Aces High Bulletin Board
General Forums => The O' Club => Topic started by: jdbecks on January 07, 2010, 05:29:35 PM
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imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
? :noid
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imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
? :noid
ben watchng mythbusters have ya?
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ben watchng mythbusters have ya?
lol I was going to ask the same thing.
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ben watchng mythbusters have ya?
LOL , me too ..
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Obviously he hasn't, they answered that one, all the while they were rolling their eyes. It looked, and sounded like" we know the answer, smart people know the answer but we will do this myth for the less than intelligent."
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lol I was going to ask the same thing.
me too
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I seem to remember a thread about this a while back.... :D
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imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
? :noid
Better ask yourself why wouldn't it be able to take off?
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Not this subject again.
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No. No Airlift, if your going 70mph but no were, there is no air moving on the wings, thus not able to lift.
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Ugh, someone please find this thread. Yes, it flies because wheels on planes don't work like wheels on cars. They are rolling backwards at whatever speed the conveyor belt is going, yet the plane moves forward because thrust is generated from somewhere other than the wheels. (the prop) Therefore, the plane still picks up speed, still generates lift, and still takes off. Again, the wheels can move in whatever direction they want -- it matters not. :aok
Also, if this is bait, I'll take it. :cheers:
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Ugh, someone please find this thread. Yes, it flies because wheels on planes don't work like wheels on cars. They are rolling backwards at whatever speed the conveyor belt is going, yet the plane moves forward because thrust is generated from somewhere other than the wheels. (the prop) Therefore, the plane still picks up speed, still generates lift, and still takes off. Again, the wheels can move in whatever direction they want -- it matters not. :aok
Also, if this is bait, I'll take it. :cheers:
As requested:
http://bbs.hitechcreations.com/smf/index.php/topic,197189.0.html
enjoy
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It DOES fly, idiots...
The pegasus engines direct their thrust downward, the US Marine Corps pilot increases the thrust, and off they go.
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Just saw this but I first thought no, there's no lift under the wings, since you aren't going anywhere.
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It flies. It doesn't accelerate because of the wheels, they just roll. The prop/jet thrust accelerates the aircraft through the air and that isn't affected by any treadmill contraption. All the treadmill would do is make the wheels spin faster as it took off.
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now that thats outta the way, i was flippin thru channels and on MTV (most retarded channel ever) some gangster chick keeps talkin trash insulting a drunk guy (acting like she was gonna do somethin :rolleyes:) and gets punched in the face. now im not for hitting girls but that made me laugh.
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imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
? :noid
You know , if one could build a runway like this it would save a lot of ground space , lol .
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You know , if one could build a runway like this it would save a lot of ground space , lol .
No because it would still need to be as long as any runway now.
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Oh , I understand now .Had to re-think that one ..
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It flies. It doesn't accelerate because of the wheels, they just roll. The prop/jet thrust accelerates the aircraft through the air and that isn't affected by any treadmill contraption. All the treadmill would do is make the wheels spin faster as it took off.
Karnak, The reason planes have wheels and roll to take off is to get air moving over the wings at speeds high enough to create enough lift to life the planes off the ground, if the plane was not moving, it could not get wind moving over its wings fast enough.
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Karnak, The reason planes have wheels and roll to take off is to get air moving over the wings at speeds high enough to create enough lift to life the planes off the ground, if the plane was not moving, it could not get wind moving over its wings fast enough.
What he is saying is that the force is acting on the air, not the treadmill, therefore the speed of the wheels is irrelevant, it is the speed of the air that matters.
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No. No Airlift, if your going 70mph but no were, there is no air moving on the wings, thus not able to lift.
How can you be going 70 mph and not going no w(h)ere ? ... Why would you be going no w(h)ere ? ... What is preventing you from moving forward and obtaining lift ?
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Karnak, The reason planes have wheels and roll to take off is to get air moving over the wings at speeds high enough to create enough lift to life the planes off the ground, if the plane was not moving, it could not get wind moving over its wings fast enough.
And in this scenario, what is preventing the plane from moving forward ? ... think about it.
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For anyone confused think of it this way:
Boeing 747 (4x Jet engines) push the plane forward. The tires are there to make the trip between 'Stop' and 'airborne' smooth. The plane could take off with those tires in a dead stopped position, but it would be a very bad ride and would defeat the purpose of having tires on the thing. Since the Jet engines are pushing the plane those tires even in a 'Locked' position would just make the trip bumpy and maybe take longer, the tires if matched in speed with a belt system would basically look like they were standing still, unless the belt is matching the plane speed not the tire RPMs.
One test would be, Get a high-powered (and light) fan and put it on wheels, find a way to prevent the wheels from spinning. The fan should still move due to getting it's power from pushing air, not from the spinning of tires like a car does (lock those tires and you'll have a face full of dashboard).
Then again, I'm likely just confusing myself! :D
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UGH ...
For the confused ... http://www.youtube.com/watch?v=YORCk1BN7QY&NR=1
Even the pilot thought that he wouldn't take off ... :rolleyes:
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Karnak, The reason planes have wheels and roll to take off is to get air moving over the wings at speeds high enough to create enough lift to life the planes off the ground, if the plane was not moving, it could not get wind moving over its wings fast enough.
Check out 15 seconds in:
http://www.youtube.com/watch?v=0ul_5DtMLhc
There's not enough room for the plane to take off, but see how it still moves FORWARD off the end of the treadmill? Later in the video they do a full-scale test as well.
For the life of me I just can NOT understand how difficult a concept this is for people to wrap their skulls around.
A car on a treadmill won't move forward because its forward acceleration is driven BY its wheels, which there's a drive train and transmission that control how fast they turn. A plane's wheels are COMPLETELY free spinning, and it receives forward acceleration via thrust from its prop or jet engine. The wheels are there for support on the ground ONLY. It'd be like putting the car in neutral. Which, btw, they also did:
http://www.youtube.com/watch?v=S377HwOthjo
The conveyor belt can NOT stop the engine from producing thrust. As long as it does, the plane is going to move forward REGARDLESS of how fast the treadmill is moving. Jamie could have used a frelling ROCKET SLED to drive the conveyor belt and the plane would have taken off within the same amount of runway.
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It can't fly, because it can't move forward. As soon as the plane begins to accelerate, the treadmill will instantly accelerate to a nearly infinite speed. Since rolling resistance will keep increasing as the treadmill increases in speed, the plane simply can't get moving through the air because the rolling resistance will increase to exactly counteract the engine thrust. If the rolling resistance isn't strong enough, the plane will start to creep forward, causing the treadmill to speed up, which will increase the rolling resistance which will halt the plane again.
There is only one exception, and it is the same thing that makes modern hard drives work... A moving surface will create a little bit of wind over that surface.
http://www.pcguide.com/ref/hdd/op/heads/op_Height.htm
So the treadmill itself will create a little layer of moving air along the surface. This is how computer hard drive read/write heads "fly" on the very thin layer of air on the top of the spinning hard drive discs, and why it is a really bad idea to try to completely seal a computer hard drive or run it in a vacuum. Assuming that the wheels don't explode from the nearly infinite rotation speed or the bearings don't give out, at some speed (very very fast), the treadmill itself would probably create enough wind to lift the plane up off of the treadmill, at which point the rolling resistance on the landing gear would drop to zero, and the plane would begin to move forward. It's a bit like if a stationary plane was hit with a gust of wind... It would essentially leap up and backwards relative to the ground unless the engine was strong enough to accelerate it forward through the air and assuming the wind didn't stop.
But you have to assume a whole lot of things that simply can't happen, primarily that the landing gear, wheels, bearings, etc. would survive the treadmill's acceleration to a near-infinite speed, as required by how the original question is stated. That simply can't ever be tested in full scale.
It would be possible to test a subscale model at lower speeds though... Just slowly increase both the engine thrust and treadmill speed so that the drag created by the rolling resistance always equals the thrust of the engine. At some point, the rolling resistance will be as great as the engine's maximum thrust, and there will be an equilibrium point where the plane doesn't move because the thrust of the engine at max power is exactly counteracted by the wheel drag from the treadmill. In a full-scale test, I'm not sure the rolling resistance would ever be enough without the treadmill either moving far faster than physically possible using known materials, or if the wind effect caused by the treadmill itself would create enough airflow over the wings to allow the plane to lift off of the treadmill.
I used to know what that wind effect was called, but I can't remember it and I'm apparently not smart enough to google it correctly either.
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the tires if matched in speed with a belt system would basically look like they were standing still, unless the belt is matching the plane speed not the tire RPMs.
The wheels would always move in the opposite direction of the moving conveyor belt, at the speed of the conveyor belt, until the wheels had no more contact with the conveyor belt ... the wheels are "free wheeling" on their axle.
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I vote Skuzzy just lock this thing now, because it's only going to get nasty from here.
It doesn't matter HOW much proof you give people this myth is a crock. They're not going to accept anything but their own narrow view of the mechanics involved.
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Since rolling resistance will keep increasing as the treadmill increases in speed, the plane simply can't get moving through the air because the rolling resistance will increase to exactly counteract the engine thrust. If the rolling resistance isn't strong enough, the plane will start to creep forward, causing the treadmill to speed up, which will increase the rolling resistance which will halt the plane again.
The rolling resistance on a free-wheeling axle would not become a factor soon enough to prevent eventual rotation.
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(http://techblog.dallasnews.com/mrfusion.jpg)
where we're going, we don't need runways.
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The wheels would always move in the opposite direction of the moving conveyor belt, at the speed of the conveyor belt, until the wheels had no more contact with the conveyor belt ... the wheels are "free wheeling" on their axle.
I was never good at anything above basic thinking, so I didn't expect to be right on all, or any accounts. :D
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The rolling resistance on a free-wheeling axle would not become a factor soon enough to prevent eventual rotation.
The plane can't go "forward" through the air without the wheel rotating faster than the treadmill, but the premise is that the treadmill always matches the speed of the wheel rotation. That is why the treadmill will accelerate essentially instantaneously to an extremely high speed.
Example. The treadmill is stationary, and the plane moves forward at .5 mph, so the wheel is rotating at .5 mph faster than the treadmill. The treadmill will begin to rotate at the same speed as the wheel. Rotational drag goes up a bit, and the plane (assuming no further thrust increase) stops it's forward motion, but the wheel and treadmill are now both going at .5 mph. Thrust is increased to whatever level is required to get the plane moving again, but once again, the treadmill instantly speeds up as fast as necessary to cause enough of an increase in rolling resistance to exactly counteract the engine's thrust, which brings the plane to a halt again. If the plane is moving through the air, then by definition the wheel is rolling faster than the treadmill is turning, which can't happen according to the question. So the treadmill will instantly accelerate to whatever speed is necessary to create enough rolling drag to exactly counteract the thrust, thereby keeping the plane stationary.
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And that's where the mythbusters test went wrong... They assumed that since the plane takes off at 60mph (or whatever), all they had to do was put the plane on a 60mph treadmill. But for the plane to get to 60mph airspeed on a 60mph treadmill, the wheels would have to be rotating at 120mph! Which can't happen in the original question. The wheels always rotate at the treadmill speed plus the speed of the plane through the air. Since the treadmill speed is always equal to the wheel speed, the airspeed must remain zero (assuming no wind).
Again, the only way this can be changed is if the wind created by the treadmill surface is enough to create enough wind over the wings to lift the plane off of the treadmill. But the thickness of that wind is very very very thin over the moving surface, so I'm not sure enough wind would be produced by the time the rolling resistance equalled the max thrust of the engine.
If there was no rolling resistance, the treadmill would accelerate to an infinite speed due to a divide-by-zero condition. Do the math...
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X = tire rotation speed
Y = treadmill speed
Z = ground speed at takeoff
By definition, X = Y, since that is how the question is worded. HOWEVER...
For the plane to take off, X must equal any hypothetical surface speed (the ground or a treadmill) plus the speed through the air required to get off the ground.
For a plane to take off, X = Y + Z, so Y would have to be some amount lower than X, violating the conditions of the test. For X to continue to equal Y, ground speed at takeoff would have to be 0. The only way for Z to be 0 at liftoff would be if there was some wind, but again the test condition specifically states that wind is zero. The only way to create a wind condition is to move the treadmill fast enough for the surface effect to generate enough moving air over the treadmill surface to create enough lift over the wings for the plane to lift off. I don't think you can get a treadmill moving fast enough to do that (for a human-sized aircraft), using current materials technology. Obviously since hard drive read/write heads "fly" over the surface of the platter using this same effect, it could be possible on a small enough scale.
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And oh by the way, HT posted basically the same conclusion the last time this came up :) Assuming that it was possible to reproduce the conditions of the test (which isn't possible for a human sized aircraft), the plane can't fly and the very basic math formula I posted above proves it conclusively. Add in other factors as I describe, and it maybe could be tested with a small enough plane where the forces involved aren't so strong that they exceed the strength of the treadmill, wheel, tire, and bearings.
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So the treadmill will instantly accelerate to whatever speed is necessary to create enough rolling drag to exactly counteract the thrust, thereby keeping the plane stationary.
then the wheels will simply drag. There is no way the treadmill + wheel bit would be enough to overcome the thrust of an aircraft.
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The plane that they used in the Ghost Busters had a take off speed of 60 mph ... Jamie could pull that tarp at 200 mph and that plane would still have taken off within it's designated rotation distance ... rolling resistance would have no effect.
The conveyor belt and the wheels have no direct influence on the thrust provided by the propeller ... there is no frictional drag that will stop the plane from moving forward.
Answer me this ... if what your saying is true, then the fact that Jamie was providing the concept of the conveyor belt (but not to your exact specifications) ... why did the plane take off in the exact distance it rotated when there was no conveyor belt ... you would have thought that Jamie's actions would have at least provided some sort of a disruption in it's normal rotation distance ... yet it didn't.
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Another way to look at it...
Plane on treadmill that isn't moving, takes off. At liftoff, it has 60 mph ground speed. What was the tire rotation speed at liftoff? 60mph. Easy huh?
Ok, now the treadmill is moving at 100,000mph. The plane again takes off, at the same 60mph relative to the ground that the treadmill is sitting on. How fast are the tires rotating at liftoff? 100,060 mph. See? For the plane to take off, the wheels have to be rotating faster than the surface they're sitting on. No matter how fast you turn that treadmill, if the wheel is rotating exactly as fast as the treadmill is going, then the wheel moves... nowhere. It just sits there. If you try to drag it forward down the treadmill, you just made it turn faster than the treadmill. But by the test condition, the treadmill will instantly accelerate to match the tire rotation speed, so you simply can't drag, push, whatever, the wheel down that treadmill because the treadmill will keep accelerating to an infinite speed if necessary, to keep you from making that tire rotate faster than the treadmill is going. If rotational drag is zero (impossible), then the treadmill will instantly go to an infinite speed (impossible).
Therefore, the test can't be carried out on a human sized aircraft using current materials technology. And if it could, the plane still couldn't budge an inch because the treadmill would keep increasing it's speed.
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then the wheels will simply drag. There is no way the treadmill + wheel bit would be enough to overcome the thrust of an aircraft.
Nonsense. You're not bothering to read the test condition. The treadmill always exactly matches the tire speed. The tire speed cannot exceed the treadmill speed. If you say rolling resistance is negligible, then the treadmill simply keeps accelerating to an infinite speed, before the plane budges an inch, because that's how the test is set up.
The wheels cannot drag because the treadmill moves at the same speed as the wheel rotation.
I suppose you can cheat the test by locking the wheels and dragging them along the treadmill surface, but the test also assumes that the wheels can rotate. If you say that the wheels being allowed to skid on the treadmill is an allowable deviation from the test conditions, then you might as well be asking if a plane on skids can take off from a treadmill rotating at an infinite speed (or from a normal surface), because now your engine must simply overcome the friction of the dragging wheels on the treadmill.
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The plane that they used in the Ghost Busters had a take off speed of 60 mph ... Jamie could pull that tarp at 200 mph and that plane would still have taken off within it's designated rotation distance ... rolling resistance would have no effect.
The wheels would have been rotating at 260mph, which violates the test condition that the wheels rotate at the same speed as the treadmill.
The conveyor belt and the wheels have no direct influence on the thrust provided by the propeller ... there is no frictional drag that will stop the plane from moving forward.
The frictional drag will increase to infinity if you have infinite thrust, because the treadmill speed exactly matches the wheel speed. The plane can not move unless the wheel speed is faster than the treadmill speed! This is the point you are completely ignoring. How can you explain the wheels moving faster than the treadmill, since that violates the test condition? There is no explanation you can give for the plane moving an inch that does not require the wheel to rotate faster than the treadmill. Name any condition where the plane takes off that does not involve skidding (you might as well put the plane on skis if that is allowable) and you will find that the wheels are moving faster than the treadmill. Which is not allowable because the treadmill will always accelerate to keep the speeds the same.
Answer me this ... if what your saying is true, then the fact that Jamie was providing the concept of the conveyor belt (but not to your exact specifications) ... why did the plane take off in the exact distance it rotated when there was no conveyor belt ... you would have thought that Jamie's actions would have at least provided some sort of a disruption in it's normal rotation distance ... yet it didn't.
Because the treadmill in the test was moving at maybe 60mph, the plane took off at 60mph, so the wheels were turning at only 120mph. Rolling resistance is significant at that speed (probably around a hundred pounds of force) but it wouldn't show up in their test conditions because that's still a fraction of the engine thrust. Move the treadmill at, say, 100,000 mph, and the wheels would have to spin at 100,060 mph... That would be a massive amount of rolling resistance, and it STILL violates the test conditions which state that the wheels can't rotate faster than the treadmill.
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The plane that they used in the Ghost Busters had a take off speed of 60 mph
I don't remember that scene. :D
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http://www.straightdope.com/columns/read/2638/an-airplane-taxies-in-one-direction-on-a-moving-conveyor-belt-going-the-opposite-direction-can-the-plane-take-off
As you point out, one problem here is the wording of the question. Your version straightforwardly states that the conveyor moves backward at the same rate that the plane moves forward. If the plane's forward speed is 100 miles per hour, the conveyor rolls 100 MPH backward, and the wheels rotate at 200 MPH. Assuming you've got Indy-car-quality tires and wheel bearings, no problem. However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation." This language leads to a paradox: If the plane moves forward at 5 MPH, then its wheels will do likewise, and the treadmill will go 5 MPH backward. But if the treadmill is going 5 MPH backward, then the wheels are really turning 10 MPH forward. But if the wheels are going 10 MPH forward . . . Soon the foolish have persuaded themselves that the treadmill must operate at infinite speed. Nonsense. The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.
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X = tire rotation speed
Y = treadmill speed
Z = ground speed at takeoff
By definition, X = Y, since that is how the question is worded. HOWEVER...
For the plane to take off, X must equal any hypothetical surface speed (the ground or a treadmill) plus the speed through the air required to get off the ground.
For a plane to take off, X = Y + Z, so Y would have to be some amount lower than X, violating the conditions of the test. For X to continue to equal Y, ground speed at takeoff would have to be 0. The only way for Z to be 0 at liftoff would be if there was some wind, but again the test condition specifically states that wind is zero. The only way to create a wind condition is to move the treadmill fast enough for the surface effect to generate enough moving air over the treadmill surface to create enough lift over the wings for the plane to lift off. I don't think you can get a treadmill moving fast enough to do that (for a human-sized aircraft), using current materials technology. Obviously since hard drive read/write heads "fly" over the surface of the platter using this same effect, it could be possible on a small enough scale.
You have got to be kidding me ... you're gonna create your own formula to prove your theory.
Wheel rotation (X) and treadmill speed (Y) are disconnected from ground speed at takeoff (Z) because X is free-wheeling. The wheels on a plane have nothing to do with take off speed ... they only make it easier to reach takeoff speed because they are ... free-wheeling. Put skids on a 747 instead of wheels ... it will take off but it will take a hell of a lot more runway than it would need if it had wheels.
It's more like ...
X = thrust
W = wheels speed (which are free-wheeling)
Y = conveyor belt speed
Z = distance needed to travel to obtain ground speed at takeoff
X - (W-Y) = Z
When you take off in a plane, is your ground speed measured by how fast your wheels are rotating on the runway ? or is it how fast you are being thrusted forward ?
How is that Bush planes are able to rotate off bog marshes with skids ... imagine that ... they have no wheels.
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i'm probably completely off base here but...seems to me...
to stop the plane going forward, the conveyor belt would need to be moving in a way that counteracts the thrust of the engines...regardless of what the wheels are doing. which, i believe, would require going massively faster, not matching what the wheels or plane itself is doing. but it is possible.
eagls looks fine if the belt is matching the wheels rotational speed...and what the mythbusters did is correct, if the belt matches the wheels forward speed.
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then the wheels will simply drag. There is no way the treadmill + wheel bit would be enough to overcome the thrust of an aircraft.
And by the way, if you allow this, the wheels would drag across the treadmill at... you guessed it, takeoff speed. When was the last time you saw a plane capable of taking off with the wheel brakes locked? Not many planes in the world can accelerate to takeoff speed with the brakes locked.
X = Y + Z. Very simple equation. Wheel speed = treadmill speed + takeoff airspeed (no wind). It does not add up any other way. No matter what numbers you plug in, Z must equal zero (the plane doesn't move) if the wheel speed equals the treadmill speed and there is no skidding. If you allow skidding, then no matter how fast the treadmill is going, the wheels would skid across the treadmill at takeoff speed, which would be this equation: wheel speed = treadmill speed + takeoff airspeed - skidding speed. Skidding speed would equal takeoff speed, no matter how fast the treadmill was going, even if it was stopped.
Again, when was the last time you saw a plane that could take off with the wheel brakes locked? I've never flown one that could do it, and I flew large-motor F-15Es. The plane would come apart long before takeoff speed was reached due to to tire/wheel/strut failure. Heck, at idle thrust, the tires will blow and destroy the wheels if the wheels are locked as low as 30 kts. At max thrust the wheels would probably grind down to nothing before you got to 100 kts, and then you'd be dragging the struts across the runway...
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Again, when was the last time you saw a plane that could take off with the wheel brakes locked? I've never flown one that could do it, and I flew large-motor F-15Es. The plane would come apart long before takeoff speed was reached due to to tire/wheel/strut failure. Heck, at idle thrust, the tires will blow and destroy the wheels if the wheels are locked as low as 30 kts. At max thrust the wheels would probably grind down to nothing before you got to 100 kts, and then you'd be dragging the struts across the runway...
That only would prove that the failure was due to other factors ... and not that the aircraft would not take off if tire/wheel/struts didn't fail.
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Everyone needs to read the OP slowly and carefully. I think there is an argument over different concepts here.
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Eagl ...
Go find an inclined treadmill and set it to it's highest angle and set it to it's fastest speed.
Get a pair of roller skates ... strattle the treadmill facing downwards at midpoint in the treadmill ... hold on to the bars and get on the treadmill ... get your balance, let the wheels get up to speed, and then let go of the bars ... do you stay there ... do you go up ... or do you go down ?
With you logic ... you should go stay right there in the middle of the treadmill because your wheels speed is exactly matched to the treadmill speed.
Please film this because I would bet the ranch ... that you go down.
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Sombra's quote is "truth" except for the last sentence. The test condition is "impossible" to set up. But if it WAS possible (and it would be with a small enough plane), the plane wouldn't move an inch.
Slapshot, you're going to have to define "free wheeling", since no such thing exists in this universe. Anything that moves will take energy to keep it moving since we have no frictionless bearings and no perpetual motion devices. You still haven't explained away how you think it is acceptable to violate the test condition that the wheels rotate at the same speed as the treadmill. If the plane moves forward, the wheel has to be moving faster than the treadmill.
100mph treadmill, plus the plane moves at 100mph, means the wheels are moving at 200mph, which violates the test condition so you *fail*. This is why the mythbusters test was meaningless.
And I didn't "invent" an equation. If you don't understand that, then you lack the basic grasp of newtonian physics required to hold any sort of valid opinion on this whole subject and you might as well invoke witchcraft to explain how the plane moves.
Still, I will humor you. Let's say we have a plane that takes off at 10 mph (it is very light) and it takes 100 ft to get to that speed, and it can accelerate to that speed in 6 seconds. We have a treadmill that moves at 10 mph. That means in 1 hour, 10 miles of treadmill moves under the plane. Using simple math, that means the treadmill moves 1 mile in 6 minutes, 1/6 of a mile in 1 minute (60 seconds), and 1/60th of a mile in 6 seconds. A nautical mile is really close to 6000 ft, so let's call 10 mph approx 100 ft in 6 seconds. If the plane is stationary, the wheel must also travel 10 miles in that 1 hour, which means that the wheel rotates 100 ft every 6 seconds. But by your idea, the plane must accelerate to 10 mph in 100 ft and in 6 seconds, which means that the wheel must rotate a total of 200 ft in that same 6 seconds. Therefore a plane on a 10 mph treadmill with a 10 mph takeoff speed must have a wheel speed of 20 mph.
That violates the test condition, which specifically states that the treadmill will accelerate to match the wheel speed.
You can make the equation as complicated or as simple as you like, but it will still add up to the same - the test is worded in such a way that it requires an impossibility to occur - that the treadmill be capable of going at an infinite speed. I say that due to rolling resistance the treadmill speed will be somewhat under infinity, yet it will still be so fast as to exceed the strength of any material we might possible use to make the treadmill, wheels, tires, bearings, etc.
I am still wondering what you mean by "free wheeling", since it sounds like you are assuming a frictionless bearing or some other sort of perpetual motion machine that takes no energy to keep moving. No such thing exists - everything that moves takes power to start or stop the movement by pure newtonian physics, and keeping anything moving in the real world requires you to overcome friction or bearing drag (or both). And that drag increases by the square of the velocity. Even if you melt the bearing and are now looking at a fluid bearing with a different friction coefficient, the drag still increases by the square of the velocity.
That's how real math and physics work. You're trying to prove witchcraft :)
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Eagl ...
Go find an inclined treadmill and set it to it's highest angle and set it to it's fastest speed.
Get a pair of roller skates ... strattle the treadmill facing downwards at midpoint in the treadmill ... hold on to the bars and get on the treadmill ... get your balance, let the wheels get up to speed, and then let go of the bars ... do you stay there ... do you go up ... or do you go down ?
With you logic ... you should go stay right there in the middle of the treadmill because your wheels speed is exactly matched to the treadmill speed.
Please film this because I would bet the ranch ... that you go down.
You're arguing witchcraft (a common-sense approach to problem solving that completely ignores real physics). If the treadmill was going fast enough to create enough rolling resistance to counteract the gravity force vector, you bet your bellybutton you'd go up. And by the way, you set up the experiment backwards. The treadmill would have to be negatively inclined or the treadmill would have to be running backwards. Since you made that simple error, you clearly can't visualize the forces involved.
Test it yourself. Tilt a board at a slight angle, place a towel on top of it, and put a ball on the towel. Adjust the angle of the board until the ball slowly rolls down the board. Now repeat the test, while pulling the towel up the board. The ball will still roll in the "down" direction, but will climb up the board. That's the simplest junior high experiment in the world... And if you pull the towel at the same speed as the ball rotates, the ball *gasp* stays in the same spot. Amazing.
6 years of focused education in physics, math, engineering mechanics, and aerodynamic, aerospace, and mechanical engineering are backing up my arguments... Where I went to school, 7th graders did these sorts of experiments and I paid close attention.
Your arguments assume that you zero out some forces, and that leads to divide-by-zero conditions resulting in an invalid "infinite" solution for the treadmill speed.
We've come up with only 2 ways to make this work... Either the treadmill itself creates enough wind for the plane to fly (would only happen at very very very high treadmill speeds), or the wheels skids along the treadmill surface at exactly takeoff speed and it doesn't matter what speed the treadmill is turning at (the wheel is essentially locked relative to the treadmill surface). Those are the only two cheats I've heard to get around the "impossible" test condition.
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(http://www.scienceblogs.de/astrodicticum-simplex/2009/05/07/double-facepalm.jpg)
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ok think of it this way, consider the roller skates on a treadmill idea(u remain motionless), but thrust is a completely different force relative to pulling the air (think of it as holding a rope atatched to a truck). So if your on a treadmill with skates it does not matter how fast the wheels or treadmill are rolling, because the truck(thrust) will pull u foward anyways..... and the rope is invisible. hopefully this will paint a helpfull picture :aok
also a helicopter does not need wheels at all pure thrust pulls it up, think of a plane as a horizontal helicopter pulling it foward.------------->
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I vote Skuzzy just lock this thing now, because it's only going to get nasty from here.
It doesn't matter HOW much proof you give people this myth is a crock. They're not going to accept anything but their own narrow view of the mechanics involved.
You were right.
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OK, so this hurts my brain, but let me try to wrap my head around it.
The OP states the wheels would be going the same speed as the treadmill, but this (as someone else put it) would be a paradox. This means the treadmill would move relative to the speed of the wheels. However the wheels move relative to the ground they are rolling over, because the wheels move independently of the engine. They usually move relative to the engine but that's because the ground usually isn't a variable, because it doesn't move.
So once you add power to the engine, it is impossible to keep the speed of the wheel and the speed of the treadmill equal. You can keep speeding up the treadmill but because of the power provided by the engine, the wheel will just increase at the same rate.
Right?
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This is so funny!
The 2nd time this argument has been brought up, and people still argue till blue in the faces.
Saying a conveyor belt that only turns the wheels, is saying the wheels generate lift. So when the plane is supposedly in the air after the wheels reach takeoff speed, the wheels (from what I've seen on any aircraft) stop rolling eventually, right? Does the plane crash? Don't most modern aircraft have retractable wheels that don't turn while in the air?
This is really still an argument?
:airplane:
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No. No Airlift, if your going 70mph but no were, there is no air moving on the wings, thus not able to lift.
you want somethng fun to try dude?
take a regular sport plane up on a windy day.........40 size. if you get everythign right, you can fly in place, and even backwards. :aok
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This discussion has crossed the line into irrationality. Once you have to accept that the speed of the belt become so large that the friction of the wheels can keep the plane stationary, this discussion looses all sense of plausibility and testability. It is impractical to consider that the original premise considered this in the first place, and discussions down this line are not worth discussing. You are bending the argument to support your preconceived notion of what would happen. Try to limit your discussions to that which is at least close to the realm of possibility and reality.
There is an error in the original construction of the argument. Accelerating the belt to nearly infinite speed in fractions of a second is not what was anticipated in the original conception of the argument. This is the great discovery, not whether the plane will fly. The plane will always fly, you should be delighted to know that it would take an impossible feat to prevent the plane from flying. Don't miss the beauty of the argument because you are so set on a different conclusion.
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1. If you believe myth busters pseudoscience on this experiment or any other then you need to have your head examined. Its obvious they didn't even have the means to attempt to simulate the proposed situation.*
2. Everyone knows lift is generated by the wings and not the wheels, to point this out in this argument is pointless.
3. Everyone that thinks the plane can take off ALSO assumes the belt can not accelerate to infinity, nor can it accelerate as quickly as the wheels. This was simply not restricted in the original question. This is a theoretical question, and to limit the answer based on what man can currently construct is pointless in regards to the original question.
There are different answers to the question depending on how exactly it is worded. The OP in this topic said, "The conveyer belt is designed to exactly match the speed of the wheels at any given time". If this is true then the plane simply does not move forward. The wheels and the conveyor both simultaneously quickly accelerate to infinity. The drag created by the tire on runway, bearings, etc also increases to infinity. Thrust is thrust, weather it comes from wheels or props, and no amount of increasing thrust can overcome drag simultaneously increasing to infinity.
If you word the question differently you can come out with different outcomes.
*They assume rotation speed is 60mph, so the wheels are moving 60mph, and the belt should move 60mph in the opposite direction. The wheels are now moving 120mph, but the belt is still only going 60. Completely flawed logic in regard to the original question.
The bottom line is there is just no way to disprove this in an experiment becuase we can't simulate the proposed parameters. As long as it can't be disproven in a real world experiment, people will insist its possible. You've just got to remember this is a theoretical question that doesn't limit itself to our inability to actually build the proposed conveyor belt.
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Good God, I'm evil! :devil
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The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
If the conveyor belt exactly matches the speed of the wheels, then the plane is stationary.
I think the common description of this problem states that the conveyor belt matches the speed of the airplane. In which case, the plane does take off.
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If the conveyor belt exactly matches the speed of the wheels, then the plane is stationary.
I think the common description of this problem states that the conveyor belt matches the speed of the airplane. In which case, the plane does take off.
Exactly! :)
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One, must only...ask why do planes fly! Grasshopper.
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One, must only...ask why do planes fly! Grasshopper.
Because of relative wind moving over an airfoil with a positive angle of attack, pure and simple.
If the plane can't move, then it can't generate relative wind, and it can't fly.
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Besides the original question being a paradox, the plane would take off. Notice in the mythbusters example that the plane took off exactly like it would without the tarp? By your theory it should have at least taken longer to achieve flight.
If you put a car on a conveyor belt under the same principles, it would not move. Now... the applicable question for the airplane would be:
If you put a fan behind an aircraft that instantly created a downwind equal to the speed of the airplane, would it take off? :rolleyes:
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this is very simple.
if for whatever reason, there is no airflow over the wings, the plane will/can not fly.
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Besides the original question being a paradox, the plane would take off. Notice in the mythbusters example that the plane took off exactly like it would without the tarp? By your theory it should have at least taken longer to achieve flight.
Wheels moving at 60mph or 120mph generate very little difference in drag. I would guess the plane in mythbusters did take more time to take off but by a very small amount.
If they had a way to accelerate the tarp to several hundred miles per hour then you would start to see significant increase in distance. If they could accelerate the tarp to infinity as the original question presumes, then the plane would simply never move.
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It's called rotational inertia (look it up) and unless the plane has enough thrust to get to takeoff speed while the wheels are locked, it won't fly.
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Wow.
This is a yearly topic three years in a row now:
O' Club January 2008 (http://bbs.hitechcreations.com/smf/index.php/topic,225421.0.html)
O' Club January 2007 (http://bbs.hitechcreations.com/smf/index.php/topic,197189.0.html)
wrongway
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(http://i.smiles2k.net/angry_smiles/banghead.gif)
There is no hope for humanity
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One, must only...ask why do planes fly! Grasshopper.
Money
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It flies. It doesn't accelerate because of the wheels, they just roll. The prop/jet thrust accelerates the aircraft through the air and that isn't affected by any treadmill contraption. All the treadmill would do is make the wheels spin faster as it took off.
Lot of posts on this, but go to bottom of page 1, Karnak has it spot on.
The plane will fly, however on a conveyor belt the wheels will be rolling twice as fast...simple.
At time T=0, the plane is stationary however the conveyor belt is making the wheels role at a given speed.
Thrust is then applied by propeller. The plane and air are independent of the conveyor belt. So as the plane begins to move forward, the wheels now are moving even faster.
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Another way of visualizing why it takes off:
1) Connect the plane (with engine off) to a powerfull glider winch launcher, the plane doesn't care where the forward pointing force comes from.
2) The winch launcher is placed on solid ground, so it will pull the plane closer - the opposite moving conveyor can't do anything about the shortening of the rope.
3) As the winch rotates faster the plane accelerates forward - and takes off.
(The winch however must work a *tiny* bit harder compared to take off without moving conveyor, due to the change in the wheels rotational inertia and friction in wheel bearings)
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DRED,
What they're asking is what happens should the treadmill continue accelerating to keep pace with the speed of the wheels, ignoring ALL mechanical impossibilities and looking at it completely from a theoretical standpoint. I misread the OP on that, and it IS a completely different question than what the Mythbusters tested. The Mythbusters were testing the argument that the plane wouldn't take off on a treadmill matching its takeoff speed.
However even viewed as theoretically, the airplane WILL take off because its wheels are ALWAYS moving faster than the conveyor belt. Look at it this way:
An aircraft has a takeoff speed of 60mph with the conveyor belt at 0mph. The plane's wheels spin at 60mph on takeoff. You accelerate your conveyor belt to 60mph, which would match the speed of the wheels at the time the conveyor belt was at a stopped state. However because the aircraft's propulsion system is independent of its wheels, which are free-spinning, it still continues accelerating to its 60mph takeoff speed, with its wheels now spinning at 120mph.
So you accelerate your conveyor belt to 120mph. The plane continues to accelerate until its wheels are spinning at 180mph.
When the conveyor belt is at 180mph the wheels are at 240mph, etc, etc, etc.
No matter how fast the conveyor belt is spinning the wheels are ALWAYS going to be 60mph faster and the plane will take off.
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Everyone who has said that it will not take off is wrong. Here's why:
Imagine you have two conveyor belts next to each other. One has a car on it, the other has an aeroplane on it.
You then start up both the aeroplane and the car, move their engines to full power, and turn on the conveyor belts to match the speeds of the vehicles.
Now, you're probably wondering: why bother with the car?
Here's the answer: the car accelerates by rotating its wheels against the surface of the road. KEY POINT: the car moves forward by rotating its wheels against the surface of the road - if you lift it off the road, it will stop accelerating. If the road beneath it is moving backwards at the same speed as the wheels are moving the car forward, then there will be NO net motion of the car.
HOWEVER, the aeroplane accelerates by using it's propeller to push air backwards. KEY POINT: the aeroplane moves by pulling on the air with its propeller - if you lift it off the ground, it will continue accelerating. Even if the runway beneath the aeroplane starts to move backwards at the same speed as the aeroplane is moving forward, the propeller will STILL be pulling the aeroplane forwards, REGARDLESS of what is happening with the wheels.
Hence, the plane will accelerate, and take off.
EDIT: Saxman beat me to it :P
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what does the speed of the wheels have to do with anything? Plane flys independatly of the wheels anyways. You dont need wheels to take off. I dont even get it i guess.
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By the way...
http://www.xoxma.com.ua/img/flash/zadachka001.swf
Text at the end of flash says "The Great Physicists".
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The plane will take off......
Here is another way to look at it..
Go put a toy car on the treadmill at home. IT doesnt matter if you (SPIN THE WHEELS ) at 4 miles per hr or 4 million you can still push the car forward with your hand. The force of the wheels is up and down not horizontal and has no effect on acceleration.
Once you have enough thrust to overcome the friction of the wheels against the converyor you can turn up the speed on the conveyor to 4 million mph. IT wont move the plane backward it just spins the wheels.
Your are spinning Wheels at what ever speed does not affect forward movement. The effect of spinning wheels is no different that if you had hover skids on the plane.
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This is why I want a seaplane.
I agree with eagl's explanation.
The original post by jdbecks states
imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
? :noid
As far as this argument is concerned the conveyor belt will always match the speed of the forward thrust. The plane cannot roll forward because rolling forward would mean the wheels are going faster then the conveyor belt. The argument states the wheels and belt always match speed.
the plane becomes an expensive over-sized box fan.
(http://www.homedepot.com/catalog/productImages/300/b4/b4324287-99f2-4793-92b0-43f977054948_300.jpg)
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Even if the treadmill goes to infinity, the thrust from the engine will take the wheels to infinity, and beyond.
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I guess some people will argue with a wall...
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DRED,
What they're asking is what happens should the treadmill continue accelerating to keep pace with the speed of the wheels, ignoring ALL mechanical impossibilities and looking at it completely from a theoretical standpoint. I misread the OP on that, and it IS a completely different question than what the Mythbusters tested. The Mythbusters were testing the argument that the plane wouldn't take off on a treadmill matching its takeoff speed.
However even viewed as theoretically, the airplane WILL take off because its wheels are ALWAYS moving faster than the conveyor belt. Look at it this way:
An aircraft has a takeoff speed of 60mph with the conveyor belt at 0mph. The plane's wheels spin at 60mph on takeoff. You accelerate your conveyor belt to 60mph, which would match the speed of the wheels at the time the conveyor belt was at a stopped state. However because the aircraft's propulsion system is independent of its wheels, which are free-spinning, it still continues accelerating to its 60mph takeoff speed, with its wheels now spinning at 120mph.
So you accelerate your conveyor belt to 120mph. The plane continues to accelerate until its wheels are spinning at 180mph.
When the conveyor belt is at 180mph the wheels are at 240mph, etc, etc, etc.
No matter how fast the conveyor belt is spinning the wheels are ALWAYS going to be 60mph faster and the plane will take off.
Spot on. Well said.
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This is why I want a seaplane.
I agree with eagl's explanation.
The original post by jdbecks states
As far as this argument is concerned the conveyor belt will always match the speed of the forward thrust. The plane cannot roll forward because rolling forward would mean the wheels are going faster then the conveyor belt. The argument states the wheels and belt always match speed.
the plane becomes an expensive over-sized box fan.
(http://www.homedepot.com/catalog/productImages/300/b4/b4324287-99f2-4793-92b0-43f977054948_300.jpg)
Kotrenin,
Reread my post. The conveyor belt can NEVER match the wheel speed of the aircraft. No matter how fast the treadmill is going, the nature of the plane's manner of acceleration guarantees that its wheels will ALWAYS be faster.
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The conveyor belt can NEVER match the wheel speed of the aircraft. No matter how fast the treadmill is going, the nature of the plane's manner of acceleration guarantees that its wheels will ALWAYS be faster.
Or:
Since we agree the plane in reality will move forward, the only way to make the argument true, is to allow for slip between wheels and belt. So - since the conveyor belt is designed to exactly match the speed of the wheel it must be a belt with a very special lubricated surface. (how they designed it however is not our concern).
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Or:
Since we agree the plane in reality will move forward, the only way to make the argument true, is to allow for slip between wheels and belt. So - since the conveyor belt is designed to exactly match the speed of the wheel it must be a belt with a very special lubricated surface. (how they designed it however is not our concern).
Here's an important point to consider:
The chief arguments of the people against are dependent on limiting the mechanical capabilities of the aircraft while REMOVING such restrictions from the treadmill. IE: The treadmill is capable of accelerating to infinite velocities, but the aircraft must contend with the mechanical limitation of its wheels, such as friction of the axles.
To be fairly considered and represented in its purest state, the ONLY factors that can come into play with this problem are: The speed of the treadmill, the speed of the aircraft's wheels at takeoff, (you ca NOT solve this without knowing at what speed the aircraft takes off) and that the aircraft's engine produces thrust independent of the rotation of its wheels--it doesn't even matter how MUCH thrust the aircraft's engine produces.
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OK I fell for this one.ooooops.
Can g...d create a rock so big he cant' move it.
I can't type a word of English.
The problem is in the argument.
An argument is only logically valid if it follows the premise. So while the argument is valid. It is what is call unsound because the premise is wrong or invalid.
It violates the law of non contradiction.
Can you match speed exactly. Our definition of forward movement violates the definition of matching speed. At the same time.
The argument is unsound and cannot be tested as stated.
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I guess some people will argue with a wall...
no they won't
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Saxman,
reread the Original Post by jdbecks and pay attention to the parts I made bold they are the rules for this argument.
Quote from: jdbecks on Yesterday at 05:29:35 PM
imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
The Myth Busters had different rules that are not part of the argument in this thread. The Myth Busters are limiting the conveyor belt to the planes take off speed but the wheels are not.
The equation of jdbecks argument is x = y + -y
where x is air speed
y is wheel speed
and -y is conveyor belt speed or the negative wheel speed because The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. according to jdbecks rules.
that being said if the wheels are going 2 mph the conveyor belt is going -2 mph and airspeed is 0. Enter any integer for y and you must enter the same integer for -y.
In the Myth Buster equation the rule limits the conveyor belt to the air speed for take off. As long as your airplane can provide enough thrust to go at least twice as fast as its takeoff speed it will take off.
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Sonic,
make sure you explain it to your brother, he's not as quick as you. :D
<S>
Ko
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The Myth Busters had different rules that are not part of the argument in this thread. The Myth Busters are limiting the conveyor belt to the planes take off speed but the wheels are not.
because that is all you can test in the real world. :devil
<S> kot
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Kotrenin,
you are essentially saying that the pilot is not allowed to apply throttle. Because if he does, the plane will take off, thus violating your interpretation of the conditions in the original statement.
But the original statement do allow movement:
"The conveyor belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation"
Movement will only happens if he apply throttle, and if he does - he will take off. There now seems to be a problem with the matching speed condition, so there must be slip between the wheels and conveyor. Or - since speed is defined relative to the observers position, we could be observing from a vehicle driving faster than the conveyor but slower than the plane.
This reminds me of Zeno's "Achilles and the Tortoise" paradox - we know what will happen in reality, therefore we must understand the shortcomings of the paradox. It's not reallity that is broken.
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What if your conveyor belt goes the other way? Let's say 150mph forward (the wheels are spinning 150mph backward). Throttle up, prop pulls plane forward, wheels now spinning slower but still backward. Plane takes off. Eh?
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The question I have is, say a jet lifts off at 150mph and there is a wind that increases at the exact same rate as the jet accelerates. (For example, when the jet is going 60mph, the wind is 60mph against it). Would the jet be able to lift off? If so, would the jet lift off the ground in place without moving laterally? :headscratch: I say yes, but I don't know for certain.
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What if your conveyor belt goes the other way? Let's say 150mph forward (the wheels are spinning 150mph backward). Throttle up, prop pulls plane forward, wheels now spinning slower but still backward. Plane takes off. Eh?
:) No, it doesn't change anything in the paradox
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(http://hphotos-snc3.fbcdn.net/hs157.snc3/18456_230670963175_599988175_3338048_4496827_n.jpg)
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Kotrenin,
you are essentially saying that the pilot is not allowed to apply throttle. Because if he does, the plane will take off, thus violating your interpretation of the conditions in the original statement.
But the original statement do allow movement:
"The conveyor belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation"
Movement will only happens if he apply throttle, and if he does - he will take off. There now seems to be a problem with the matching speed condition, so there must be slip between the wheels and conveyor. Or - since speed is defined relative to the observers position, we could be observing from a vehicle driving faster than the conveyor but slower than the plane.
This reminds me of Zeno's "Achilles and the Tortoise" paradox - we know what will happen in reality, therefore we must understand the shortcomings of the paradox. It's not reallity that is broken.
lengro,
you are reading too deep into the problem. Yes this is a paradox. No matter what the pilot does to increase or decrease thrust, the conveyor belt must always be the same speed as the wheels, that is the central rule to this paradox. You can throw whatever you want under the wheels, oil, grease, butter, sand paper... the rules of the universe for this problem dictate that the rotation of the wheels and the rotation of the conveyor belt must cancel each other out. Thus airspeed will always equal 0. An airplane does not take off the instant forward thrust is applied. It must travel some distance to reach that speed.
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Yes this is a paradox.
The above we can agree in, I will leave it at that :)
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The problem with this puzzle is that many feel the answer is so obvious that they fail to see the implications of the requirements of the experiment.
As Eagl has shown with his very simple equation, the plane does not fly and the owner and creator of this game agrees.
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imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
as the plane starts to move forwards, the wheels start to rotate. the conveyor belt instantaniously matches their speed in the opposite direction of rotation counteracting the rotation of the wheels. the plane accelerates down the conveyor to takeoff speed and takes off. at all times during the roll, the conveyor belt is moving at the same speed and the same direction as the aircraft, the wheels are stationary relative to the conveyor.
:banana:
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Not this subject again.
comes up every 1.5 years or so :neener:
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hmmm gone a bit quiet now hasnt it?
<< pwns this problem :neener:
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I think it's more a matter of people being tired of debating this issue.
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The plane will not takeoff because it will not be allowed to roll forward. As soon as the plane moves forward, it will be PUSHED backwards by the conveyor belt.
Now, if you place this experiment in a wind tunnel and set the wind speed to be equal or greater to the airspeed needed for takeoff, then the plane will takeoff even if on a conveyor belt that is matching the wheel speed of the airplane.
(Also, if you are going to try this experiment with a R/C airplane, please ensure that the motor is retarded to a power level that is just enough to achieve takeoff airspeed.)
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As soon as the plane moves forward, it will be PUSHED backwards by the conveyor belt.
no it wont, the points of contact between the plane and the conveyor are freewheels (its reasonable to assume that the brakes are off). in fact the wheels wont rotate at all. see my answer above.
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no it wont, the points of contact between the plane and the conveyor are freewheels (its reasonable to assume that the brakes are off). in fact the wheels wont rotate at all. see my answer above.
You, like all who think they know the answer, are not reading the problem correctly. The conveyor belt matches the speed of the wheels, not the plane. Although you are correct that there would be relatively little friction involved with rolling the wheels, you are not taking into account the force required to overcome the wheel's rotational inertia (the wheel's desire to stay at rest). Newton would agree with me on this one.
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indeed it does, however the wheel speed is 0, so the speed of the conveyor relative to the wheels, and hence also relative to the plane is also 0.
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indeed it does, however the wheel speed is 0, so the speed of the conveyor relative to the wheels, and hence also relative to the plane is also 0.
Wrong. The wheel speed is not zero; the difference is zero in the opposite direction. It is a mathematical impossibility whereas the instant the wheel begins to move the conveyor belt counters by ramping up to infinity.
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nope, from earlier:
as the plane starts to move forwards, the wheels start to rotate. the conveyor belt instantaniously matches their speed in the opposite direction of rotation counteracting the rotation of the wheels.
edit: starting from rest, the wheel speed would indeed tend to infinity if the conveyor belt matched the wheel speed in the same direction of rotation, however it matches it in the opposite direction of rotation so it tends to 0.
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I can't help you.
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There is still debate on this?
The plane takes off, as a professional engineer I am certain of this.
Lets look at it a different way. Suppose the conveyor belt is made of snow or ice, and instead of wheels, the plane has skids on the bottom of it. Would the plane take off then?
Of course it would, because no matter how fast the conveyor belt is moving, the plane's skids simply skate over the top of it no matter what the speed.
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There is still debate on this?
The plane takes off, as a professional engineer I am certain of this.
Lets look at it a different way. Suppose the conveyor belt is made of snow or ice, and instead of wheels, the plane has skids on the bottom of it. Would the plane take off then?
Of course it would, because no matter how fast the conveyor belt is moving, the plane's skids simply skate over the top of it no matter what the speed.
So... You're an engineer and you don't know about Newton's law and rotational inertia.... Scary.
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So... You're an engineer and you don't know about Newton's law and rotational inertia.... Scary.
the conveyor belt counters by ramping up to infinity.
How much energy does it take to ramp a conveyor belt up to infinity, and what happens with time at light speed?
Can there only be one answer to a paradox - is their an answer at all?
:bolt:
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Did you know you can measure how much acceleration energy was absorbed by an airplane's wheels by how long they spin after the plane takes off?
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I can't help you.
you might be able to if you can supply a formal proof for:
"starting from rest, the wheel speed would tend to 0 as the conveyor belt matches the wheel speed in the opposite direction of rotation."
my calculus is a bit rusty :)
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you might be able to if you can supply a formal proof for:
"starting from rest, the wheel speed would tend to 0 as the conveyor belt matches the wheel speed in the opposite direction of rotation."
my calculus is a bit rusty :)
If the wheel is turning clockwise the conveyor belt is most definitely turning counter-clockwise...
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Did you know you can measure how much acceleration energy was absorbed by an airplane's wheels by how long they spin after the plane takes off?
Really, I was pretty sure I also would need to know something about the friction in the wheel bearings, and the mass of the wheel?
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Really, I was pretty sure I also would need to know something about the friction in the wheel bearings, and the mass of the wheel?
What does mass of the wheel have to do with it? you say the plane on the conveyor belt will fly, right?
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So... You're an engineer and you don't know about Newton's law and rotational inertia.... Scary.
In point of fact, Newton has three laws of motion, not one. Also, when you describe rotational inertia, I'm going to assume you mean 'angular impulse and momentum'.
The 'intertia' of the wheels as you put it however, is not applicable in this situation, and only acts as a resistance to the thrust of the airplane while spinning, similar to how air friction (drag) would.
This hypothetical scenario can be mathematically described with a free body diagram. The reason I used the sled example was to try to show it in laymans terms so it can be understood more easily. The wheel and sled both serve to highlight the frictionless interaction between plane and undercarriage.
Of course in a real world situation, the sled and snow conveyor belt would generate 'some' friction which the planes thrust would have to overcome. Same goes for the wheels, the planes thrust would have to overcome the inertial resistance you mention.
There is no debate, the plane flies. It is a mathematical certainty.
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in fact for this example the wheel bearing friction and inertia of the wheel are irrelevant, because the rotational speed of the wheels is 0.
just in case anyone hasnt got it yet:
(http://kottkegae.appspot.com/images/plane-takes-off.gif)
:aok
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What does mass of the wheel have to do with it?
I think mass would be handy if I was given some friction coefficients in the wheel bearings - but so far you stated that spin time after take off was all I needed?
you say the plane on the conveyor belt will fly, right?
Hey - it's your turn to answer one of my questions :)
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The relative speed is not 0 because the myth stated 'opposite rotation'. The myth is based on the false assumption that the plane cannot move because the acceleration is done by turning the wheels like in a regular automobile and hence plane allegedly cannot move if wheel spin is exactly matched at all times (like going to measure the engine power in a dyno except this one doesn't resist at all) or imagine trying to run on wet glass-surface ice - you'll just flap around not moving anywhere.
But this is not a problem because a) The plane does not generate or need any thrust using the wheels b) The maximum wheelspin would be double the regular takeoff speed, hardly a problem. c) plane still moves on conveyor belt despite belt rolling in opposite direction in accelerating motion exactly matching aeroplane speed in reverse, remember if plane is not moving, neither is conveyor belt. All motion is generated as counter motion of the plane itself d) when enough airspeed is reached the plane takes off normally.
Imagine having your car roll in a dyno that has zero resistance. You can spin the wheels all you want and it wont move since wheel speed is always matched in opposite. Push it with your hand gently and it will move despite of rotational speeds. Put 20 000lbs of thrust in it and it'll fly with no wings.
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err by "myth" I assume you're referring to the Mythbusters plane/conveyor belt problem. their assertion was "An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff."
although it looks similar at first glance, this problem is completely different.
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The relative speed is not 0 because the myth stated 'opposite rotation'. The myth is based on the false assumption that the plane cannot move because the acceleration is done by turning the wheels like in a regular automobile and hence plane allegedly cannot move if wheel spin is exactly matched at all times (like going to measure the engine power in a dyno except this one doesn't resist at all) or imagine trying to run on wet glass-surface ice - you'll just flap around not moving anywhere.
But this is not a problem because a) The plane does not generate or need any thrust using the wheels b) The maximum wheelspin would be regular takeoff speed, hardly a problem. c) plane still moves on conveyor belt despite belt rolling in opposite direction in accelerating motion exactly matching aeroplane speed in reverse, remember if plane is not moving, neither is conveyor belt. All motion is generated as counter motion of the plane itself d) when enough airspeed is reached the plane takes off normally.
Imagine having your car roll in a dyno that has zero resistance. You can spin the wheels all you want and it wont move since wheel speed is always matched in opposite. Push it with your hand gently and it will move despite of rotational speeds. Put 20 000lbs of thrust in it and it'll fly with no wings.
the conveyor doesn't match the airplane's speed, it matches the wheel's speed.
I honestly think you guys are dismissing the premise because you think you've got it figured out. but you don't.
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The relative speed is not 0 because the myth stated 'opposite rotation'. The myth is based on the false assumption that the plane cannot move because the acceleration is done by turning the wheels like in a regular automobile and hence plane allegedly cannot move if wheel spin is exactly matched at all times (like going to measure the engine power in a dyno except this one doesn't resist at all) or imagine trying to run on wet glass-surface ice - you'll just flap around not moving anywhere.
But this is not a problem because a) The plane does not generate or need any thrust using the wheels b) The maximum wheelspin would be regular takeoff speed, hardly a problem. c) plane still moves on conveyor belt despite belt rolling in opposite direction in accelerating motion exactly matching aeroplane speed in reverse, remember if plane is not moving, neither is conveyor belt. All motion is generated as counter motion of the plane itself d) when enough airspeed is reached the plane takes off normally.
Imagine having your car roll in a dyno that has zero resistance. You can spin the wheels all you want and it wont move since wheel speed is always matched in opposite. Push it with your hand gently and it will move despite of rotational speeds. Put 20 000lbs of thrust in it and it'll fly with no wings.
Wrong
imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.
Can the plane take off?
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the conveyor doesn't match the airplane's speed, it matches the wheel's speed.
I honestly think you guys are dismissing the premise because you think you've got it figured out. but you don't.
The wheel turns only as fast as the aeroplane moves. The conveyor belt duplicates that movement in reverse.
I don't know, could it be then that the final wheel spin is takeoff speed to the second order (magnified by matching belt speed?) in which case the wheels woult bust apart before takeoff.
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nope, the wheels start with a rotational speed of 0 at rest, and continue with rotational speed 0 throughout the takeoff.
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nope, the wheels start with a rotational speed of 0 at rest, and continue with rotational speed 0 throughout the takeoff.
Lol if the belt rotates in the opposite direction how do you get rotational speed of 0? That would happen only if the rotation was happening in the same direction as plane moves.
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someone mention that earlier.
No the conveyor belt acts like a break. But instead of using friction like in a caliper brake or drum brake it acts like a wheel chock. It is more like a hill or incline.
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someone mention that earlier.
No the conveyor belt acts like a break. But instead of using friction like in a caliper brake or drum brake it acts like a wheel chock. It is more like a hill or incline.
The way I see it any wheelspin is generated by the aircraft actually moving. Unless aircraft is not moving there is no wheelspin and hence no counter rotation.
The instant a/c moves, wheels start to turn and this is counteracted by treadmill. Result is speed x speed afaik so in the end takeoff speed would mean that wheels and the treadmill would spin at speed of takeoff speed to the second order (i.e. 140 knots x 140 knots = 19600 knots). At that speed the wheels would burst and friction from bearings etc. would be considerable but given enough thrust it should still make it.
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err by "myth" I assume you're referring to the Mythbusters plane/conveyor belt problem. their assertion was "An airplane cannot take off from a runway which is moving backwards (like a treadmill) at a speed equal to its normal ground speed during takeoff."
although it looks similar at first glance, this problem is completely different.
Not actually. It just set limit of speed with conveyor belt can move, but it doesnt change the fact what conveyor belt cannt prevent plane from moving forward.