Can the Plane Take-Off?

[Madison]

Originally Posted by sciguy125

Ah, I now see the error in my thinking..

Let's say we drop a missile from an airplane. Better yet, from a helicopter that is hovering in place. The missile will still go forward even if it has no contact with the ground. So, the ground has nothing to do with the airplane's ability to take off.

Or, to mix metaphors, strap the missile to Elgin's dyno. The missile isn't going to make the dyno spin.

[Arkorobotics]

NO! This is not possible because the speed of the trake and wheels (which usually are not powered, it's the engine that pulls the plane if I am not wrong. But lets go with the example) will cancel out and the airplane will stay in place. Airplanes have airfoils, where high pressure builds on the bottom and low pressure on the top as air passes by. This creates lift, but how can the airplane lift if there is no air moving across the wings? The answer is it can't! In this sitution it can't.

[EricH]

As many people have said, there is no connection between engines and wheels in an airplane. The treadmill could be going backward at lightspeed, but if that plane's engine(s) is (are) going full blast forwards, or close to it, then the plane will move forward, thus getting the lift it needs. Wheel drag will not be enough to stop the plane from taking off unless the pilot is standing on the brakes, in which case he wouldn't take off anyway.

Now, people have used an F-18 as an example, but I'll try a Harrier for an example. Harrier won't go forward at all, but it will take off in its VTOL manner (that's Vertical TakeOff and Landing) and take off. No question about that, right? (Yeah, I'm cheating a bit, but if it's any airplane, I'll just go with a VTOL and take off without the debate. )

[greencactus3]

Originally Posted by EricH

Now, people have used an F-18 as an example, but I'll try a Harrier for an example. Harrier won't go forward at all, but it will take off in its VTOL manner (that's Vertical TakeOff and Landing) and take off. No question about that, right? (Yeah, I'm cheating a bit, but if it's any airplane, I'll just go with a VTOL and take off without the debate. )

The plane moves in one direction, while the conveyer moves in the opposite direction.

vtol is not allowed. the plane's thrust is angled down so the conveyor belt can't move in the opposite direction anymore. illegal move

[EricH]

Originally Posted by greencactus3

vtol is not allowed. the plane's thrust is angled down so the conveyor belt can't move in the opposite direction anymore. illegal move

I was thinking outside the box. And if the conveyor belt isn't supported well enough, it will move--downwards--until it can't stretch anymore--and then it won't move anymore.

If someone has a powered model airplane with landing gear (improvised or included), they should put it on a treadmill (in a big open area), run both, take a video, and post it here. I think that's the only sure way to answer the question.

[Mike Schroeder]

That wont work, you will never ever get the type of precision that you would need to get a computer to say okay plane is moving 60mph make conveyor move 60mph even a nano-second delay means that the plane will move forward, and with a treadmill that makes things that much more difficult because there is no way to get close to the syncronization needed for this to work

[KenWittlief]

Originally Posted by EricH

If someone has a powered model airplane with landing gear (improvised or included), they should put it on a treadmill (in a big open area), run both, take a video, and post it here. I think that's the only sure way to answer the question.

It would be an interesting test. With the plane at a fixed throttle position you could measure the airspeed of the plane, with the treadmill still, and with it running, and see what effect it has on the planes airspeed.

It doesnt match the original problem/question, but you shoud be able to calculate the force the moving threadmill is applying to the plane (by the amount its airspeed is reduced) and extrapolate from there.

I think the answer you will get will be the same we have been saying since post #2 on page 1. The plane will take slightly longer to lift off than normal.

[fishgurl]

Yes.

[RP Robotics]

The plane could take off. Planes do not put any force into the wheels when it taxis. It moves the same way it does in the air, prop or jet. Due to inertia you could say the plane would stay in the same position relative a third stationary observer. the conveyer belt could move back and forth rapidly and the plane would still stay still. All the wheels do is just keep the plane from falling and give it support when it has 0 lift. If you turn on the prop the plane would move foward, it doesnt matter at all what the conveyer belt does, the only force it can put on the plane is up. If you had a converyer belt long enought he plane would take off for it would be moving compared to the 3rd stationary oberserver. The only way it would not work is if the conveyer belt was not long enough or there was a strong tail wind. but a plane can not take off without a conveyer belt in thos conditions.

[DonRotolo]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

The question is:

Will the plane take off or not? Will it be able to run up and take off?"

Originally Posted by RP Robotics

The plane could take off.

The only way it would not work is if the conveyer belt was not long enough or there was a strong tail wind. but a plane can not take off without a conveyer belt in thos conditions.

Here we go again.
The key to this puzzle is the statement in the initial conditions "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)."
As soon as the thrust of the engine pushes the plane forward some small distance (thus it has some speed), the conveyer moves at whatever speed necessary to counteract that speed. It can move at any speed!

So, the force on the plane from the belt is limited by the friction of the wheel bearings. Move that belt at a high enough speed (a fraction of lightspeed, for example) and the force transmitted to the aircraft through the freely rolling wheels will be enough to counteract the force of the engine.

By definition, the plane cannot move because the belt is free to move at any speed, and while the rolling wheels don't transmit a lot of force to the airframe, the force is nonzero and therefore you must account for it.

Oh, in a real, practical world, you are correct: The plane will move before the belt can reach 0.1c. But, that wasn't the puzzle question...

Don

[EricH]

Originally Posted by Don Rotolo

Here we go again.
The key to this puzzle is the statement in the initial conditions "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)."
As soon as the thrust of the engine pushes the plane forward some small distance (thus it has some speed), the conveyer moves at whatever speed necessary to counteract that speed. It can move at any speed!

Ummm... Don, I don't see how you can say that the belt can move at any speed.
The condition says that the speed of the conveyor is (effectively) the negative of the speed of the airplane. It does not say that the conveyor goes fast enough to stop the plane with friction. It does not say that the conveyor is attached to the wings or tail (which would be necessary to stop the plane from taking off unless the brakes were on).

All it says is that the conveyor moves at the same speed as the plane in the opposite direction.

That said, guess who's going to be watching Mythbusters next Wednesday if at all possible?

[wfoard]

I know that I might bnot seem reliable because this is my first post, but I wish to dispel this fear by mentioning that I have seen this done in real life, not on Mythbusters. First, the airplane did take off. For all of you people who talking about lift, yes the lift comes from the wings moving through the air. As far as the airplane moving in relation to the ground, it does. The reason for this is that the thrust comes from the engines (whether they be old-fashioned propellor or modern turbine) and has nothing to do with the wheels whatsoever. This means that basically the wheels can spin freely. Because of this, the treadmill is basically nonexistant. If you do not believe me, put a model airpland on a big treadmill and notice that it does take off. I have personally seen it done many times.

[Protronie]

Originally Posted by sanddrag

As mentioned previously, a plane's flight is based on it's relation to the air, not to the ground. Thrust comes from engines pushing on the air, not from wheels pushing on the runway.

Yes but... the thrust from the prop pulls the plane forward so air rushes over the wings to produce lift.
Would the prop-wash alone be enough to produce the required air moving over the wings flight surface in order to produce the lift needed?

If the plane is on a belt moving under it at the same speed as the plane then the air above and under the wing would depend on the prop wash.

[EricH]

Originally Posted by Protronie

Yes but... the thrust from the prop pulls the plane forward so air rushes over the wings to produce lift.
Would the prop-wash alone be enough to produce the required air moving over the wings flight surface in order to produce the lift needed?

If the plane is on a belt moving under it at the same speed as the plane then the air above and under the wing would depend on the prop wash.

You aren't getting it. The wheels are NOT repeat NOT connected to the engine and prop. Not in any way, shape, or form (save the mounts to the fuselage).

Think of it this way. You put your car into neutral on a flat surface. It won't move, right? Now, do that on a hill (conveyor belt). It moves, right? The wheels are free-spinning when the car is in neutral and the brakes are off. Same for an airplane, except that there isn't a drive gear.

Prop wash (air pushed by prop) has nothing to do with it. It's whether the plane can move forward on the ground. That is determined by the thrust. The thrust is how strongly the prop pushes on the air. Now, the thrust only has to overcome air resistance (definitely a bit of this) and friction from the wheels (assumed to be negligible) to get the air flowing over the whole wing. If the plane can move forward (thrust > air resistance, or drag), then the lift can come into play against gravity. Because the wheels are free spinning, they can't resist forward motion, even with a conveyor belt. So the plane moves forward, lift comes into play, and the plane takes off. I don't care what speed the conveyor belt is moving. Unless it's moving fast enough to cause a wheel to seize up, the plane will take off.

[Wayne Doenges]

Mythbusters proved that the plane can take off. Nuff said, end the thread