plane on a conveyor belt?

[BlckMgk]

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?

So just drawing this out, the conveyor's belt will be moving in the same direction of the plane.

Hmm APDrone does have a point.

Kurt is thinking the conveyor will be moving opposite of the plane, but its opposite the wheels which is in fact in the same direction of the plane. What is incorrect up there is that it says "Like a treadmill" but thats incorrect a treadmill goes the same direction of the force applied.

But what will happen and has been said incorrectly, the plane wheels won't move but the plane will take off.

[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 half the 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?

[john9001]

well, Al's plane will never get off the ground because all the energy from his engine is being used up putting rotational inertia into the wheels and his bearing's will get hot and melt.


however, a rapidly moving wheel will have the top of the wheel moving at a faster airspeed that the bottom of the wheel, the faster airspeed will create a low pressure area on top of the wheel which will provide lift (bernoulli's law), this will cause a upward lift vector to the wheel actually pulling the plane into the air.

this has been proven on open wheeled race cars with their wide race tires, that's why open wheeled race cars have the big spoilers to resist the lift vector and hold them down on the track.

[lukster]

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?

Well, the initial energy impartation of a belt capable of instanteous infinite speed to the wheels of the plane would probably destroy it, the airfield, the city, and the planet it was on but barring that the plane might be bounced off the belt with enough forward speed to reach jupiter before the belt could grab it.

[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.

[lukster]

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.

There is never a transfer of energy without friction involved.  Forget the axle for a moment. Assume you have a tire (minus the wheel and plane) sitting on the belt. When the belt turns energy is transferred to the tire causing it to rotate and move. No energy can ever be transferred from one object to another without friction. Friction is thereby the conduit through which energy is transferred.

[lukster]

BTW, I'm not disagreeing with you Minid and I'm not sure that either Eskimo or Hitech are either. 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.

[chancevought]

the question says a plane, not Starships...so lets think basically here (since I'm not Stephen Hawkings).

Since direction and speed of air around aircraft is not mentioned, we can assume it is zero...
the plane starts it's engine...nuthin happenin right, since it's not producing enough torque at idle to move the weight of the aircraft....conveyer not moving

throttle is applied, propeller bites the air, thus causing thrust through the air...not in anyway connected to speed of wheels...they jus hang on the airplane

as the aircraft moves the wheels start to spin...the conveyer moves in opposite direction of the wheels, not the plane, thus makin the wheels sit still....

the thrust generated by prop still cuts throught the air producing thrust in the air...still has no relation to wheels which are jus hangin on the airplane, the aircraft will move...

the wheels will still remain still, since the conveyer will start to move in opposite direction, preventing them from gaining any speed...

at say 60 kts the aircaft will be movin fast enough for the wings to generate lift..the plane will take-off...

what you would see is the plane would start movin, the conveyer would counteract this movement and actually move in same direction as plane..(the wheels wont move at all because the conveyer trying to counter act wheel will keep it still, not move it in opposite direction...get it?  If the conveyer sensed a rearward spin of the wheels, it would apply forward movement. We can assume that there may be a slight bobble, but it would eventually catch itself, and there would be no spinning of the wheels. all this would happen in small increments, but totally independent of what the prop is doing the air)

You all keep thinkin the conveyer will move opposite of plane, but to counteract motion of wheel, it would actually move in same direction as plane, thus wheels sit still....prop cuts air making thrust regardless of wheel speed...plane takes off

This Thread is Closed!!!

*doh...lookin back now I see others came to the same conclusion...sorry i couldnt wait through 5 pages of Trek dribble to make my post....

[lukster]

The original question did not say that the belt moved to counteract the speed of the wheel but simply that it moved at the same speed in the opposite direction. There is room for interpretation as to what that means exactly and yours is as good as any.

[kamilyun]

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

I'm sorry for revisiting this, but I only post drunk and tired...and I'm not trying to argue, just trying to edjumicate myself after ignoring physics for 10 years:

Isn't drag due to friction between a fluid and a surface?  This is what I was thinking of when posting the equation above.  And is drag a force?

[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

[Golfer]

Chance Vought I'm not sure that you're helping any argument with anything you're saying...


In fact I like to think I understand what you're trying to say...but you're thinking the airplane is simply riding along a moving walkway or treadmill until it gets enough airspeed to fly.  That has nothing  to do with the original question with the intent of the treadmill to be similar to one on which you walk at the gym.

[lukster]

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

Wheel speed can mean two different things and you can see what I mean in a drawing I posted earlier. Did he mean the speed of the wheel relative to the air (no breeze)? This would be the speed of the entire aircraft. Or did he mean the rotational speed of the wheel, like you would measure a cars speed by measuring the wheels rpm. If you assume that the belt will try to counter that speed then the result is the same in both instances.

If you assume that the belt will move in the opposite direction from that of the tire then the result is dramatically different in both resulting in one with the plane lifting off at 70mph while it's tires are rotating at 140mph and the other with the tire instantly spinning as fast as the belt can go.

[2bighorn]

Originally posted by eskimo2
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.

Put 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.

So let me help you.
The mass moment of inertia of the wheels would not easily overcome the mass of the plane because there's not enough friction between the wheels and conveyor and even less between wheels and the plane. Now add the forces of the forward thrust and you'd have even larger force to beat.

[eskimo2]

Read my story, answer the questions, and ye shall see the light.