be all that prevented the aircraft from moving. In the absence of friction, it will move.
Originally posted by hitech
mijac: Note the Frictional force is independent of speed. Power required changes with speed, but the force remains the same.
he only real drag is created between the wheel and the bearings.
Originally posted by hitech
In the airplanes case, this force would be transmitted to the airplane, and if the force of the conveyor equaled the thrust of the plane. The plane would not move.
HiTech
Originally posted by hitech
Wish eskimo2 sander had a variable speed control so you could see the rubber band stretch at constant speed, see it stretch more during the 2nd acceleration,and then return to it's steady state stretch at the new higher speed.
HiTech
Originally posted by hitechYes, I am. But that acceleration needs to be transfered to drag on the airframe just as friction is required to transfer the energy from the jets to the rotating wheel. Ask yourself this: what requires more work, pushing a airplane or spinning a suspended wheel. You're ignoring the primary energy source and focussing on the less significant source.
Mini D:
Correct in the Drag piece, but drag is not the only force. You are not quite seeing the force created by the acceleration of the wheel.
Originally posted by lukster
Ok, let's say this plane has very strong landing gear with a nose wheel. All you need to do is nudge it backwards a bit, the belt rotates forward instantly launching the plane due to the energy imparted to the wheels. Problem solved. ;)
Originally posted by rabbidrabbit
A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.
The question is:
Will the plane take off or not?
Originally posted by Wes14
but as soon as it launches forward wouldnt the belt catch it and fling it back,then forward,then back, like a swingset or sumthing?:confused: :noid
Originally posted by Mini D
Let's get away from analogies and think about the most simple aspect of this...
The force on the wheel.
At some point, to prevent 30,000 lbs of thrust from moving the aircraft forward, there is going to have to be an equivelent thrust applied to the airframe. Not the wheel... the airframe. The only point this would occur would be at the moment where the bearings are inducing a force on the wheel to attempt to move it forward as another force is moving it backwards. At this point, the counter to 30,000 lbs of thrust must be occuring. This must occur in the form of friction as there is no other means for a bearing wheel to transfer energy to an imobile object. This means that there is no infinite motion... the wheel disintigrates. If you remove the friction, then there is no acceleration nor deceleration... the wheel does not move and the plane slides forward faster than if it were rolling.
Originally posted by hitech
kamilyun: That equation you posted is drag, not friction. They are 2 completely different equation forms and concepts.
Surface friction does not have a Velocity component.
HiTech
Originally posted by chancevought
it says it tracks WHEEL SPEED, then tunes the speed of the conveyor to be the SAME in the opposite direction. I'm pretty sure if it reached 1 mph forward, the conveyor would go 1mph backward...1-1=0
once the wheel moved the slightest bit..the conveyor would move the slightest bit opposite of that...producing a balance of zero...as the aircraft gained speed the wheel would TRY to accelerate, but wouldn't because the conveyor would COUNTERACT that movement again producing a net gain of zero....
once the wings got enough Airspeed...it would lift off, it's wheels never having rotated, if they even moved at all
Originally posted by eskimo2Put a model plane on a sheet of paper. Remove the sheet as fast as you can. Sit down and think about inertia and friction. Then think some more about what it would take to make model move with the sheet.
This question, however, has the conveyer matching the wheel speed. When the plane moves forward 1 inch, its wheel is going faster than the conveyor. This cannot be allowed so the conveyor must speed up like mad trying to move it back. By dumping enough rotational energy into the wheel, it can and will keep the plane in place.
Originally posted by luksterWhat makes you think there has to be overheating to make drag? If there is friction, there is drag. The energy of this friction must be significant to prevent the airfoil from moving forward since THAT IS THE ONLY MEANS OF COUNTERING THE THRUST.
I think they are making a distinction between the force applied from the tire to the axle with a well greased bearing functioning as intended vs a bearing that is inducing drag due to overheating.
The belt will most definitely apply force to plane in the direction the belt is moving through the tire and axle and as we said previously the only way for this force to be transferred is through friction.The belt can only apply force to the wheel, not the plane. Of course this HAS TO HAPPEN VIA FRICTION. We're talking infinite rotation, infinite friction here. That means infinite heat. That means disintigration, no matter what it is. Remove the friction... there is absolutely no wheel motion and absolutely nothing to stop the aircraft from moving.
Originally posted by Mini D
The belt can only apply force to the wheel, not the plane. Of course this HAS TO HAPPEN VIA FRICTION. We're talking infinite rotation, infinite friction here. That means infinite heat. That means disintigration, no matter what it is. Remove the friction... there is absolutely no wheel motion and absolutely nothing to stop the aircraft from moving.
Originally posted by chancevought
it says it tracks WHEEL SPEED, then tunes the speed of the conveyer to be the SAME in the opposite direction. I'm pretty sure if it reached 1 mph forward, the conveyer would go 1mph backward...1-1=0
once the wheel moved the slightest bit..the conveyer would move the slightest bit opposite of that...producing a balance of zero...as the aircraft gained speed the wheel would TRY to accelerate, but wouldn't because the conveyer would COUNTERACT that movement again producing a net gain of zero....
once the wings got enough AIRspeed...it would lift off, it's wheels never having rotated, if they even moved at all
Originally posted by deSelys
In theory (no friction), the conveyor belt wouldn't be able to stop the plane from moving no matter how powerful it (the conveyor belt) is.
in practice (friction from spinning wheels*), the wheels would disintegrate quickly and you'd loose the perks of the plane.
* edit: as MiniD explained, friction in the ball bearings between the wheels and the axles
Originally posted by SteveBailey
I just read this whole thread and it is unbelievable that some of you don't get it.
The initial question is a bit oddly worded. The belt is spinning in the opposite direction of the wheels.
The plane would move forward at a normal take off as thrust is applied(to the air, not to the belt). The conveyer wouldn't double the speed of the wheels.. it would bring the speed of the wheels to 0. The plane would move down the conveyer at normal acceleration and take off departure behaviors, the speed of the wheels would be 0.
The only way the wheels would accelerate is if the converyer was spinning the SAME way as the wheels.
Basically, the plane would move along the conveyer at 0 ground speed until it achieved enough air speed to lift off.
The fact that you guys don't get this is staggering.
Originally posted by lukster
Ok, I'm glad you used the word spinning in regards to the conveyor belt. For the purpose of clarity take the belt out of the picture and just set the plane on the convey belt's wheel. This is the part of the belt that "spins", the belt itself only moves horizontally. If the belt spins in the opposite direction but at the same speed from the plane's wheel then plane's wheel will stay in place. Spin the belt wheel in the same direction and the plane's wheel and you will get the effect you described.
Originally posted by SteveBailey
Right! For the sake of the argument, our original poster is implying that the conveyer belt will match the speed(energy) of the wheel spin, but in the opposite direction, right?
At the point where the wheel touches the conveyer, the wheel is moving toward the back of the plane. Therefore, if the conveyer is countering the movement(speed) of the wheel, the conveyer would move at the exact same speed toward the front of the plane.
Ergo, the wheel speed would be 0 while the conveyer would match the air speed of the accelerating plane.
lurker, I'm thankful you thought this through. :)
Originally posted by SteveBailey
The rotation would be the same but that's not what the original poster is implying and that's why I siad it was oddly worded.
The OP is implyiong the the conveyer is acting against the spin of the wheels. As a result , we need only consider the top of the conveyer for this, not the rotation of the conveyer.
Originally posted by lukster
Tangent time, here's a little puzzle. Why can you always say that a plane's wheel will always rotate clockwise when the plane is rolling forward on the ground?
Originally posted by lukster
Tangent time, here's a little puzzle. Why can you always say that a plane's wheel will always rotate clockwise when the plane is rolling forward on the ground?
Originally posted by lukster
bzzzzt! wrong
Does a plane have a left and right wing? Do they swap places relative to where someone outside the plane is standing?
I just realized only the left wheel will spin cw, the right will always spin ccw, my bad.
Originally posted by rabbidrabbit
A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.
The question is:
Will the plane take off or not?
If the question is answered litterally as written the airplane will take off normally with the wheels motionless. The belt moves in opposition to wheel spin.
Originally posted by Casca
The last possible interpretation is the conveyor moves equal and opposite to wheel speed irrespective of the speed of the airplane. If you read it like that you wind up in your basement filming a belt sander documentary.
Originally posted by hitechThis is not what I said.
Mini D: In agreement so far.
And I also agree the tire static surface friction (I.E. Brake's locked how much force can the tire hold back) has to be as great as the total thrust, or the wheel would start to slide on the belt.
But once again, that is not what is normally considered as the rolling friction force transmitted to the plane. The rolling friction force is a constant at any speed the wheel turns.Whether it is a constant or not is irrelevant, once again. There needs to be a counter force acting on the airframe that has the engine mounted to it. Basically, there is no rope in this situation.
As you accelerate the wheel a force is needed for this acceleration (energy is stored in the rotation of the wheel). That force can not be larger than the STATIC friction of the tire to the belt or the wheel would begin to slide. If it is not greater the force is transmitted to the belt and the plane. But note, that force is not what eskimo and I are considering friction.The static friction, once again, is irrelevant. In order to move the wheel.. that is to get the energy from the engine to the wheel, you have to move your moment on the wheel relieving a bit of the downward force on the bearings and moving it to a portion of the bearings that is elevated causing the wheel to turn. Please understand that this is friction that is causing this rotation. You aren't turning an axle, you are moving it forward against the wheel. In order to prevent the plane from moving, an equal force would have to be pushing the wheel back against the axle which would actually cause even more forward acceleration on the wheel and be infinately absorbed. The act of trying to keep the wheel from moving forward would require infinite rotation, but it would not be enough because it would not be able to transfer that energy to oppose the thrust of the engine.
Originally posted by Casca
Help. I'm stuck in the conveyor belt thread and cant get up.
Originally posted by eskimo2
Here’s a story that illustrates my idea: (Note that the term wheels in this story refers to wheels and tires)
Identical triplets Al, Bob and Chuck buy three identical bush planes. Since they live in Alaska, all three brothers buy and install large balloon “tundra tires” and wheels. The wheels, planes and brothers are identical. All three planes will take off from a normal runway in exactly 100 feet and at exactly 50 mph. The brothers fly their planes to an air show in Wisconsin. At the air show Bob finds and buys a set of fantastic wheels. These wheels are exactly like the wheels he has on his plane in every way except they have half the mass. Their mass is distributed in the same proportion as the wheels that he plans on replacing. Al thinks Bob is silly and is content with his old wheels. Bob thinks that Al will eventually want a set, so he buys a second set to give to Al on their birthday.
Bob finds a buyer for his old heavy wheels and installs a set of his new lightweight ones. He loads the second set into his plane so that it is balanced just as it was before. Bob’s plane now weighs exactly the same as Al’s and Chuck’s, but its wheels have no mass.
Meanwhile, Chuck runs into a magician who sells him a set of magic wheels. These wheels are exactly like the wheels he has on his plane in every way except they have no mass. Chuck installs his magic wheels. He loads the second set into his plane so that it is balanced just as it was before. Chuck’s plane now weighs exactly the same as Al’s and Bob’s, but its wheels have half the mass.
When the brothers leave the air show they request a formation take off. They line up wing tip to wing tip and apply power at exactly the same time. All three planes weigh exactly the same and must hit 50 mph to lift off. When Chuck’s plane lifts off his wheels stop spinning instantly since they have no mass. Since they have no mass, they also have no rotational inertia. When Al’s plane lifts off his heavy wheels are spinning at 50 mph and have considerable rotational inertia. When Bob’s plane lifts off his half-weight wheels are spinning at 50 mph and have exactly half the rotational inertia as Al’s wheels.
Where did the rotational inertia and energy in Bob’s and Al’s wheels come from?
How did the rotational inertia and energy now stored in Bob’s and Al’s wheels affect the take off distance of their planes?
We know that Al’s plane will still take off in exactly 100 feet; where will Bob’s and Chuck’s planes take off?
Originally posted by hitech
lukster: Can you help either me or Mini D understand what we are missing from each others posts?
Originally posted by hitech
lukster: Can you help either me or Mini D understand what we are missing from each others posts?
Originally posted by lukster
If the belt is allowed an accleration which induces a force on the wheel greater than the thrust the plane is capable of producing then the plane will never move forward. Assuming you allow an acceleration somewhat less than what the plane is capable of countering then you would calaculate the resulting difference over time to determine if the plane could reach flying velocity before the wheels were spinning so fast that wheel bearing heat became a factor.
I know all of this has been said. I'm really just summarizing.
Originally posted by Terror
An airplane can overcome the inertial forces without the conveyor belt, right? It still has to accellerate the wheel. Why would it not be able to overcome the force applied by the conveyor that is also accellerating the wheel? I would estimate the inertial force applied by the conveyor to be double the normal forces needed to be able to take off. Why would it be anymore than that?
Wouldn't the inertial forces be calculable based upon the weight of the wheel, the radius from the center of rotation to the point of friction?
Something like: (wikipedia ftw!)
I=k*M*R^2
where
I is the moment of inertia
k is the inertial constant (1/2 for a solid disk .. closest approximate to a wheel),
M is the mass, and
R is the radius of the object from the center of mass.
I just dont see the wheels (a small percentage of the plane's overall mass) generating enough inertial forces to keep the plane reaching take off speed. The engine and thrust were designed to get the plane+load+20% to flight speed.
Terror
Originally posted by lukster
We're talking rotation speeds much much great than what the plane's thrust is capable of generating. I'm not an engineer so I can't give you some realistic esitmates off hand but I will dig out my old physics book and get back to you with some numbers.
Originally posted by Terror
Why would the rotational speeds be so great? If the wheel needs to spin at 1000rpm (just a number) to get the plane airborne, then if "the control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction", wouldn't the wheel be doing 2000rpm at the plane's take off speed?
I submit that to get the wheel spinning 2000rpm, it would take twice the amount of energy than to get it to 1000rpm.....
Terror
PS. Of course, this is if you translate "opposite direction" to making the wheel spin faster, not sit still....
Originally posted by eskimo2No, miniD is right. Your experiment is flawed.
MiniD and anyone else who does not understand the forces involved in accelerating the rotational velocity of a wheel needs to explain where the energy in Al’s and Bob’s planes’ wheels came from. This really simplifies the problem.
Originally posted by 2bighorn
No, miniD is right. Your experiment is flawed.
If you want to move plane backwards on the conveyor you have to overcome not only the wheel's rotational inertia (which is easy), but also the plane's inertia. The maximum amount of USABLE force to move plane created by conveyor acceleration EQUALS that of friction between wheels and axles (if we assume that grip between conveyor and wheel is optimal). Add the thrust to equation and plane will take off without difficulties.
Keep in mind that the greater its mass, the less a body accelerates under given force.
Originally posted by Terror
Why would the rotational speeds be so great? If the wheel needs to spin at 1000rpm (just a number) to get the plane airborne, then if "the control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction", wouldn't the wheel be doing 2000rpm at the plane's take off speed?
I submit that to get the wheel spinning 2000rpm, it would take twice the amount of energy than to get it to 1000rpm.....
Terror
PS. Of course, this is if you translate "opposite direction" to making the wheel spin faster, not sit still....
Originally posted by lukster
I don't want to dig into the numbers, too many cobwebs in my mind and I don't like spiders. Just assume the belt acceleration is capable of sustaining a backwards force on the plane equal to half it's thrust. While the plane would need a much longer rollout it would eventually takeoff. It would move as if half power were applied.
Originally posted by Terror
I also think that the forces the plane are overcoming during a normal take off roll (without the conveyor) are not generated by rotational inertia. A small percentage is, but not much. The forces in order being overcome in a normal takeoff roll is: AirFrame Inertia (total plane mass), Aerodynamic Drag, Wheel Friction, Bearing Friction, and lastly Rotational Inertia from the wheels.
The conveyor will potentially double the Rotational Inertia needed to be overcome, but doubling the Rotational Inertia of the wheels is still a small portion of the overall forces that need to be overcome.
Terror
Originally posted by eskimo2
Imagine that you are the control system.
You see the plane’s wheel roll forward a foot so you turn on the speed control for the conveyor; it moves a foot. But, when the conveyor moved back a foot, the wheel also moved a foot. So the wheel has moved two feet, but your conveyor has only moved one foot. The only way that they will ever be equal is if the plane’s wheel moves back a foot. So, you turn the conveyor acceleration control like mad. Now the wheel is accelerating at the rate of 100,000 rpm per second. That increase in acceleration moves the wheel back a foot so you adjust the rpm acceleration rate to 99,000 rpm per second and the plane is still, at full throttle.
Originally posted by luksterNot quite. If your conveyor is just trying to match the wheel rotation than rolling resistance and static friction forces would never come even close to thrust force. Rolling resistance coefficient would not significantly change either with increased speed.
It's quite easy to add a belt acceleration that will cause the tire to "skid" due to the tire's inability to match the belt's velocity, even without a plane or rubber band attached. That means the plane's thrust would have to overcome the friction of a tire at least unmoveable relatively to the surface it is sitting on. That would be like taking off with the brakes fully engaged.
Originally posted by 2bighorn
Not quite. If your conveyor is just trying to match the wheel rotation than rolling resistance and static friction forces would never come even close to thrust force. Rolling resistance coefficient would not significantly change either with increased speed.
At any given time and speed, to match the wheels rotation conveyor would accelerate at about the same rate as at the beginning (assuming that plane's acceleration caused by thrust would be linear).
Where Eskimo fails is that he think acceleration equals speed.
Originally posted by Terror
So you are saying at full throttle, the plane will just sit at one spot on the conveyor runway with all the energy being generated by the engine being lost to rotational inertia of the wheels? So the plane would be thrusting against the air sitting still and the wheels whirring and conveyor screaming along?
Terror
Originally posted by eskimo2
More and better movies.
Here is a paper treadmill; the source off acceleration is a falling shoe tied to the paper. On the paper treadmill are a mouse ball, a copper pipe with a rubber band glued around it for traction, and an acrylic ball that may have skid/slip some.
Originally posted by 2bighorn
Not quite. If your conveyor is just trying to match the wheel rotation than rolling resistance and static friction forces would never come even close to thrust force. Rolling resistance coefficient would not significantly change either with increased speed.
At any given time and speed, to match the wheels rotation conveyor would accelerate at about the same rate as at the beginning (assuming that plane's acceleration caused by thrust would be linear).
Where Eskimo fails is that he think acceleration equals speed.
Originally posted by 2bighorn
Nice film but you're not trying to match the rotation of a circular body attached to the large forward moving object.
Your experiment is about creating rotational inertia with sufficient initial acceleration and has little to do with the original question.
Remember, in order to match the wheel RPM you'd never have to accelerate at the greater rate than plane is accelerating down the conveyor no matter the speed.
Originally posted by 2bighorn
Wheels on a plane are there to reduce the friction between plane and runway. They do not propel plane forward.
P.S
I have never seen message board where laws of physics would be changed as often as here.
Originally posted by eskimo2
Yup!
Hard to imagine, isn’t it?
Imagine that you’ve made a human powered fan car. You have bicycle type pedals that turn a propeller for a drive system. You’ve made the frame drive, prop, seat, etc. at just under a hundred pounds. You put four 50 pound wheels on it, however. Luckily it’s got great bearings though. If you pedal really hard you can get it up to 10 mph.
Now you park your contraption on this super treadmill, get on and pedal. I turn the acceleration control so that your wheels are gaining 1,000 rpm per second (at the end of 1 second they are going 1,000 rpm, at the end of 2 seconds they are going 2,000 rpm, etc). Do you think that you can move forward? Or will you go shooting back?
Now just imagine the same concept of a grander scale!
Originally posted by Terror
So for this to happen, the conveyor runway would have to be able to accellerate to infinity. Because the only way to counter the forward rotation of the wheels is to use accelleration. So according to your theory, if I had extremely light wheels (not massless, just very light ) the belt would accelerate quickly, but if I had very heavy wheels, the belt would accelerate more slowly. If I leave the throttle set, the belt would continue to accellerate because that is the only way to counter the energy (from thrust) being added to the mix.
I guess I'm not convinced that:
If the wheel travels 20in in one RPM and the conveyor matches that distance with 20in, and 2 RPM moves the conveyor 40in that it would produce enough counterforce from inertia to keep the plane from moving. The plane would accelerate, the wheels would rotate at twice the RPM for any given speed, and the conveyor would accelerate to match the wheels RPM.
Terror
Originally posted by lukster
The mass of the wheel does not determine the speed of the belt. Since mass cannot accelerate to the speed of light that is our limiting velocity. If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity, and the force applied to the plane by the accelerating belt equaled the thrust of the plane, then the plane would sit stationary at full power until the belt and wheel got really close to lightspeed. Then the plane would begin to roll forward and take off normally.
Originally posted by Terror
So for this to happen, the conveyor runway would have to be able to accellerate to infinity. Because the only way to counter the forward rotation of the wheels is to use accelleration. So according to your theory, if I had extremely light wheels (not massless, just very light ) the belt would accelerate quickly, but if I had very heavy wheels, the belt would accelerate more slowly. If I leave the throttle set, the belt would continue to accellerate because that is the only way to counter the energy (from thrust) being added to the mix.
I guess I'm not convinced that:
If the wheel travels 20in in one RPM and the conveyor matches that distance with 20in, and 2 RPM moves the conveyor 40in that it would produce enough counterforce from inertia to keep the plane from moving. The plane would accelerate, the wheels would rotate at twice the RPM for any given speed, and the conveyor would accelerate to match the wheels RPM.
Terror
Originally posted by lukster
The mass of the wheel does not determine the speed of the belt. Since mass cannot accelerate to the speed of light that is our limiting velocity. If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity, and the force applied to the plane by the accelerating belt equaled the thrust of the plane, then the plane would sit stationary at full power until the belt and wheel got really close to lightspeed. Then the plane would begin to roll forward and take off normally.
Conveyor matches the plane’s speed: plane will fly!
Originally posted by APDrone
Actually.. if the speed of the belt and wheel got close to lightspeed.. wouldn't they expand in size as they approached light speed? Then the gear would probably rip through the wing or fuselage and produce wreckage.
Still won't fly.. :rofl
Originally posted by luksterOK, I'll try one last time.
If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity
Originally posted by lukster
The mass of the wheel does not determine the speed of the belt. Since mass cannot accelerate to the speed of light that is our limiting velocity. If it were possible to build a belt, wheel, and bearing that could withstand lightspeed velocity, and the force applied to the plane by the accelerating belt equaled the thrust of the plane, then the plane would sit stationary at full power until the belt and wheel got really close to lightspeed. Then the plane would begin to roll forward and take off normally.
Originally posted by lukster
As an object approaches the speed of light it's mass approaches infinity. Don't confuse mass with size (volume) though. Black holes can be relatively small in size but very massive.
Originally posted by Terror
I did not mean to say the mass determines the actual speed, but it determines the rate at which the belt will accelerate. (also, the diameter of the wheel also...)
Limitations of any of the physical parts are not a part of the discussion. You have to assume the parts (conveyor, plane, engine, wheels, controller) are of limitless capabilities and will not fail under any circumstance. Energies are tranferred, but the energies cannot cause failures within the systems they are being transfered to/through. Otherwise you can only speculate as to which system or part will fail first.
I think eskimo2 may have converted me. As long as energy can be tranferred from the conveyor to wheel to the plane, then there is the possibility that the plane would sit still on conveyor. As long as thrust is input via the engine, the conveyor would accelerate at a rate to completely counter the input energy.
The question remains in my head as to whether the above situation matches the original "the controller monitors plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction" would allow for the belt to accelerate at a rate high enough to impart enough counter force....
Terror
Originally posted by APDrone
D'oh! Ok.. But mass does equate to weight, right?.. so if the wheel attains more mass, then the airplane engine has more mass to move, reducing it's acceleration.
I know I'm going somewhere I don't belong.
Simple vectors I can handle.. Relativity, however, is one of those relatives I'd rather not pick.
Or was it their nose?...
Originally posted by lukster
Well, an object that has 1 kilogram of mass on the surface of the earth will also weigh 1 kilogram but the same object will "weigh" less on the moon but still have a mass of 1 kilogram.
Originally posted by lukster
Maybe I'm misunderstanding you. Perhaps you are saying that a lesser acceleration will be required to hold the plane in place for a wheel with more mass?
Originally posted by Gunston
I can’t believe this is still going on.
There have been several similar examples like this but here is another. The plane moves because of the thrust generated by the engine (prop, jet) not the wheels so in my example we remove the engine. Ok you have your conveyor runway set up, your plane on the runway and a truck at the far end of the runway with a rope tied to the front of the plane to provide thrust. The truck starts rolling gaining speed the conveyor runs in the opposite direction matching wheel speed as you pull the plane the truck reaches 60 mph guess what the plane is going to lift off. On the next run you use the engine instead of the truck and rope for thrust the same thing will happen it will lift off.
However the energy is added to the aircraft (engine, rope, etc), an equal and opposite reaction will occur. The belt will accellerate the wheel at such a rate to completely "nullify" the input energy
Originally posted by Terror
However the energy is added to the aircraft (engine, rope, etc), an equal and opposite reaction will occur. The belt will accellerate the wheel at such a rate to completely "nullify" the input energy. It is the ONLY way for the X=-X equation to balalce. X=wheel speed, -X=conveyor speed. (ie. converyor moves at exact opposite speed of the wheel) The energy applied to the aircraft is "absorbed" by the inertial forces of the conveyor accelerating the wheel at extremely high rates. If you continue to add energy (ie. pulling the aircraft with the truck) the belt will continue to accelerate at a rate the nullifies the input energy.
The rate of acceleration of the belt depends on several factors:
1. the rate of energy being added (engine thrust, rope from truck, etc)
2. the radius of the wheel from axle to contact patch
3. the mass of the wheel and where the mass resides within the radius.
Of course, this is a philisophical/theoretical discussion. There is no way to simulate/experiment this kind of setup. The "perfect" systems do not exist. A conveyor that has an unlimited acceleration capability, a wheel that has a perfect friction (slipless) contact with a surface, a wheel that can spin at an unlimited rate, etc, etc. Real world systems would disintegrate rather quickly....
[edit for run-on sentence...]
Terror
Ps. eskimo2, Am i explaining things anywhere close to correctly?
Originally posted by eskimo2
Here is a paper treadmill; the source off acceleration is a falling shoe tied to the paper. On the paper treadmill are a mouse ball, a copper pipe with a rubber band glued around it for traction, and an acrylic ball that may have skid/slip some.
AVI:
http://hallbuzz.com/movies/paper_treadmill.AVI
QuickTime
http://hallbuzz.com/movies/paper_treadmill.MOV
Originally posted by SteveBailey
If the truck is not on the conveyer, the airplane will move forward and take off.
The original poster didn't say anything about energy, only about wheel speed. The conveyer would match the speed of the wheels, so say... doubling it. the friction and rolling resistance of the wheels is miniscule compared the to thrust of the motor, doubling this incredibly small resistance would not significantly affect the takeoff speed of the plane.
Again: The original poster did not say the conveyer was matching the planes energy, only wheel speed. One has nothing to do with the other.
Originally posted by Terror
Actually, it takes energy to accelerate the plane. Which would spin the wheels, which would make the conveyor spin to match the wheels, which also accelerates the wheels, which adds energy in the opposite direction. To match the wheels speed, the conveyor will continue to accelerate at an extreme rate to match the energy input from the engine. It is the only way the conveyor can match the wheel speed.
If the wheel rolls forward 36in in 1 revolution, and the belt moves backward 36in to "match the speed" of the wheel for each revolution, how far does the wheel/airplane move forward over the ground(or the air)?
Terror
Originally posted by rabbidrabbit
A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's wheel speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.
The question is:
Will the plane take off or not?
Originally posted by Gunston
I should have been more clear in the example of course the truck is not on or in any way affected by the conveyer, as thrust from the prop would not be. It is straight ahead on the far side of the contraption with a really long rope going back to the plane. Now tell me that the truck could not move the plane due to equal and opposite reaction of the conveyer
Originally posted by SteveBailey
At this point, anyone who still thinks the plane will not fly should either take Physics 101 or maybe ground school, although if they are still convinced the plane will not fly I doubt they are teachable/ willing to learn.
Originally posted by TerrorI'm happy I do not live in your universe :lol
Doesnt matter where the "thrust" comes from. If the original bounds of the question are maintained. Then the plane CANNOT move. The wheel and conveyor must maintain the same speed in opposite directions and if thats true, then the wheel cannot move down the conveyor.
Originally posted by 2bighorn
I'm happy I do not live in your universe :lol
Originally posted by SteveBailey
Terror, how do the wheels start turning? What makes the wheels on the plane turn?
AP drone, feel free to answer this question as well. :)
Originally posted by eskimo2
Steve & deSelys,
Watch the Movie!
AVI:
http://hallbuzz.com/movies/paper_treadmill.AVI
QuickTime
http://hallbuzz.com/movies/paper_treadmill.MOV
Originally posted by SteveBailey
so, the only way for the wheels to move is for the plane to move forward, we agree on that.
Based on what you are saying:
So the plane is now moving forward(wheels rolling), then it stops? It stops [ actually, never really started ] in relation to the world outside of the conveyor. If the engine is still generating thrust, the thrust is trying to move forward, which is translated into a rolling effect, which is countered. The only way for the wheel to stop spinning is if the engine is turned off ( or idled or whatever it takes to make the prop stop trying to go ) or the brakes are applied. If it was moving forward a 1 mph and then stopped, the wheels would then slow down right? By 1 mph? Then the conveyer would slow down right?
Originally posted by deSelys
Lol, i've just found this on a science-oriented forum:
More than 400 pages! (http://forum.physorg.com/index.php?showtopic=2417&st=6960&#entry170648)
We're waaaaaaay behind! :D
Originally posted by Terror
From a Physics Forum on this exact topic:
459 Pages, 6876 replies!!
Physics Forum Thread (http://forum.physorg.com/index.php?showtopic=2417&st=0)
Hitech, this thread may just fill your HD space !!
Terror
Originally posted by deSelys
Lol, i've just found this on a science-oriented forum:
More than 400 pages! (http://forum.physorg.com/index.php?showtopic=2417&st=6960&#entry170648)
We're waaaaaaay behind! :D
Look.. I have explained this stuff about 4 or 5 times and Eskimo has explained more often and better than I.
Originally posted by SteveBailey
Here's the simple truth and you agreed already, although you won't like it.
The only way for the wheel to start spinning.. THE ONLY way, is for the plane to start moving forward.(there is no thrust at the wheel)
If the plane does not move forward, the wheels never start spinning. In your model APdrone, the wheels magically begin turning yet the plane never moves to start the wheels turning in the first place.
That you guys continue to argue this is amazing to me.
Explain to me how the wheels start turning if the plane does not move forward? The conveyer matches the speed of the wheels. If the plane never moves as you state, the wheel speed muist be 0. Explain this.
Whitehawk, what happens to the plane... does it move at all?
Then if it never moves at all, the plane still doesn't fly.
Originally posted by SteveBailey
So your answer to my debate is: since it never moves it doesn't fly. how exactly does the conveyer keep the plane from moving? The conveyer matches the wheel speed... that means the wheels have to turn first, even if for an instant. Which means the plane has to move forward.
You are getting somewhere if you would heed our discussion instead of worrying about "winning".
Think about the initial question. The conveyer matches the "speed" of the wheels, not the thrust of the airplane. If the thrust of the plane is 1000 pounds at first push to full throttle, are you saying the conveyer thrusts back against the wheels at 1000 pounds? If so, what does this have to do with wheel speed, which was the original posit?
Again, the conveyer matches the speed of the wheel. The only way for the wheels to move is for the plane to move forward. What stops the plane from rolling forward and how?
Originally posted by lukster
Steve, your missing the point of one perspective of this situation allowed by the ambiguity in the question. The force applied through acceleration of the wheel may meet or exceed the thrust of the plane if we apply no limitation to how fast the belt can accelerate.
The plane would move forward of it's initial starting ponint only once.
Originally posted by SteveBailey
What stops the plane from moving forward? I ask you this.... again.
and explain exactly how the conveyer accerates off into infinity.
If the plane isn't moving forward as your model states, why can the conveyer never match the wheels speed? If the wheel is merely spinning and not moving forward, how is it possible not to match it's speed? Explain this please.
Originally posted by deSelys
LOL....It will move. Because the conveyor belt is bringing energy into an 'energy tank' (the spinning wheels) which has next to zero influence upon the other energy tank feeded by the plane engines (cinetic energy= plane mass*(plane speed squared)/2).
The next to zero influence is the friction between wheels and axles. I've looked it up and friction is max when there is no relative movement. As soon as the two objects begin to 'slip' on each other, the friction lowers a bit and stays constant. The energy generated by the friction will be turned into heat. Will heat stop the plane?
You could consider the problem like this: a truck is filled with water. As soon as the truck engine tries to move it, heaters in the water tank kick in to try and stop the truck. Will it move?
Originally posted by john9001
the belt can not apply any force to the plane , all the belt can do is spin the wheels.
Here's the premise. The plane moves forward and the wheel is now spinning forward at 10rpm. The wheel monitor senses this and tells the belt to move in the opposite direct at 10rpm. Since the A/C engine is applying thrust at least holding the plane in place or moving forward for now, the belt now moving in the opposite direction at 10rpm causes the wheel to rotate at 20 rpm. The wheel monitor/belt controller senses this and speeds the belt to 20rpm, this increases the wheel to 40 rpm. The rate at which the belt is allowed to accelerate to it's max speed determines the force applied to the plane countering it's thrust.
Originally posted by SteveBailey
There is no mention of the conveyer countering the force of the motor, only the speed of the wheels. The conveyer is acting against the rpm of the wheel, not the thrust of the motor(per the initial post). The only way for the wheels to be moving faster than the conveyer(causing it to accelerate) would be if the plane had a foward vector.
Originally posted by SteveBailey
So your answer to my debate is: since it never moves it doesn't fly. how exactly does the conveyer keep the plane from moving? The conveyer matches the wheel speed... that means the wheels have to turn first, even if for an instant. Which means the plane has to move forward.
You are getting somewhere if you would heed our discussion instead of worrying about "winning".
Think about the initial question. The conveyer matches the "speed" of the wheels, not the thrust of the airplane. If the thrust of the plane is 1000 pounds at first push to full throttle, are you saying the conveyer thrusts back against the wheels at 1000 pounds? If so, what does this have to do with wheel speed, which was the original posit?
Again, the conveyer matches the speed of the wheel. The only way for the wheels to move is for the plane to move forward. What stops the plane from rolling forward and how?
Originally posted by SteveBailey
So where did you come up with the belt countering the thrust of the motor?
How else are you gonna stop the plane from moving forward?
Originally posted by SteveBailey
That's just it, the plane doesn't stop moving forward. It accerates down the conveyer until liftoff.
Originally posted by Kuhn
The wheels wont roll if an airplane does not move forward..Being that the wheel movement is is controlled by the prop moving through the air ,the plane will move forward nomatter what the wheels are doing The conveyer can try all it wants to keep the wheels spinning fast enough to keep up with the rotation but in no way is it possible even in theory for it to keep up with the wheel speed. It will keep accelerating as the plane moves forward and the plane will lift off. So actually the original question is flawed and cannot be answered the way it was asked. It is not possible for the plane not to move forward when the prop is pulling it through the air. It is not possible for the plane to move forward if the wheels dont rotate when in contact with the ground.
I could go on but I wont :D
Originally posted by Golfer
Or a throttle lock for that matter :rolleyes:
The agruments for the airplane not flying are getting more and more absurd
Originally posted by Golfer
"If I understood their madness then I would be insane too."
I said that 5 years ago and I think it clearly still applies.
If the plane does not slow, the wheel will then be sped up to 2 RPS
Originally posted by SteveBailey
Well according to you the plane does slow. If the plane slows, this allows the conveyer to catch up to the wheels, thus no longer accelerating. If the plans slows to a stop, as you claim, then the wheels are no longer going forward so there is no speed in the wheels for the conveyer to match. Since the conveyer has now caught up, and the plane is slowing to stop, the wheels will slow down. Now the conveyer has to match the wheels, so the conveyer stops too.
Thus, according to you, the wheels will not be moving, the conveyer will not be moving, and the airplane will somehow be not moving even though it is at full throttle.
Originally posted by SteveBailey
Well according to you the plane does slow. If the plane slows, this allows the conveyer to catch up to the wheels, thus no longer accelerating. If the plans slows to a stop, as you claim, then the wheels are no longer going forward so there is no speed in the wheels for the conveyer to match. Since the conveyer has now caught up, and the plane is slowing to stop, the wheels will slow down. Now the conveyer has to match the wheels, so the conveyer stops too.
Thus, according to you, the wheels will not be moving, the conveyer will not be moving, and the airplane will somehow be not moving even though it is at full throttle.
Originally posted by SteveBailey
Whitehawk, what happens to the plane... does it move at all?
Originally posted by APDrone
Then if it never moves at all, the plane still doesn't fly.
The question states the wheel speed is matched.. Assumingly instantly. How it does that is not really part of the question. It just happens. That's the aggravating aspect to the whole question because we know that physics ( as we know it ) wont let that happen after the tolerance for heat, matter, friction, .. etc are exceeded.
So then it dives into an argument about when is the thrust applied to when and where and by how much.
Which is not part of the question.
Bottom line, again, is that if wheel speed is matched by a counter speed which leaves the net speed being zero, then necessary airspeed is never achieved and flight is not attained.
Plane doesn't fly.
Period.
End of sentence.
End of argument.
Originally posted by WhiteHawk
Sure, the only way the plane would not move would be if the drag factor of the wheels between the plane and the conveyor is 100%. Or if the plane were fixed to the conveyor.
Originally posted by APDrone
At your request to 'heed our discussion', I will continue.
You're probably right in that there may be some actual initial movement, then the conveyor would match the rotational speed and it would bring net movement back to 0. It really depends on the tolerances of the equipment used. With laser scanning and such, it could even be down to a billionth of an inch. The wheel would still be spinning and constantly accelerating, but this speed, whatever it is at a given time is matched, as stated. Any additional acceleration would be starting with 0 net speed.
I think I see where you're going with this.. in that, eventually, the aircraft may inch its way across the conveyor and eventually be liberated from it... at which point the normal physics kick in and it takes off normally..
Unless it is in AH's world and hits the customary tree or hill at the end of the runway.
Originally posted by Golfer
You're allowed to agree that it will fly, lukster. It's not too late for that.
Originally posted by Golfer
I'm trying to uncomplicate things.
Answer the question. Come up with something that invalidates the boat example and we'll name the theory of aircraft conveyor relativity after you.
The examples are valid. Air is the fluid through wich an airplane passes. Water is the fluid through which a boat travels. The conveyor works the same way.
Invalidate the boat.
Originally posted by lukster
if the wheel speed is what the belt matches in the opposite direction and is capable of accelerating at a rate capable of inducing into the wheel a force equal to the engine's thrust it will not.
Originally posted by lukster
I don't understand how your boat scenario is any different from the plane. I'm sorry but I couldn't follow your explanation. Tell me how your boat scanario is different from that of the plane and let's talk about that.
Originally posted by Golfer
Exactly how do you plan on getting a spinning wheel on a spinning treadmill to counteract 6,800lbs of thrust for something like a Citation Encore?
Originally posted by Golfer
Since the the belt is supposed to match the wheel speed...if the airplane is moving forward at 1,000,000mph then your belt would still only double the wheelspeed.
You think it's not possible for the engine to be at full power without rolling the wheels forward? Try tying together two planes facing opposite directions. Anywhere is fine but tie their axles together and you will most closely simulate the force being applied by an accelerating belt. If both planes are identical neither will move with engines at full power.
Originally posted by eskimo2
I think part of the problem here is that when people finally realize that there is a real force involved in accelerating a rotating object they just can’t get past that it could potentially rival the power of an aircraft engine.
Originally posted by 2bighorn
True, what you don't realize is that maximum amount of force you can transfer from conveyor to the airframe is equal that of rolling resistance which is defined as following:
Rolling resistance = ((Aircraft weight X gravitational acceleration) / number of wheels) X coefficient of rolling friction
Coefficient of rolling friction used in aircraft industry is about 0.0035 in average.
Now do some math and you'll see that amount of your usable force is nothing in comparison to the thrust.
Originally posted by luksterI did the math.
You do the math and see if your engine can overcome the coefficient of friction of your skidding tires. If it can I just increase the acceleration to reduce your tires to smoke in a microsecond. Then you have to figure the drag of the gear. I already addressed this.
Originally posted by 2bighorn
I did the math.
You assume that wheels will blow due to speed but not the conveyor. For fairness assume that if conveyor is indestructible so are the wheels.
To eskimo.
Repeat your experiment with little wheel cart or car model instead of wheel alone. Put some weight on it so that ratio between wheels weight and total weight of your cart will match that of airplane/wheels ratio.
Since your model won't have sophisticated axle/bearings and sander has sand paper on it, your Crr (Coefficient of rolling friction) will be few times bigger. Even so, your rubber band will be stretched considerably less than if you're using wheel alone.
Originally posted by Golfer
Eskimo in all of your experiments the conveyor is initiating the action onto the wheel. In the question its the wheel that initiates the conveyor.
If your points regarding this are valid I have just one more question...
Why can't you dyno an airplane?
That right there blows any argument could make right out of the water. You dyno an airplane in a still-air environment then I might listen to what you have to say. Other than that...can't be done.
get an airplane to dyno on one of these...your arguments are all valid. You won't...
(http://www.archang3l.net/pics/dyno2.jpg)
Originally posted by SteveBailey
No, i'm saying that the wheels can only begin moving by the plane rolling forward. If the hweels of the plane stop moving forward, as you said they would,(you said the plane would stop) then since the wheels were no longer acceolerating, the conveyer would catch up. If the wheels are no longer moving forward, and the conveyer does not spin the wheels, but merely paces it, the wheels will slow down and stop since there is nothing causing the wheels to spin. Again, in youir model, the plane stops, the wheels stop, the conveyer stops yet the plane goes nowhere.
Originally posted by luksterThat's the point, you can exercise force no greater than rolling resistance force and given the indestructible gear, rolling resistance is constant no matter how fast would you accelerate the conveyor.
Very good. We have an indestructable belt and wheels. Give me a coefficient of friction and I'll give you an acceleration.
As I said many posts ago, this is calcuable. Given the weight of the plane, coefficient of friction, and thrust of the plane we can determine what acceleration of the belt is required to counter the planes max thrust.
Originally posted by eskimo2
No,
As long as the plane’s engine is generating thrust the conveyor and wheels must be accelerating! Even at a power setting that would normally make the plane taxi slowly, the conveyor and wheels must be accelerate by hundreds or maybe thousands of rpm per second! The rpm must always be increasing until the pilot shuts off the motor or the plane runs out of gas.
Originally posted by WhiteHawk
So its settled, the plane will take off, Or is there more?
Originally posted by 2bighorn
True, what you don't realize is that maximum amount of force you can transfer from conveyor to the airframe is equal that of rolling resistance which is defined as following:
Rolling resistance = ((Aircraft weight X gravitational acceleration) / number of wheels) X coefficient of rolling friction
Coefficient of rolling friction used in aircraft industry is about 0.0035 in average.
Now do some math and you'll see that amount of your usable force is nothing in comparison to the thrust.
Originally posted by 2bighorn
I did the math.
You assume that wheels will blow due to speed but not the conveyor. For fairness assume that if conveyor is indestructible so are the wheels.
To eskimo.
Repeat your experiment with little wheel cart or car model instead of wheel alone. Put some weight on it so that ratio between wheels weight and total weight of your cart will match that of airplane/wheels ratio.
Since your model won't have sophisticated axle/bearings and sander has sand paper on it, your Crr (Coefficient of rolling friction) will be few times bigger. Even so, your rubber band will be stretched considerably less than if you're using wheel alone.
Originally posted by WhiteHawk
So its settled, the plane will take off, if the wheels and conveyor dont fail? Or is there more?
Originally posted by SteveBailey
Why must they be accelerating? The conveyer is designed to match the speed of the wheels. It does not accelerate the wheels. the wheels have no thrust on their own. They can only spin when the plane moves forward. In your model the plane doesn't move. yet we know that the wheels can only start to spin if the plane moves. How do you explain this?
The conveyer is not designed to offset the thrust of the motor, but only to offset the speed of the wheels. Tell me again how the conveyer went from matching the speed of the wheels to causing the wheels to spin faster?
Originally posted by eskimo2For Christ's sake Eskimo, you're a teacher. Try to include some deductive reasoning in here. For once try to understand that there's a linkage between the wheels and plane which can transfer only limited amount of force.
You’re not recognizing the real force at work. This is not about friction, it’s about rotational inertia. Watch the video; see the force.
Originally posted by 2bighorn
For Christ's sake Eskimo, you're a teacher. Try to include some deductive reasoning in here. For once try to understand that there's a linkage between the wheels and plane which can transfer only limited amount of force.
No matter the force applied to the wheel, there's a limited amount transfered to the plane and that amount can not be greater then force of rolling resistance. In our case rolling resistance of ball bearings. Ball bearings are your force transfer bridge and it can transfer finite amount of force only. This amount can be easily calculated.
In your wheel movie you're transferring force from the sander to the wheel but everything else is missing. If you want your experiment to be valid, you need to TWO force bridges. One between belt and wheel and one between wheel and the airframe.
Originally posted by Golfer
Exactly how do you plan on getting a spinning wheel on a spinning treadmill to counteract 6,800lbs of thrust for something like a Citation Encore?
Originally posted by APDrone
Explain how chocks work, then.
And brakes too, while you're there.
Originally posted by luksterFirstly, where did you guys get an idea that it'll reach speed of light? Even if the speed of conveyor is matched to that of the wheel at relatively low frequency it should never exceed speed that twice of the plane at take off. Mathematically, if I would sync the belt every few seconds only, the speed differential would never be as such that the short term acceleration would significantly change the transfer of force.
Are you intentionally ignoring a belt that might actually approach light speed assuming enough force can be applied? Before it reaches an insurmountable speed a tremendous force will be exerted upon that wheel.
Originally posted by 2bighorn
For Christ's sake Eskimo, you're a teacher. Try to include some deductive reasoning in here. For once try to understand that there's a linkage between the wheels and plane which can transfer only limited amount of force.
No matter the force applied to the wheel, there's a limited amount transfered to the plane and that amount can not be greater then force of rolling resistance. In our case rolling resistance of ball bearings. Ball bearings are your force transfer bridge and it can transfer finite amount of force only. This amount can be easily calculated.
In your wheel movie you're transferring force from the sander to the wheel but everything else is missing. If you want your experiment to be valid, you need to TWO force bridges. One between belt and wheel and one between wheel and the airframe.
Originally posted by 2bighorn
Firstly, where did you guys get an idea that it'll reach speed of light? Even if the speed of conveyor is matched to that of the wheel at relatively low frequency it should never exceed speed that twice of the plane at take off. Mathematically, if I would sync the belt every few seconds only, the speed differential would never be as such that the short term acceleration would significantly change the transfer of force.
It's true that faster the wheel spins the bigger the wheel angular momentum is. Angular momentum will keep wheels spinning for a while after they get retracted into wheel wells. If the angular momentum is large enough it may cause gyroscopic effect similar to that of flywheel and it will resist the directional changes due to the stored energy, however it will not stop plane gaining the speed nor the resistance to directional change would be sufficient to prevent pitching the nose high for take off.
While you guys are always hanging on the wheel momentum, do not forget that the plane inertia itself will resist to any change of its forward movement.
If you glue the tires to the belt and slam the brakes, then yeah, something weird might happen.
What speed wiil match the speed of the wheels?
Originally posted by eskimo2Simplified definition: inertia is amount of resistance to change in velocity which is determined by mass.
The plane has no inertia, it does not move.
Originally posted by eskimo2That, honestly doesn't make any sense. As long as the energy created by generators isn't used for braking force, plane will still move.
The plane on the treadmill has big electric generators driven by the wheels. When the treadmill spins the airplanes’ wheels they spin the generators which make as much energy as the output of the planes engines. In that situation the plane sits still, even though the plane’s engines are on full power.
Originally posted by SteveBailey
Answer the question. The question is, when did the conveyer go from matching the speed of the wheels to acceratign them? I can't find anywhere in the original post where it says the conveyer acelerates the wheels.
Originally posted by luksterLets assume the same is with destructibility of conveyor and wheels.
Assuming this as I will, consider then the masses involved as velocities approach the absurd, namely light speed. Any jet or rocket engine currently in existence is inconsequential in comparison.
Originally posted by eskimo2
It has to, because it’s the only thing that works. Should the conveyor speed up to only 10 mph? Should it not even try? Should it be happy at 90%? 99%?
Imagine that you are the control system.
You see the plane’s wheel roll forward a foot so you turn on the speed control for the conveyor; it moves a foot. But, when the conveyor moved back a foot, the wheel also moved a foot. So the wheel has moved two feet, but your conveyor has only moved one foot. The only way that they will ever be equal is if the plane’s wheel moves back a foot. So, you turn the conveyor acceleration control like mad. Now the wheel is accelerating at the rate of 100,000 rpm per second. That increase in acceleration moves the wheel back a foot so you adjust the rpm acceleration rate to 99,000 rpm per second and the plane is still, at full throttle.
If the riddle stated that the plane had mass less wheels, we could only assume that the conveyor and wheels would instantly spin at an infinite speed while trying to do its job. It would clearly fail and the airplane would take off.
Originally posted by 2bighorn
Simplified definition: inertia is amount of resistance to change in velocity which is determined by mass.
That, honestly doesn't make any sense. As long as the energy created by generators isn't used for braking force, plane will still move.
Originally posted by SteveBailey
Now you are talking distance(feet). The original scenario talks of speed. You have already admitted the plane will move forward, regardless. Plane moves, wheels spin, plane takes off.
Again you are allowing for the conveyer to accelerate the wheel... it only paces the wheel. You cannot change the rules.
Also, why are you ignoring the fact that rolling resistance is constant?
Originally posted by hitech
eskimo2:
Not a bad analogy eskimo. And obviously they would accelerate at different speeds do to the rotational inertia stored in the wheels. Since 1 has 0 mass, 1 has 1/2 mass, the 0 would be in front , followed by the 1/2 mass. Followed by the original plane.
Originally posted by 2bighorn
Lets assume the same is with destructibility of conveyor and wheels.
If your conveyor is able to reach the speed of light, let my plane be able to do the same.
It is unfair to ignore or bend laws of physics for the parts of equation needed to prove your hypothesis, at the same time apply them rigorously to the rest.
Originally posted by SteveBailey
I'm not confusing anything, you are trying to ignore certain points because they do not fit your argument.
you have already said the plane will roll foward, and you've already said the plane will not move. You want to have it both ways as it is convenient for you.
Which is it?
Originally posted by lukster
Allow me. For the belt to initiate movement the plane must move forward. How much forward motion is allowed is dependent upon how much thrust the engine can generate and how much opposing force the belt can apply. It may only be trillionths of an inch.The plane may only have to think about moving. At the quantum level of course. ;)
Originally posted by SteveBailey
Then again it might move far enough and fast enought to take off, in your model.
Originally posted by lukster
All you have to do is say the thrust of the plane is greater than the force applied by the belt. I'm ready to let you decide.
Originally posted by SteveBailey
I've been saying that. :)
Originally posted by eskimo2Ok Eskimo, let me see if I got that right
Read the story, I think it’s what made the guys on the physics board finally get it.
Originally posted by SteveBailey
LOL Luk, I didn't feel like we were debating/arguing enough to have a "winner"... just a friendly tete-e-tete.
I do understand, FWIW, the point you are making about the net drag/resistance at the wheels. I'm glad we concluded this thread on a friendly note, irrespective of the outcome. I do so very much enjoy civil discourse.*
To all in here who treated me so kindly during this lil affair, I thank you.
Steve..........out!
* I am sometimes guilty of needlessly ratcheting up the volume.
Originally posted by Kuhn
I wish I was in a bar with y'all debating this. Sounds like it be a good time.
Originally posted by eskimo2
Depends on the question:
If the question were that the conveyor were matching the plane’s speed in the opposite direction, the conveyor would be traveling at the rate of 50 mph at take off and so would the plane. The plane would take off normally except its wheels would be rolling at twice their normal take off rpm. That consumes a tiny bit of energy that really can’t be felt by the pilot.
Conveyor matches the plane’s speed: plane will fly!
Our question, however, states that the conveyor must match the plane’s wheel’s speed.
In this case if the plane moves forward, its wheel has gone further, and therefore faster than the conveyor. The only thing that the conveyor can do about this is to speed up like mad!
Conveyor matches the plane’s wheel speed: plane will NOT fly!
Originally posted by Kuhn
There are too many variables added to the question at this point. If we keep adding "what ifs" or "and this too" the question can never be answered correctly. If we stick with the original question my answer still applies. :D
Originally posted by Kuhn
:lol you know what? If the propeller spins at the speed of light you wont be able to see it untill the future. In real life the conveyer would make that plane nose over and crash like a bad landing.
For those who dont understand physics, youre thinking too hard about it.
I wish I was in a bar with y'all debating this. Sounds like it be a good time.
Originally posted by 2bighorn
Ok Eskimo, let me see if I got that right
You have created the problem where there isn't any. You work under assumption that conveyor possesses some fuzzy logic and is unable to match the acceleration rate of the plane therefore it outaccelerate at such rate that wheels would be spinning so fast to be able to store enough rotational energy to act as a giant flywheel resisting to any kind of movement of a plane.
Lets say it is so and that the conveyor is accelerating faster and faster and that the wheel can actually sustain unlimited rpms.
Now, device such as this fast spinning wheel would resist all latitudinal but not longitudinal change of direction of it's axis of rotation, hence if plane moves forward, it wouldn't stop it, no matter how much force you apply to conveyor, but it would resist if plane would try to weer off to the right or left.
Before you ask me where all this energy goes, let me tell you it wouldn't go anywhere. The wheel would simply store all that energy and keep rotating until all energy would slowly wear off due to friction and other smaller forces such as drag etc.
That ability to store the energy is the reason we got flywheels and they were used since centuries.
And the plane... Ahh yeah, it would still take off.
Originally posted by Casca
(http://pcspray.com/pics/tombstone.jpg)
Originally posted by eskimo2
No, I didn’t create the problem; I created a solution.
Take a look at this question:
A plane is standing on a runway that can move like a giant conveyor belt. The plane applies full forward power and attempts to take off. This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same but in the opposite direction, similar to a treadmill.
The question is:
Will the plane take off or not?
(Note the word “wheel” has been removed)
In this situation the plane would take off normally except its wheels would be rolling at twice their normal take off rpm. That consumes a tiny bit of energy that really can’t be felt by the pilot. - But wait, how can the conveyor speed be exactly the same as the plane? If it waits for the plane to move even the slightest bit it is behind, which is not the same speed…? Or should we just assume that the conveyor stops trying as soon as the plane moves at all? Who cares if the conveyor matches the speed perfectly or within a millimeter or a foot? Focusing on how it might anticipate such movement or accuracy is irrelevant. The point is to understand that in this situation 99+ % of the airplane’s engine energy will be devoted to acceleration and increasing air resistance.
We could play with the elements of the question to help other’s understand why the plane will take off. Such as: Suppose if the conveyor were traveling at twice the speed of the plane; In this situation the plane would take off normally except its wheels would be rolling at three times their normal take off rpm. That would consume twice as much energy (stored in the form of rotational energy) as the same speed conveyor scenario, but it still wouldn’t be felt by the pilot.
The wheel speed question is playing by the same rules; the solution is much more difficult to comprehend however.
Originally posted by WhiteHawk
Soo, the plane will fly should all the mechanical components hold??:(
Originally posted by lukster
Tell me which is greater, an unmovable object or an unstoppable force and I will give you your answer.
Originally posted by WhiteHawk
Nice hijack effort. I am trying to decipher eskimos answer as yes the plane will fly if it were possible to tune the conveyor to the wheel speed, or no it would not, even if it were possible. Feel free to comment.
Originally posted by WhiteHawk
Nice hijack effort. I am trying to decipher eskimos answer as yes the plane will fly if it were possible to tune the conveyor to the wheel speed, or no it would not, even if it were possible. Feel free to comment.
Originally posted by eskimo2
Depends on the question:
Our question, however, states that the conveyor must match the plane’s wheel’s speed.
In this case if the plane moves forward, its wheel has gone further, and therefore faster than the conveyor. The only thing that the conveyor can do about this is to speed up like mad!
Conveyor matches the plane’s wheel speed: plane will NOT fly!
Originally posted by WhiteHawk
Ok, but only because it is not possible for the conveyor to match the wheel speed? The question asks us to assume that to be possible.
Originally posted by john9001
it's a paradox, the conveyor can only match the speed of the plane, so the plane has to move for the conveyor to move, if the plane does not move the conveyor can not move. so there is no answer.
it's like the "time travel paradox," if you go back in time and kill your grandfather you will not be born, if you are not born you could not go back in time to kill your grandfather.
Originally posted by WhiteHawk
Ok, but only because it is not possible for the conveyor to match the wheel speed? The question asks us to assume that to be possible.
Originally posted by eskimo2
Sure its possible, the conveyor can even push the plane back if it wanted to.
Originally posted by WhiteHawk
Lets put the question another way. A plane sitting on a conveyor with alien technology 0 friction wheels. Able to withstand unlimited rpms without failure. A conveyor belt is racing below at 100mph. If there is 0 friction then there would be 0 energy transmission and the plane would be stationary. Now lets fire the engines and give say, 2000lbs thrust. what happens to the plane?
Originally posted by Kuhn
The wheels wont roll if an airplane does not move forward. Being that the wheel movement is is controlled by the prop moving through the air ,the plane will move forward nomatter what the wheels are doing. The conveyer can try all it wants to keep the wheels spinning fast enough to keep up with the rotation but in no way is it possible even in theory for it to keep up with the wheel speed. It will keep accelerating as the plane moves forward and the plane will lift off. So actually the original question is flawed and cannot be answered the way it was asked. It is not possible for the plane not to move forward when the prop is pulling it through the air. It is not possible for the plane to move forward if the wheels dont rotate when in contact with the ground.
Originally posted by JB88
i wish i was in the same bar ignoring all of you physics dweebs while making out with the hot blond in the corner.
Originally posted by eskimo2
Note; this question does NOT depend on rolling resistance or bearing friction. In this area it only requires normal traction between the tires and conveyor surface. This is the same traction that would prevent a plane from moving if it applied full power with its brakes on.
Originally posted by WhiteHawk
Ok good point, if the brakes were on, the plane wouldnt move in our problem right? Now lets fire the thrust at 2000lbs keeping the brakes on, WS=0; CS=0; increase the thrust to full power 18000lbs, brakes on, WS=0; CS=0; We take off the brakes in a fashoin as not to torque apart the plane, what happens to the plane?
Originally posted by lukster
I'm not attempting to influence the determination as to whether the plane flies or not. However, force is transmitted through friction. There can be no force transmitted from one mass to another without friction. You can define the coefficient of friction to be so small that perhaps only two atoms are interacting but you cannot eliminate it. To say that two objects acted on each other with no friction would be like saying they have no mass.
Originally posted by eskimo2
Depends on the mass of the plane. A light enough plane will skid and accelerate. If the skidding drag isn’t too much it might even take off. It has been stated; any plane capable of taking off with its brakes locked (negating tire blow out leading to prop hitting the ground, etc.) will also take on the conveyor where the conveyor matches the wheel speed (also negating tire blow out leading to prop hitting the ground, etc.).
Originally posted by WhiteHawk
although most of the forward movement is absorbed by the traction or friction or drag of the wheels on the conveyor?
Originally posted by WhiteHawk
So the WS=0; CS=0; with the brakes on, but let the brakes out and we get forward some activity. although most of the forward movement is absorbed by the traction or friction or drag of the wheels on the conveyor? The lighter the plane the lesser the effect on PS (Plane speed?)? Ok i am with you.
PS wife is home so I may have to bolt unexpectadly.
Originally posted by WhiteHawk
So the WS=0; CS=0; with the brakes on, but let the brakes out and we get forward some activity. although most of the forward movement is absorbed by the traction or friction or drag of the wheels on the conveyor? The lighter the plane the lesser the effect on PS (Plane speed?)? Ok i am with you.
PS wife is home so I may have to bolt unexpectadly.
Originally posted by eskimo2
Not absorbed, transfered.
Originally posted by eskimo2
Think about this:
The plane on the treadmill has big electric generators driven by the wheels. The generators can turned on or off.
Plane sits on treadmill. Treadmill is off. Plane engine is off. Generator switch is off.
Treadmill is turned on to 50 mph: plane basically sits still, wheels freewheeling. Plane engine is still off.
Treadmill is still on at 50 mph. Generator switch is turned on: generators make electricity. The plane starts moving back on the conveyor. Plane engine is still off.
Right?
Originally posted by WhiteHawk
Right.
Originally posted by eskimo2
Good,
Plane engine is now turned on enough to keep it from moving back. Plane stops scooting back and now sits still, engines on, treadmill on, making electricity (about the same amount of energy as the plane’s engines produce).
Right?
Originally posted by WhiteHawk
Right.
Originally posted by eskimo2
Steve & 2bighorn,
You interpret this question that a plane that normally takes off at 50 mph will take off at 50, and the conveyor will be going…
50? Is this right? Why 50? Why not 40? Why not 60?
Why? Because the total sum of the forces remains the same. Only thing that will change is amount of kinetic energy stored in the wheels which is result of different wheel mass.
Originally posted by eskimo2You need to understand the difference between rotational inertia and angular momentum and how they are related. Both play the roll.
Where did the rotational inertia and energy in Bob’s and Al’s wheels come from?
Originally posted by eskimo2The take off distance does not change since it is depended on total weight (plane + wheels). In your story the reduced mass of the wheels is offset by ballast therefore total mass sum is unchanged and equal thrust is needed.
How did the rotational inertia and energy now stored in Bob’s and Al’s wheels affect the take off distance of their planes?
Originally posted by eskimo2As already said, they all take off at the same speed and after same take-off distance.
We know that Al’s plane will still take off in exactly 100 feet; where will Bob’s and Chuck’s planes take off?
Originally posted by hitechWheel energy comes from the conveyor not from the thrust.
This is incorrect. Where did the wheel energy come from? If you have an increase in energy (which you agree) then some work had to be done.
Originally posted by hitechWhy? it works same way if we increase time (flywheel effect)
Work = Force * Time. Therefore there has to be more force.
Originally posted by hitech
The energy stored in the wheel came from the wheel being turned by the runway (or conveyor). This force is opposite the thrust of the plane (I.E. less net thrust), hence they would not all accelerate at the same rate.
More force is needed to overcome inertia, but less later, since heavier wheels stores more energy therefore later on less is needed to keep it rolling.
rotational inertia and angular momentum
Originally posted by hitechRotational inertia is not velocity depended while angular momentum is.
Some how I am missing your point here, In my view those 2 terms are 100% the same.
Originally posted by 2bighorn
Anyways, thrust has to overcome the rotational inertia.
As already said, they all take off at the same speed and after same take-off distance.
Originally posted by eskimo2Eskimo, thrust has to overcome total inertia that is wheel inertia + plane inertia. Since both are depended on mass, if you just move the mass around from plane to the wheel or from wheel to the plane and the total sum doesn't change then necessary force to get plane in motion doesn't change either.
You said the thrust has to overcome the rotational inertia…
But then you are saying the planes will take off in the same distance?
Originally posted by eskimo2It doesn't.
How does thrust “overcome” energy without consuming energy?
Originally posted by hitechYes
2bighorn: Is this what you are saying?
Moment of inertia = Rotational inertia
Angular momentum = Moment of inertia * RPM.
HiTech
Originally posted by 2bighorn
Eskimo, thrust has to overcome total inertia that is wheel inertia + plane inertia. Since both are depended on mass, if you just move the mass around from plane to the wheel or from wheel to the plane and the total sum doesn't change then necessary force to get plane in motion doesn't change either.
Eskimo, thrust has to overcome total inertia that is wheel inertia + plane inertia. Since both are depended on mass, if you just move the mass around from plane to the wheel or from wheel to the plane and the total sum doesn't change then necessary force to get plane in motion doesn't change either.
Originally posted by Golfer
The fog of this physics war has clouded everything and it makes me not want to click this thread anymore (for my sanity this is probably a good thing)
We need a table, chart or graph. The question asks will the airplane fly.
It will fly people (clowns) to the left...
It won't fly people (jokers) to the right...
Will it fly? Will it not fly? Your opinion in a one word response.
Either "Yes" (it will fly)
or "No" (it won't fly)
-------------------------------------------------------
Yes
Originally posted by Golfer
The fog of this physics war has clouded everything and it makes me not want to click this thread anymore (for my sanity this is probably a good thing)
We need a table, chart or graph. The question asks will the airplane fly.
It will fly people (clowns) to the left...
It won't fly people (jokers) to the right...
Will it fly? Will it not fly? Your opinion in a one word response.
Either "Yes" (it will fly)
or "No" (it won't fly)
-------------------------------------------------------
Yes
Originally posted by Mini D
If an airplane were sitting on a conveyor belt and the conveyor belt instantly accelerated, the plane would barely move.
If you don't believe this, try pulling a table cloth out from below the place settings some time. That is completely acceleration dependent too and shows that you cannot add more acceleration to generate more force when friction is your only mode of transferring energy.
A plane would sit still and gradually start drifting backwards if a belt instantly accelerated below it... depending on how much friction could case the moment of the axle to drift backwards from perpendicular. The same is true of trying to keep the plane in one place with the engines running. You would have to be able to generate enough friction on the hub to move the plane's axle back to a perpendicular (straight down) moment.
No.
Originally posted by lukster
Any rotational inertia stored in the wheels is not countering the planes thrust. Spin the wheels to 100,000 RPM, instantly stop the belt and the plane will launch like a UFO (you probably don't want to do any banking for a while though). However, an accelerating belt is applying a force to the plane through the wheel. If the plane's thrust is great enough to overcome the frictional coefficient of the tire/belt contact the plane can move forward. The thrust of the plane, the frictional coefficent of the tire/belt, and the acceleration of the belt are variables, any two of which is capable of determining the outcome.
Originally posted by hitechI did not forget. lets say for a moment plane goes down the runway at constant speed and it does not accelerate so all forces would remain constant including angular momentum.
This one had me for a sec, now I see what you are missing, you are not seeing the change in force between the surface of the wheel and the runway when the moment of inertia of the wheel changes.
Originally posted by luksterYou got most if it right except that (without going into materials mechanical and thermal properties) coefficient of rolling friction is constant.
Any rotational inertia stored in the wheels is not countering the planes thrust. Spin the wheels to 100,000 RPM, instantly stop the belt and the plane will launch like a UFO (you probably don't want to do any banking for a while though). However, an accelerating belt is applying a force to the plane through the wheel. If the plane's thrust is great enough to overcome the frictional coefficient of the tire/belt contact the plane can move forward. The thrust of the plane, the frictional coefficent of the tire/belt, and the acceleration of the belt are variables, any two of which is capable of determining the outcome.
- C Johnson, Physicist, Physics Degree from Univ of Chicago
…For a 747 airliner, which weighs around 400,000 pounds at landing, and which lands at about 130 mph, the numbers are all bigger but the effect is very similar:
400,000 pounds weight is equal to 12,500 slugs of mass. 130 mph is equal to about 191 ft/second. Therefore, the kinetic energy of the airplane just before touchdown is 1/2 * m * v2 or 0.5 * 12500 * 1912 or about 227 million ft-lb of energy.
That size tire is around 8 feet in diameter, and the tire and rim probably weighs around 1000 pounds. The Rotational Inertia is equal to 1000/32 * 2.52 or 195 slug-ft2.
Once the tire stops skidding, it will be spinning at 191/25 revolutions per second and so omega is 48 rad/sec. (The giant wheels actually spin more slowly than the small aircraft tires do!)
The kinetic energy of rotation that the wheel/tire will eventually have is then 0.5 * 195 * 482 or 225,000 ft-lb. The aircraft has sixteen main tires/wheels, so this total is 3.6 million ft-lb if kinetic energy needed to up-spin all the main landing gear tires/wheels..
So, if the aircraft has 227 million ft-lb of kinetic energy the moment before touchdown, it will have around 223 million ft-lb left after fully spinning the wheels/tires up! Again, an almost irrelevant effect as regarding stopping the aircraft.
Say the 747 normally takes 5,000 feet of runway to completely stop. We can easily calculate the deceleration that occurs. Another Physics formula is 2*a*d = v2. We know everything but a, 2 * a * 5000 = 189.062. Solve for a and get 3.5744 ft/second, a gentle deceleration of around 1/10 G.
Let's see how far that exact same aircraft would have taken to stop if it had pre-spun the wheels/tires, and applying the same deceleration! Same equation:
2 * 3.5744 * d = 1912. This gives 5103 feet as the needed landing distance, roughly one hundred feet longer, half the length of the aircraft. That also is certainly not any "20% longer landing distance"!
That necessary tangential force for upspinning the wheels/tires on impact must entirely be provided by friction with the ground. This gives a value that indicates how much heating and wear is likely to occur to a tire under those circumstances.
In a very small fraction of a second, the heavy wheel and tire assemblies must be spun up to the 130 mph (191 feet per second) tread speed. From the lengths of runway skid marks (seemingly under 20 feet), this seems to occur in well under 1/10 second. …
Originally posted by 2bighorn
You got most if it right except that (without going into materials mechanical and thermal properties) coefficient of rolling friction is constant.
Originally posted by Mini D
Eskimo,
Get rid of the lame analogies. They don't work.
Originally posted by hitech
eskimo2:
Not a bad analogy eskimo. And obviously they would accelerate at different speeds do to the rotational inertia stored in the wheels. Since 1 has 0 mass, 1 has 1/2 mass, the 0 would be in front , followed by the 1/2 mass. Followed by the original plane.
Originally posted by Mini D
Instead, look at something you do know. Does increasing the acceleration used to pull the table cloth out from under place settings cause the place settings to move more or less?
Then explain your answer.
Originally posted by luksterI have suggested to ignore materials used because on one side we had super unlimited acceleration capable conveyor and darn stinky Cessna on another.
Perhaps I misunderstood you bighorn. The thrust of the plane and the belt acceleration capabilites we are dealing with are unknowns. You are the one who brought a super powered engine capable of infinite thrust into this and so I am therefore anticipating your obstacle. Are we dealing with the tires of a dragster or solid rubber thin bicycle tires? How much does your plane weigh?
Originally posted by eskimo2
So, the question is: can a 747 accelerate up to 130 MPH in under 5.6 seconds?
If so, it may be able to take off on the super treadmill.
Originally posted by eskimo2Actually, you are getting confused.
Of course not, they are sliding, not rotating. I think that you are confusing the two and need to answer the question in the story:
Originally posted by rabbidrabbit
A plane is standing on a runway that can move like a giant conveyor belt.
Originally posted by Maxbeen watching this thread...
Why? :lol
Originally posted by Max
Why? :lol
Originally posted by eskimo2That's where you all nay-sayers got it wrong.
Now think about the process in reverse.
Originally posted by 2bighorn
That's where you all nay-sayers got it wrong.
If conveyor is moving BACKWARD it helps rolling wheels FORWARD, therefore plane doesn't have to fight any excess forces but increased rolling resistance. All the wheels inertia forces are actually overcome by the conveyor.
Repeat again Eskimo, if conveyor is moving in the OPPOSING direction that of the plane, it is helping to roll wheels in the SAME direction as the plane travel.
BACKWARD --> FORWARD
Originally posted by eskimo2Yes, I've explained that already
Are you saying that it is easier for an airplane to take off on a conveyor that travels the opposite direction (and same speed) than it is for an airplane to take off on a conveyor that travels the same direction (and same speed)?
Originally posted by hitech
Miny D did you use pounds instead of mass to compute I? Not seeing how you got 16.1
Also you might really want to check you conversion from Radians per sec to feet per sec, because with 1 1 foot radius circle they will be equal.
Also I did make 1 typeo the Revolutions per sec should be 144 instead of 114, but the belt acceleration of 910 is correct.
HiTech
Originally posted by hitechYour drawing is fine so is calculation, but you missed on logic.
2bighorn: You really need to read up on some more physics. My sketch is exactly the way Rotational inertia and forces on bodies works.
Originally posted by hitechThe conveyor is not the opposing force, it just moves into opposite direction relative to the plane.
It is as simple as sum up the forces and torques involved, if force sum = 0 plane does not move.
Originally posted by 2bighorn
Your drawing is fine so is calculation, but you missed on logic.
The conveyor is not the opposing force, it just moves into opposite direction relative to the plane.
Look at the picture bellow. In order for force B to cancel force C, force B must work on the axis of rotation. But it doesn't. Due to circular body it changes it's direction and works same way as force A.
Therefore the sum of forces are not F = C - B but F = A + C
(http://sierra-host.net/belt.gif)
Originally posted by takeda
No conceivable arrangement that can only spin the wheels can keep the plane from gaining forward airspeed once thrust is applied.
If the threadmill is spinning the wheels with 30g of acceleration, once those engines start pushing, the wheels have no other option but accelerate at say 32g.
I can't really follow your diagram that well, I feel that a few arrows and labels are missing, but you can't oppose the force exerted by the threadmill and plane on the wheel, because they are in fact working in the same direction, making the wheel spin forward.
The only way to stop the plane using the wheels is countering that spin, ie. brakes or blocks. A threadmill running opposite will just spin them faster and the plane will fly, a threadmill running in the same direction means the plane will fly, because it would be more aptly called a catapult.
Originally posted by takeda
No conceivable arrangement that can only spin the wheels can keep the plane from gaining forward airspeed once thrust is applied.
If the threadmill is spinning the wheels with 30g of acceleration, once those engines start pushing, the wheels have no other option but accelerate at say 32g.
I can't really follow your diagram that well, I feel that a few arrows and labels are missing, but you can't oppose the force exerted by the threadmill and plane on the wheel, because they are in fact working in the same direction, making the wheel spin forward.
The only way to stop the plane using the wheels is countering that spin, ie. brakes or blocks. A threadmill running opposite will just spin them faster and the plane will fly, a threadmill running in the same direction means the plane will fly, because it would be more aptly called a catapult.
Originally posted by eskimo2No offense Eskimo, but for a teacher your reasoning is very weak. True, my drawing is extremely simplified, but follows the logic of directional change of an applied force over a circular body. I didn't want to spend time on geometric derivation just to prove something to you what you are not capable of understanding.
2bighorn, when you drop things on your planet, which way do they fall?
In order for force B to cancel force C, force B must work on the axis of rotation.
Originally posted by takeda
You forgot to have the other shoe pull your objects forward. Planes tend to have engines that make them go forward.
Originally posted by eskimo2
What’s stronger, a bear or some ants?
Originally posted by 2bighorn
No offense Eskimo, but for a teacher your reasoning is very weak. True, my drawing is extremely simplified, but follows the logic of directional change of an applied force over a circular body. I didn't want to spend time on geometric derivation just to prove something to you what you are not capable of understanding.
Originally posted by hitechNot true, force has a magnitude and direction. In case where more than one force is acting upon object, depended on the direction of forces, net sum can be of different magnitude and/or direction.
Basicly a force is a force is a force no mater where you exert it on an object.
Originally posted by JB88
a. how many ants?
b. what is being acted upon by the subjects?
Originally posted by hitechWe are dealing with rotational motion here, it is important if force passes through the center of mass or not. Conveyor's vector force is translated into rotation or to be more exact we get 'translational rotation' (since it does not pass the center of mass). Once we have rotation we have angular momentum as well.
But it states that you can convert any (force vector at a point) to the same force vector at the CG of an object along with a torque.
Originally posted by eskimo2I believe somebody already told you that your analogies don't hold the scrutiny of reasoning.
The tire/wheel flywheel will hold as much force through the rotating contact patch of its tire to the treadmill as a plane with its wheels locked and engine on full power does through its stationary contact patch of its tire and the runway. Most planes can not take off with their wheels locked and also would not be able to take off on the super treadmill.
Originally posted by eskimo2Seriously, after running those toys forward and flywheel has enough energy stored and you put them back on the ground, will they suddenly move backwards?
2bighorn,
Seriously, I’d love to read your explanation of how these thing work:
Originally posted by JCLerch
There is a problem with the super tread mill theory. For the airplane under power to not move forward with respect to the ground that the tread mill is attached to, the tread mill must accelerate at "The HiTech Number" (910 feet per second per second).
The tread mill must continually accelerate at this rate for as long as the airplane engine is producing thrust. If we supply the airplane with an externally attached fuel supply, it will only need to run for a relatively short period of time before enough energy has been stored in the system for gravitational time dilation to become something that must be considered.
If we don't consider the effects of relativistic motion in this system, in just over 2 years the linear speed of the tread mill belt will exceed the local speed of light. I'm pretty certain this will pose a problem. :huh
Originally posted by 2bighorn
I believe somebody already told you that your analogies don't hold the scrutiny of reasoning.
Would you please explain in what way does the conveyor brakes the airplane's wheels? And please, no more movies of belt sander and flying shoes. Just words if possible.
Originally posted by 2bighorn
Seriously, after running those toys forward and flywheel has enough energy stored and you put them back on the ground, will they suddenly move backwards?
Originally posted by vorticon
can a floatplane take off going up a fast moving river? and why is this different?
Originally posted by eskimo2Maybe,
You need to play with toys a bit more; you missed out.
Originally posted by lukster
Take a real Cessna 172. Does anyone doubt that a conveyor capable of maintaining a high acceleration could prevent it from taking off?
If you apply linear force to the rotating body everywhere but through its center of mass it'll be translated into rotational motion.
Originally posted by hitech
No it will be translated to a torque and a force. Depending on all other torques and forces sums the object will move in the direction of the force sum. And rotate in the direction of the torque sum.
Originally posted by 2bighorn
Here, two identical conveyors, same power, same speed. Two equal and opposing forces work on the wheel.
According to you, wheel in the middle shouldn't rotate since NET FORCE SUM = 0.
Originally posted by hitech
No it will be translated to a torque and a force. Depending on all other torques and forces sums the object will move in the direction of the force sum. And rotate in the direction of the torque sum.
Originally posted by hitechNo I'm not. It's just another of your straw arguments.
2bighorn: You are confusing the term force and torque.
Originally posted by mietlaThat's the whole point. In his first drawing he used force vector, calculated the size and cancel them out and proclaim angular moment cancels plane's forward motion.
In your diagram, the vector sum of forces is 0, therefore the wheel between the belt will not move (its CG will remain stationary, but the total torque acting on a wheel is not 0, therefore the wheel will rotate.
The point is that T on wheel exercised by both conveyors is identical and by your original claim wheel shouldn't rotate.
Originally posted by hitechIt can, but is limited to the max amount of friction between wheel and the frame.
Bighorn: Please take my diagram and analyze all forces involved .
I assume you still belive that the friction force on the belt can not be transfered to the frame?
Originally posted by hitechI did look at it again. I just don't see it as opposing forces.
Based on my sum of forces being 0 argument? Which you belive to be incorect?
Originally posted by eskimo2LOL Eskimo, since when did you become an expert on my life and its relation to the real world? But if you wanna go that way, ok. In real world, long ago, part of my career was work on gyro stabilizers, therefore I'm pretty confident that you should drop golf balls more often in order to understand why and when direction of rotation matters. Fair enough?
You need to observe the real world a bit more closely.
Originally posted by eskimo2Eskimo, your movies show how single external torque works when applied to objects with different mass and different mass distribution, we however are talking about how external torque relates to other forces in play like thrust, specifically if and when they are opposing each other and when not.
2bighorn,
Did you watch the movies I posted? Did you see the way the balls, hollow cylinder and wheel traveled under rotational acceleration from the treadmill?
Originally posted by 2bighorn
d) The maximum amount of torque acting on airframe (due to angular momentum of the wheel)) can not be greater than bearing friction allows it to.
Originally posted by luksterI was thinking that same is valid for conveyor bearings
You do realize that in almost every (probably all of them) plane built, bearing friction is quite capable of locking up the wheel, preventing it from rotating about the axle, right?
Originally posted by luksterBefore that happens, conveyor breaks into pieces
Let me anticipate your argument. The bearing is destroyed and the wheel then continues to rotate, rather roughly, in it's absence.
Originally posted by luksterand you'd be fixing the damn conveyor, if I remember right, many posts ago....
My answer to that is that the wheel itself will be ground away in short order as was the bearing. As I said many posts ago you are then left with the plane dragging itself on the wheeless gear.
Originally posted by hitech
(http://www.hitechcreations.com/hitech/rveq.jpg)
Originally posted by Holden McGroin
If static friction were vectored opposite of rolling friction, static friction would be a net thrust.
Static and rolling friction are both drag forces and acting in the opposite direction of thrust. They do not occur simultaneously. As soon as rolling starts, static friction vanishes and rolling friction appears.
Originally posted by Holden McGroin
This whole thread is amazing to me, as the thought that a set of rollers to counter the spin of tires could hold back a thrusting 747 is rediculous.
Originally posted by JCLerch
This is the crux of the problem. Your question, and the original thought experiment, mix "real world" physical items with intellectual constructs. (Non-existent hardware).
For instance, You put a "real Cessna 172" on a intellectually constructed, artificially constrained, treadmill. Aka a "Super Treadmill" Then to answer your question to the finish, we have to add some additional constraints.IF you answer yes to all the above, then the airplane will not move forward (with respect to the ground the treadmill is attached to) while producing thrust. Of course, this will be an interesting experiment to an outside observer, as the surface of the treadmill will go from 0 to 700mph in about 1 second. 11 seconds later (12 seconds after opening the throttle on the airplane engine) the treadmill surface speed will be traveling at Mach 10 and still accelerating. In a little over 2 years, the linear surface speed of the super tread mill will exceed the speed light. As I said, answering YES to all the thought constraints will be very interesting to observe from a distance!
- Are the Cessna wheel assemblies indestructible? Yes/No
- Are the Cessna wheel bearings and axles indestructible? Yes/no
- Is the super tread mill indestructible? yes/no
- Is the super tread mill capable of continually accelerating at 910ft/sec/sec? Yes/no
- shall we ignore the effects of Relativistic Quantum Field theory? Yes/no
IF you decide to answer yes to everything except the last question, and include Einesten's theory of relativity, then the end result is sufficient energy in the system to be equivalent to enough mass to create a Black Hole.
IF you answer yes to everything except the last two questions, then the treadmill will eventually stop accelerating, and the airplane WILL take off.
IF you answer no to ANY of the first three questions, you will roll the Cessna up into a little metal ball.
IF we constrain the experiment to things that CAN BE FABRICATED, then I predict it is impossible to build a treadmill assembly that can accelerate at 910ft/sec/sec for anything longer than a few fractions of a second. Once the treadmill stops accelerating, the airplane will move forward relative to the ground the treadmill is attached to, eventually gain enough airspeed and takeoff. How unusual the takeoff appears depends on the rate of acceleration and the top speed the treadmill is capable of. [/B]
Originally posted by eskimo2
Golfer,
How would you answer this question (and simply, why?):
“A plane is standing on runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).
The question is:
Will the plane take off or not? Will it be able to run up and take off?”
Note: “Wheel” has been removed.
Originally posted by eskimo2
Guys, the main force at work here is NOT friction! The plane is held in place by rotational acceleration. It need friction/traction to work, but that is not the only work.
Originally posted by john9001
need to define "wheel speed" is it the rotational speed of the wheel or the forward speed of the wheel in relation to the earth which would be the ground speed of the plane.
Originally posted by eskimo2Flow: belt -> friction1 -> rotation -> friction2 -> airframe
Guys, the main force at work here is NOT friction! The plane is held in place by rotational acceleration. It need friction/traction to work, but that is not the only work.
Originally posted by john9001
it talks about the belt counteracting the speed of the plane,
Originally posted by Kuhn
The wheels wont roll if an airplane does not move forward. Being that the wheel movement is is controlled by the prop moving through the air ,the plane will move forward nomatter what the wheels are doing. The conveyer can try all it wants to keep the wheels spinning fast enough to keep up with the rotation but in no way is it possible even in theory for it to keep up with the wheel speed. It will keep accelerating as the plane moves forward and the plane will lift off. So actually the original question is flawed and cannot be answered the way it was asked. It is not possible for the plane not to move forward when the prop is pulling it through the air. It is not possible for the plane to move forward if the wheels dont rotate when in contact with the ground.
I could go on but I wont :D
Originally posted by takeda
I finally undestood it by reading this:
http://www.straightdope.com/columns/060303.html
n order to achieve the hypothetical angular velocity (close to light speed) we would have to ignore torque caused by friction of the bearings at some time.
At that time our model becomes flawed since we lose the only linkage between angular momentum and the airframe, therefore no matter how fast wheel would rotate it would not counter plane movement since now we have two completely separate systems instead of one.
Also the angular vel is not what holds the airplane. It is angular acceleration. And I just like Cecil I am almost to the point of swearing on having to repeat physics.
Are you saying the wheels departs the airplane?Limits like those are not really part of the question. Infact I belive I could build a flyable plane that would only have to have the belt accelerate the wheels at about 1/4 FPS per sec or less. At those rates you could build a system that would acctualy demonstrate the principle.That sounds awful small number.
Originally posted by hitech
Eskimo: I think it is really just a perspective thing, weather you view it as friction or rotational acceleration. The force outside the plane is based on static friction tire to the ground. What creates that friction is the rotational acceleration.
Originally posted by mietla
Eskimo, hitech
The problem as stated is illdefined and ambiguous. I dont want to go back and read this entire thread again, so can you please state how do you understand the problem setup, and where do you guys get your infinities.
Case 1.
To me, "the belt matches the wheel's speed" simply means that the wheel does not skid on the belt. The belt is not powered, it simply moves along dragged freely by the wheel. The wheel is the driving element. The wheel can translate left or right, and/or it can rotate clock, or counterclockwise. The belt does not care. The belt is a passive element and just follows.
Case 2.
"the belt matches the wheel's speed but in the opposite direction" is only possible if the wheel's rotational speed is 0. The wheel can not rotate clockwise while the belt is moving to the right (or counterclockwise while the belt is moving to the left. The friction prevents that. If the forces exceed the friction, the wheel simply separates and both the belt and the wheel move independently.
The plane moving to the left is pushing the bottom of the wheel to the right. Just because the wheel is pushed does not mean that it starts to rotate. The belt senses the force pushing it to the right (by the wheel), and compensates by applying an equal force ( the belt can not apply "a movement", it can only apply a force which may or may not result in movemnet) to the left. Both forces cancel each other and the result is that the wheel does not rotate at all. It translates to the left along with the plane. An outside observer sees the plane taking off to the left with wheels that do no spin. You could get an analog situation if you put a plane on teflon. The friction between the teflon tarmac and the wheel is miniscule in comparison with a friction in the axle bearing so the wheel does not spin and the wheels skid on teflon.
What is your understanding of the problem statement?
Why does "your" belt need to be powered is case 1.
Where do the infinities come from?
For comparison GE90-115B fan rotates at about 2500rpm at full thrust and core at over 10.000rpm and it did create 127,000lb of thrust in test run and there's not much of a torque acting on frame via bearings.
Originally posted by Holden McGroin
the question is flawed as posted....
Ponder this: If there is no forward speed, there is no need for the conveyor to move. 0 = 0
Now the plane begins to creep forward, but it does not because the conveyor pulls it back. But that means it hasn't moved, so 0 = 0 and therefore the conveyor is not moving.
What causes a plane to move forward? Thrust that is independant of the wheels, air cushion, skiis, or whatever else it has to keep the paint from being scratched on the tarmac. Thrust is independant of the landing gear.
Net thrust causes acceleration, regardless of whatever rotational acceleration does to the wheel.....
End this thread now!
Originally posted by nexus69
My Question is. Where is the plane going?
Originally posted by WhiteHawk
So there is really not enough information to answer this question. We need a drag variable for the wheels or if the wheels are sufficient, the plane will take off. .
Originally posted by 2bighorn
I was thinking that same is valid for conveyor bearings
Before that happens, conveyor breaks into pieces
and you'd be fixing the damn conveyor, if I remember right, many posts ago....
Originally posted by hitechNot sure if you mean the same, but formula bellow should work since both are expressed in unit of force
Just had a thought, can you draw a diagram of how bearing friction turns into drag?
Originally posted by eskimo2all planes take off at the same distance at the same speed.
How did the rotational inertia and energy now stored in Bob’s and Al’s wheels affect the take off distance of their planes?
We know that Al’s plane will still take off in exactly 100 feet; where will Bob’s and Chuck’s planes take off?
thrust has to overcome total inertia that is wheel inertia + plane inertia. Since both are depended on mass, if you just move the mass around from plane to the wheel or from wheel to the plane and the total sum doesn't change then necessary force to get plane in motion doesn't change either.
Originally posted by eskimo2You applied external torque to your objects. What's so magical about?
Please explain how the balls and hollow cylinder moved to the left when a paper treadmill moved to the left at an acceleration rate of a little under 9.8 meters per second per second if a force other than pure rotation did not act upon them? Rotation is not the only resultant of rotational acceleration when a force is applied to only one place.
Originally posted by 2bighorn
all planes take off at the same distance at the same speed.
This is not my theory, this is according to conservation law.
If you disagree I'd like to see some physical definitions which would support your claim.
Originally posted by Golfer
If the treadmill only matches the speed of the wheels...its never going to work.
Originally posted by 2bighorn
You applied external torque to your objects. What's so magical about?
Originally posted by eskimo2Didn't you say that planes were balanced accordingly and that planes with lighter wheels had heavier airframes for the same amount?
Chuck’s plane lost nothing to rotational inertia and took off first and in the least distance.
Originally posted by 2bighorn
Didn't you say that planes were balanced accordingly and that planes with lighter wheels had heavier airframes for the same amount?
Why?
Originally posted by eskimo2
Golfer,
In our question, why do your believe the conveyor will move at 100 knots if the plane is also moving at 100 knots? How can that result in the conveyor matching the wheels’ speed?
Originally posted by eskimo2
When the three planes take off, Al’s and Bob’s planes wheels are spinning, right?
Originally posted by 2bighorn
Chuck's machine weighs exactly the same (diff mass distribution tho), has same lift, same drag, same friction, same thrust, yet it magically takes off in least distance?
Congrats, you just invented perpetual motion machine...
Originally posted by eskimo2
If you think that the planes took off all together you have to be able to explain how their wheels have different states of energy.
Originally posted by Golfer
I've read the stories.
The wheels are along for the ride on the airplane and again...like your example of the pull string toy cars don't apply to an airplane the way I think you hope.
How...how how how...do you overcome the thrust applied by an airplane capable of propelling itself through the air by applying a equal opposite force to the wheels?
Telling me a story about bob tom and joe taking off in formation isn't going to do it. Telling me look at a 2 second blurb of a shoe falling won't cut it either. Your examples are not what the question asks. They're fine examples for showing how a conveyor underneath some objects affects them but that isn't the question. You're solving problems that aren't applicable to the question.
The question has the conveyor matching the speed of the wheels effectively taking the wheels out of the equation. The wheels, still spinning and presumably holding together while they accelerate through the speed of light, don't don't don't don't don't don't have anything to do with the airplane allowing itself to accelerate as long as they're allowed to spin.
It doesn't matter what equal opposing force you apply to the wheels...you're cancelling out the wheels. It would work wonders if you had a car on your treadmill. Your examples are perfect for a car. You'd have to have an effect on the fluid (air) surrounding the airplane to make it not fly.
You never did answer what I asked a few pages ago...
Why or why not are you able to dyno test an airplane at your friendly neighborhood automotive engine shop? Tell me how your plan to do that and I'll start listening to you.
Originally posted by 2bighorn
Not sure if you mean the same, but formula bellow should work since both are expressed in unit of force
Friction = ((plane mass + wheel mass) * gravitational acceleration) * bearing rolling friction coefficient) divided with number of bearings (one per wheel)
rolling friction coefficient is about 0.025 for very low quality radial friction only bearings (creates most of the friction)
Originally posted by hitechDefinately
Would you agree the rolling friction is generating a torque about the axle?
Originally posted by 2bighorn
They have all the same total mass, different mass distribution, hence difference in energy for a particular part ie wheels.
The heavier wheels with more inertia (more angular momentum) are attached onto lighter airframes with less inertia (less kinetic E), or
the lighter wheels with less inertia (less angular momentum) are attached onto heavier airframes with more inertia (more kinetic E).
Total E needed to lift them up and fly is the same. Just because some of the E ain't as visible it doesn't mean it ain't there.
Originally posted by hitechIf belt is stationary than thrust has to oppose both bearing friction and belt/wheel friction. If we add another source of E like conveyor and if it contributes to wheel rotation, then both share the workload of fighting bearing friction.
2bighorn: I'm not interested in the values, what I am interested in , is you drawing a diagram showing how the torque friction of the bearing you are calculating turns into a directional force.
The reason I am asking this, it is the first step to understanding how the force accelerating the wheel acts exactly the same as the bearing friction.
Originally posted by hitechDoesn't really matter, you can attach that (pseudo) vector anywhere on the wheel as long as it is pointing (tangentially) in direction of rotation.
And this is where you made your original mistake , when you transcribed the belt friction force to a torque, you were still thinking of it as a force , but you rotated it to the top of the tire, with a torque that is no problem, but not with a force.It is why you came to the conclusion that the rotational inertia helped the plane moved vs held it back.
Doesn't really matter, you can attach that (pseudo) vector anywhere on the wheel as long as it is pointing (tangentially) in direction of rotation.
But basically, if a torque works in direction of rotation it adds to rotation and oppose those which work in the opposing direction (like bearing friction torque).
Originally posted by 2bighorn
Since none really acts radially to our wheel, none contributes to movement of a wheel, just rotation and as such the belt/wheel friction torque does not contribute nor oppose in any way to forward/backward motion of the plane except for the amount of bearing friction torque.
Originally posted by hitechSince it's a pseudo vector of a torque, you have to move that vector along the rotation until the both pivot points are in the same spot.
Do you agree that my drawing is an equivalent to yours and the outcome would be the same?
Originally posted by hitechNot quite. Whenever you break down torque into that pseudo vector as the force, your force will have component which acts radially, and a component which acts tangentially.
Not sure if we are in agreement here. If you apply a force (not a torque) anywhere on a wheel and in any direction, it will move (translate/accelerate (F=MA) unless countered by another force.
Not quite. Whenever you break down torque into that pseudo vector as the force, your force will have component which acts radially, and a component which acts tangentially.
Radial component of the pseudo vector is f cos alpha. Your belt friction torque angle is 90 degrees (relative to the line from pivot to the center) which gives cos 90, hence the radial component of the moment of force is 0.
Originally posted by Holden McGroin
So are we convinced yet that the original question is horribly flawed?
It is....
1st.
The conveyor belt can only overcome rolling friction. The main drag on an airplane is aerodynamic, rolling friction max is probably less than 10% during take off. The thrust is entirely aerodynamic, the only rolling resistance is bearing friction, and the heating of the tire as it is deformed under load.
2nd.
If the airplane begins to roll forward, the conveyor belt pulls it back. The forward speed of the airplane is completely countered by the speed of the conveyor. Since my accelerating force is aerodynamic, my speed will be measured in the air. The only place where this magical conveyor can perfectly balance is at zero.
Any other airspeed is out of the range of the original problem. It does not matter what the RPM of the wheels, if the speed balance is maintained, the only thrust I can apply must be equal to the rolling friction for the balance to be maintained.
Either one must agree that the original problem is fundamentally flawed or the answer is that the airplane can never have any airspeed and therefore will never fly, unless it is a VTOL.
3rd.
As most airplanes have the thrust to overcome rolling friction the aircraft in this problem (which has thrust only equal to rolling friction) could not accellerate to takeoff speed regardless of a moving conveyor. Even if catapulted it would not have the thrust to maintain flight.
Please end this horrible thread.
Holden
Oregon Registered Professional Engineer
Originally posted by Holden McGroin
Magic wheels?
Lightweight wheels allow for a quicker accelleration. This is why TDF bicycles have $5000 carbon titanium rims. The rotational equivalent of F=Ma is Torque = (WK^2) angular accelleration. Yes, there is some energy you are storing in the flywheel effect of the spinning wheel / tire set. I can add a relatively small amount of energy I put into spinning my wheels (which is what this thread has been doing since the first post) I could build a machine that probably could have a wheel set of a 747 spinning to an equivalent of 100 mph on 30 seconds or so with a 5 hp motor.
So, I accelerate, but I don't, because I can only apply enough power to overcome the rolling friction due to this rediculous conveyor on which I find myself. I CANNOT APPLY FULL THROTTLE BECAUSE THRUST AND ROLLING FRICTION* MUST BE BALANCED IN ORDER TO STAY WITHIN THE CONTRAINTS OF THE PROBLEM regardless of the magical qualities of the wheels.
*+ 1/2*k*M*R^2*(ratational speed Rad/sec)^2 This is the flywheel energy stored in the spinning wheels... you would need to diffrerentiate with respect to time to get the force at any one specfic time.
Either the aircraft is horribly underpowered or the original question is flawed.
Originally posted by eskimo2
Now use the same principle to build a hypothetical machine and use a 60,000 hp motor to get those wheels up to 1,200,000 mph in 30 seconds. Do you think that would give the 747 motors a challenge to work against?
Originally posted by eskimo2
If you hate this thread so much, why do you keep coming back? Are you forced to read every response and thread? When most folks get tired of a thread they stop reading and posting in it.
Since you are posting in this thread, please explain why a treadmill of unlimited speed and strength could not keep a normal plane (with wheels that have unlimited strength) from moving.
Originally posted by JCLerch
This is the crux of the problem. Your question, and the original thought experiment, mix "real world" physical items with intellectual constructs. (Non-existent hardware).
For instance, You put a "real Cessna 172" on a intellectually constructed, artificially constrained, treadmill. Aka a "Super Treadmill" Then to answer your question to the finish, we have to add some additional constraints.IF you answer yes to all the above, then the airplane will not move forward (with respect to the ground the treadmill is attached to) while producing thrust. Of course, this will be an interesting experiment to an outside observer, as the surface of the treadmill will go from 0 to 700mph in about 1 second. 11 seconds later (12 seconds after opening the throttle on the airplane engine) the treadmill surface speed will be traveling at Mach 10 and still accelerating. In a little over 2 years, the linear surface speed of the super tread mill will exceed the speed light. As I said, answering YES to all the thought constraints will be very interesting to observe from a distance!
- Are the Cessna wheel assemblies indestructible? Yes/No
- Are the Cessna wheel bearings and axles indestructible? Yes/no
- Is the super tread mill indestructible? yes/no
- Is the super tread mill capable of continually accelerating at 910ft/sec/sec? Yes/no
- shall we ignore the effects of Relativistic Quantum Field theory? Yes/no
IF you decide to answer yes to everything except the last question, and include Einesten's theory of relativity, then the end result is sufficient energy in the system to be equivalent to enough mass to create a Black Hole.
IF you answer yes to everything except the last two questions, then the treadmill will eventually stop accelerating, and the airplane WILL take off.
IF you answer no to ANY of the first three questions, you will roll the Cessna up into a little metal ball.
IF we constrain the experiment to things that CAN BE FABRICATED, then I predict it is impossible to build a treadmill assembly that can accelerate at 910ft/sec/sec for anything longer than a few fractions of a second. Once the treadmill stops accelerating, the airplane will move forward relative to the ground the treadmill is attached to, eventually gain enough airspeed and takeoff. How unusual the takeoff appears depends on the rate of acceleration and the top speed the treadmill is capable of. [/B]
Originally posted by Holden McGroin
The construction of the aircraft or the conveyor does not matter, it is the construction of the problem that is fundamentally flawed, at least from a physical if not philosophical POV.
In order for the premise of the problem to remain true, the aircraft cannot move as any movement by the aircraft is counteracted by conveyor speed.
If thrust is counteracted by wheel drag, (the only counteracting force available) the thrust and drag vectors cancel so that no net force is available to accelerate the plane.
No net force means there is no acceleration, the conveyor is at a constant speed. A constant speed, the rotational inertia force (which is only required to accelerate) is zero, so we are back to just rolling resistance.
Even if you accept that acceleartion can occur, it is only acceleration of the wheels, and that is perfectly balanced by engine thrust..... somehow
Once again, we are back to immovable vs. irresistible and round and round we go.