http://www.texags.com/main/forum.reply.asp?topic_id=760762&page=1&forum_id=12

that answer is no, simple.

Wow, that thread is at 9 pages. I'm not reading all that. My guess would be no.


[This message has been edited by blake2007 (edited 12/4/2006 3:48p).]

Of course it won't take off. If the plane is not moving relative to the air, no lift is created. How did this go 11 pages?

the conveyor contacts the wheels... where a plane uses a propeller to move itself forward.

Example takeoff speed:
100MPH

lets assume that we start at zero mph.

The propeller moves the plane's body (not just the wheels) forward towards that 100MPH mark, (the wheels are also increasing their speed at this moment.) But the kicker is that the wheels are free to spin as fast as they want in relation to the ground.

The plane's body will reach 100mph eventually (just about as fast as if it was on a normal runway) and then it will lift off. The wheels may be spinning at a speed of +/- 200mph because the conveyor thinks that it can actually hold the plane back.


the difference between a plane and a car is that the plane pulls itself thru the AIR, a car pushes itself forward from the GROUND. the conveyor belt system is not part of the AIR, just the GROUND.

A car would never make it off the conveyor, a plane will.

If anyone doubts me on this, go get a toy plane and try it (I know the correct answer, but i have had to demonstrate simple concepts to people using toys before)

ummm. it does take off, guys. it's not a car.

plane moves, wheels move x2. plane takes off.

Why would the wheels move x2? It said the conveyor would match the speed of the wheels.

The plane's velocity relative to the air would be 0. With no wind speed, that plane is staying on the ground. Wheels don't lift the plane. Those wheels could be spinning 5000mph and if that plane has zero velocity relative to the air, it's not going ANYWHERE.

it would take off because the prop pulls the plane against the wind without respect to the wheel speed. When it takes off, it will read double ground speed, but its actual air speed would be normal.

The plane needs a constant flow of air to the wings in order to lift. Where does this come from???

It is possible for the plane to move forward with the ground moving out from underneath it. The plane however will not take off if the conveyor could go infinite mph. In that case the wheels couldnt spin fast enough for the plane to move forward. As crappy as the video on pg 11 of that thread is, it proves the point. The skateboard is being pushed by an external force (the fan) and the ground is moving out from behind it, yet the board moves forward.

No

Ok think about it this way. When any plane is in the air, where is the external force coming from? The prop/jet engines, no? Lets say we're working with a jet engine. The engine pushes air out the back of the plane, which accelerates the plane forward (For every action, there is an equal but opposite, yadda yadda). This means the plane is now moving forward relative to an observer on the ground. Now lets say the observer is inside the plane, or even a smart observer on the ground could realize the same thing. If the air around a plane had some sort of colored dust in it that would suspend in the atmosphere, as the plane flew through it, the dust would appear to be moving backwards, relative to the plane, no? Now go back to the original question. The plane is on the ground. Replace the "dust" with the conveyor and you have the answer. It IS possible for the plane to move forward while everything around it appears to be moving backwards, thus it is possible for the plane to take off.

[This message has been edited by GarlandAg2012 (edited 12/4/2006 8:07p).]

But alas! How does the plane get any inital velocity?

In an airplane, the thrust comes from the engine exhaust for jet aircraft and from the screwing action of the propeller for propeller driven aircraft.

In response to the thrust, the airplane will move forward. What the wheels are doing is immaterial.

In the case of a conveyor belt, the capabilities of the wheels may be exceeded and they may come apart. But for the purpose of argument, assume that they will hold up okay.

Yes. The thrust of the plane is forward and the only FORCE acting on the PLANE is the friction of the bearings, which is no where near enough to keep the plane from moving, therefore, the plane accelerates forward.

I didn't think the purpose of the exercise was to determine whether the difference between the capabilities of the conveyor belt and the plane's wheels. Besides, I think you could apply the brakes on a plane and it would still get dragged to take off speed with enough throttle.

The point is that if you really did this, the wheels would be turning about twice as fast as normal.

The tires would not be designed to that great a speed and might easily be shredded before reaching takeoff speed.

quote:

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ummm. it does take off, guys. it's not a car.

plane moves, wheels move x2. plane takes off.

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Correct.

Theory of relativity. The plane takes off. If you still dont believe it from what others have said, let me try to rephrase it a little, but I am going to be saying the exact same thing:

The engine pushes the plane forward creating airspeed relative to the plane. Even though the conveyor belt is moving in the opposite direction the engine still creates airspeed. The engine (prop or jet) provides more thrust than the planes kinetic energy which is what enables it to go faster and faster in the first place. It pushes the plane forward, no matter what the wheels are doing, creating relative airspeed.

[This message has been edited by jack 98 (edited 12/5/2006 10:57a).]

quote:

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The engine pushes the plane forward creating airspeed relative to the plane. Even though the conveyor belt is moving in the opposite direction the engine still creates airspeed. The engine (prop or jet) provides more thrust than the planes kinetic energy which is what enables it to go faster and faster in the first place. It pushes the plane forward, no matter what the wheels are doing, creating relative airspeed.

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Physical education major?

my first dp ever

[This message has been edited by jack 98 (edited 12/5/2006 11:05a).]

Kinesiology major!

As others have said - the plane takes off. What the wheels are doing is irrelevent assuming the friction is negligible. The thrust supplied by the jet or propellor acts against the air surrounding the plane. The wheels are essentially invisible. Planes take off because of the lift supplied by the wings - which requires air to flow. Again - unaffected by the speed of the wheels spinning.

I'd say the conveyor belt blows up before the wheel bearings!

How is air going to move over the wings and provide lift if the plane does not move?

quote:

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the difference between a plane and a car is that the plane pulls itself thru the AIR, a car pushes itself forward from the GROUND. the conveyor belt system is not part of the AIR, just the GROUND.

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quote:

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What the wheels are doing is irrelevent assuming the friction is negligible.

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If the plane is in contact with the ground, it's pulling itself along the ground, just like a car. Therefor it has no forward velocity. The wheels are very relevant. The plane is not flying until the wheels no longer touch the ground. Planes only pull/push themselves thru the air once they no longer touch the ground.

The convery will move in the opposite direction of the plane the instant it starts to roll forward making its forward velocity ZERO.

[This message has been edited by agracer (edited 12/5/2006 11:43a).]

The plane will move forward relative to the ground that the wheels are on. The conveyor is rolling backwards keeping the plane in place relative to the air around it. The assumption for the question was that the conveyor could match the "speed" in the opposite direction. The plane does NOT move, no lift is created. It just makes noise.

Negative. The relevant difference between a car and a plane is that a car makes its forward motion by pushing against the ground. No resistance, no forward motion. A plane pushes (or pulls in a prop plane) against the air. Of course, if you put a car on a treadmill, it will spin in place because it can't push itself forward. A plane pushes against the air, not the ground.

What would happen if you put a plane on the runway and locked the brakes up, then hit the throttle? The plane would move forward, because the fact that the wheels are not spinning is immaterial. Long black strips on the runway, but the plane moves forward. The wheels aren't a part of the plane's motivational system. They do help point it the right direction and the brakes help slow it down when landing, but that's really about it.

[This message has been edited by WHOOP!'91 (edited 12/5/2006 12:11p).]

quote:

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A plane pushes against the air, not the ground.

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Then why have runways? The plane is on the ground and the ground is relevant.

In reality, the question is vague enough that there are two ways to look at it. In once case the plane does not move, in the other it moves at 100mph and the wheels 200.

If a plane is moving thru the air at 100mph and a convery it lands on it moving at 100mph in the opposite direction, does it roll off the convery or remain in place?

the wheels and conveyor can spin asymptotically to infinity; it won't keep the plane from moving forward.

Dig it: a car is on a conveyor that spins as fast as the wheels in the opposite direction. The car will never be able to overcome it. But, someone drives up behind it with an F350 Dualie with a Triton V10 and a long metal pole attached to the front and pushes the car from behind. The car moves forward because there's another force acting on it that the conveyor can't do anything about. The jet engine is the F350.

Oh, and you need runways because the plane does have to move forward to generate air flow and lift to take off. It would do so in the example given because the conveyor can't use the plane's wheels to work against the jet engine.

So you're saying a fan powered car would move nowhere on the conveyor or roll off?

EDIT;

Nevermind.

[This message has been edited by agracer (edited 12/5/2006 1:09p).]

You have to remember that the wheels on an airplane provide no propulsion whatsoever. The propulsion (or thrust) is supplied by the jet engine or propellor.

For this "problem" we assume the wheel bearings have no friction. So the conveyor moving in the opposite direction makes the wheels spin faster and faster, but the jet engine "sees" no difference. The frame of reference for the airplane is the air (assuming it is still), not the surface the wheels are touching.

Consider another scenario - the conveyor belt moves in a direction forward. If the wheels have no friction, and the jet engine is OFF, the plane will not take off because there is no thrust. If the wheels were locked, and the conveyor could reach takeoff speed - the plane would take off, but crash unless the engine was turned on. This is the aircraft carrier catapult scenario.

The plane takes off.

Why?

The plane's engines produce a thrust (whether this be from a prop, turbojet, turbofan, whatever)

This trust creates a forward motion of the plane.

On ground, the plane's forward motion will cause the wheels to rotate because there is friction force acting on the rubber of the wheels and the asphalt of the runway.

In air there is no (or little) rotation in the wheels because of the lack (more accurately very minimal) of friction between wheel and air. There is still forward motion of the plane because thrust of the engine is not dependent on rotation of the wheels. This dependency exists in other vehicles such as cars where the rotation of the wheel determines the "thrust". No rotation, no thrust, no forward motion for these vehicles

All the conveyor is doing thus is canceling the rotation of the wheel (or doubling the rotation depending on which direction the conveyor is moving, in our case it is moving in the opposite direction of the plane, thus doubling the rotation of the wheel). Again note: The wheel on the plane is free to spin as it wants and will never affect the thrust produced by the engine. No force is canceling out the thrust produced by the engine and so the plane achieves a forward motion.

Once this velocity is such that lift created by the plane equals the weight of the plane, it will take off. The wheels will be spinning twice as fast as they would have had there been no conveyor, but the plane is moving just as fast. Once in air, the wheels will eventually slow down to almost no rotation, but the plane will still be flying.

End of story.