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SD, the driven wheels would result in an airspeed of 0 and relative to the ground of 0 as well because the conveyor would have you standing still; literally and metaphorically spinning your wheels. Plus, when you did kick in the jet engine, the fixed gear ratios of your transmission would allow the conveyor to continue to control your forward motion.

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Don't complicate this any further. I only introduced the attached jet engine to further show that the wheels can only provide thrust when the car in on the ground. And like I said, take off would occur in discrete intervals... wheels first to takeoff air speed and then the jet engines kick in. If you put this contraption on the equal but opposite treadmill, it WILL just spin its wheels. That's the point. That's the case in which it WON'T take off.

But we don't have car airplanes and this is an absurd exception, since the wheels on a normal plane provide ZERO thrust. That's the point of the whole discussion.

[This message has been edited by SuperDave03 (edited 12/6/2006 10:37a).]

Oh, so your car-plane isn't on a conveyor? I missed that part. Curve ball, strike three!

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Oh, so your car-plane isn't on a conveyor?

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Only when it works.

If the velocity is measured from an observer not on the conveyor then of course the plane takes off.

If the velocity is measured by an observer on the conveyor then a problem arises in the formulation. You are trying to apply a speed constraint and a force constraint at the same time which doesn’t work. Consider just the velocity constraint first. It says that the axle velocity relative to the conveyor is equal to the conveyor velocity. This would require the wheel to stay in one place as observed by someone not on the conveyor.

Imagine standing next to the conveyor holding the axle of a tire as the conveyor rotates. You are holding the axle and tire in one position as it rotates. The velocity of the axle equals the velocity of the conveyor. The force it takes you to hold the tire in one place is equal to the friction between the tire and the ground. As you can imagine, this force wouldn’t be that great and it wouldn’t change as the conveyor speeds up. The only way for a plane to meet this requirement is to provide just enough thrust to equal friction. In other words the plane would not be at full throttle. Once you go to full throttle then the thrust force easily overcomes the friction force and the plane takes off. But you are violating the velocity constraint.

To summarize, the plane could take off but not under the velocity constraint.

THE PLANE WILL TAKE OFF! Now can we quit talking about this?

seriously. some of you people embarrass me.

hey, some concepts are hard to grasp for some people.....

*cough*business majors*cough*

wasnt one of the first posts wondering how this got to so many posts on the general board?

Half of you are right. The plane will take off.

Let me try to rewrite the premise so that the other half of you are right.

A plane is standing on a movable runway(something like a conveyor). As the plane's engines apply thrust, the conveyor moves at a velocity equal to the angular velocity of the plane's wheels in a direction such that the center of the wheel does not move with respect to the ground (or any point not on the conveyor).

Now the way I see this is that the net velocity of the plane with respect to the air is zero. Thus no lift even though thrust is present. Am I correct?

[This message has been edited by Big_Russ (edited 12/7/2006 1:51p).]

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Let me try to rewrite the premise so that the other half of you are right.

A plane is standing on a movable runway(something like a conveyor). As the plane's engines apply thrust, the conveyor moves at a velocity equal to the angular velocity of the plane's wheels in a direction such that the center of the wheel does not move with respect to the ground (or any point not on the conveyor).

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You are holding a 5 ton weight with your right hand such that the 5 ton weight is suspended 5,000 feet above the ground. You let lose and the weight does not fall. Will it ever hit the ground?

Hey! It makes just as much sense as what you wrote.

OK. Let me rephrase then. A plane is on a conveyor. The pilot in the plane has a goal of keeping the plane absolutely still with respect to the control tower, which is not on the conveyor.

The conveyor begins to spin such that the plane moves away from the control tower. The pilot applies thrust to compensate so that the plane remains still. Is there ever a velocity of the conveyor such that when the pilot applies thrust to compensate, lift is generated and the plane flies?

No. There isn't, because the thrust is keeping the plane in place, not propelling it towards the tower.

There is a difference between the speed at which the wheels are spinning and the velocity of the plane. THAT is what those who are arguing that the plane will not take off are missing.

And your rope argument is still a mystery to me. Letting go of the rope is the equivalent of taking away the thrust from the plane. Totally irrelevant to what I was getting at.


[This message has been edited by Big_Russ (edited 12/7/2006 2:18p).]

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are we all computer nerds and completely devoid of aeronautical or mechanical engineers here? Somebody with a MS ME identify yourself and tell us the answer already!

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I think we're all either 1) studying our hearts out, or 2) just too ashamed that Aggies can't figure this out to want to interject.

To understand the effects that the conveyor would have, you have to do some basic dynamics calculations. This is because of all the coefficients of friction that are involved (between the rubber and the belt, as well as between the wheel hub and the wheel axle). Barring some Martian technology with a friction coefficient of like, 20, there is no way that the conveyor belt can create enough force to counteract that of the engine. Because F=ma, the greater force produces the greater acceleration.

It would actually be interesting to see if I could derive the equations of motion for ya'll, but that'll have to wait until Saturday at the earliest, seeing as I have this odd desire to pass and eventually graduate. But yes, the plane flies...just without wheels (would be a really cool dissintigration of the wheels, though).

Big Russ - you're missing an essential element. The thrust supplied by the engine acts against the air. Since the conveyor does not cause a change in the air surrounding the plane, the conveyor does essentially nothing to counteract the engine thrust.

If the thrust were being supplied by Superman generating thrust by blowing air from his lungs, and he were blindfolded (so he couldn't see the conveyor), he would experience almost no difference because of the conveyor belt. Maybe he would think there was a bit more rolling resistance than usual, but the plane still takes off.

Yes, Spice, I know. What you are neglecting is that the engines are only concerned with the air going through them. The engines do NOT force air across the wings.

Engines supply thrust. They do not supply lift. Different animals.

Edit: To more specifically get to your point I think, if the engines are OFF and the belt rolls backwards, there will be air flow across the plane from the back. At that point, you could adjust engine thrust so that the plane is rolling forward at a velocity so that there is no air flow from the back. At this point, the plane's wheels should be spinning at the same rate as the conveyor. Therefore no net velocity. No lift.

[This message has been edited by Big_Russ (edited 12/7/2006 2:51p).]

Of course, this is all assuming there is friction. No friction on the axle, then the plane stays in place when the conveyer moves backwards and your Superman example is correct since the only force acting on the plane is the thrust of his breath.

So for the clarity of my argument, we are not in a frictionless world.

If you still say it is irrelevant then you are saying that it is IMPOSSIBLE to generate JUST ENOUGH thrust to keep the plane in place (i.e. enough thrust to equal the friction force of the axle). Bunk.

Big_russ - I understand the role of the engine in a plane. They supply thrust (by accerating air flow). The lift is generated by the wings (Bernoulli) caused by plane's relative movement through the air.

The conveyor does nothing to change this except generate what is likely to be negligible friction by making the tires spin. The only reason there is any backward force from the conveyor is friction in the wheel bearings and the tire-to-conveyor ocntact. There is plenty of evidence that friction is small because a landing plane slows down with air brakes and reverse thrust, not wheel friction.

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If you still say it is irrelevant then you are saying that it is IMPOSSIBLE to generate JUST ENOUGH thrust to keep the plane in place (i.e. enough thrust to equal the friction force of the axle).

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Actually, that is far too trivial and obvious to worry about.

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The conveyor does nothing to change this except generate what is likely to be negligible friction by making the tires spin. The only reason there is any backward force from the conveyor is friction in the wheel bearings and the tire-to-conveyor ocntact. There is plenty of evidence that friction is small because a landing plane slows down with air brakes and reverse thrust, not wheel friction.

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Dismissing friction ignores my point. Regardless of how small the force is, it is still there.

Again, are you saying that it is IMPOSSIBLE to generate JUST ENOUGH thrust to keep the plane in place? This is very important.

If you admit that you can generate JUST ENOUGH thrust to keep the plane in place, then given that the friction force is a continuous and smooth function of speed, then it is possible to keep the plane in the exact same place regardless of conveyor speed due from the thrust alone.

If you contend that it is IMPOSSIBLE to generate JUST ENOUGH thrust to keep the plane stationary, then you are saying that the frictional force is NOT a continuous or smooth function of speed.

Maybe if you put the plane on the moon where there is no air and replace the engines with rockets it will be easier for you to see this. Burn the rockets just enough to keep up with the wheel speed on the conveyor. Here it is apparent since there is no air, then the air is IRRELEVANT to the discussion. Thrust is NOT a force against the AIR. Thrust is a force against the PLANE.

[This message has been edited by Big_Russ (edited 12/7/2006 4:35p).]

One more thing. I would like to correct one of my previous statements. I said earlier that in a frictionless world you Superman example would be correct and the plane would take off. Actually, in a frictionless world there would be no lift because there would be no pressure drop across the top of the wing. The plane would not take off.

Lift is due to friction?

All this time I thought it was due to the higher speed of the air over the top of the wing compared to the air under the wing.

I am too used to working with fluids in pipes where no friction = no pressure loss. I was always under the impression that lift was due to a difference in pressures. Pressure drop in pipe world is due to friction. I looked at it through a flight point of view and see it is only a function of volecity. I will grant you that, but I would still like some input from an aerospace person that friction and delta P play no part in lift.

You still haven't addressed my question and points, though Eric. I would appreciate it if you would answer the question in my post.

Is it IMPOSSIBLE to generate JUST ENOUGH thrust to keep the plane in place?

[This message has been edited by Big_Russ (edited 12/7/2006 5:51p).]

You don't really have rheology in a frictionless world.

The plane will take off...

For all you guys thinking the plane won't take off draw me up a free body diagram and SHOW what force is present to counteract the thrust from the engine.

People get caught up with the part of the question stating the speed of the wheels & conveyor being the same. Without a force to counteract the thrust you will get acceleration of the plane, regardless of the wheel speed and conveyor speed. Just because those speeds are equal, does not mean acceleration is zero!

[This message has been edited by Tormentos (edited 12/7/2006 5:53p).]

And what you are getting caught up on is the PRACTICAL assumption that thrust >> friction.

Will someone PLEASE answer my question?

I am not making the practical assumption that thrust>>friction, that statement makes absolutely no sense.

I am making the assumption that engine thrust>>all other forces acting on the plane (drag, bearing friction in wheels, contact friction, etc). I think that is a fairly valid assumption to make.

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You still haven't addressed my question and points, though Eric. I would appreciate it if you would answer the question in my post.

Is it IMPOSSIBLE to generate JUST ENOUGH thrust to keep the plane in place?

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The answer is so intuitively obvious that no answer should be required.

But since you insist, provided that you have that fine of control over the engine, it is trivial to see that it is possible.

That assumes, of course, that no matter how small the friction may be, the thrust from an idling engine would not exceed the amount necessary to keep it in place.

So what is your point?

[This message has been edited by eric76 (edited 12/7/2006 6:24p).]

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The answer is so intuitively obvious that no answer should be required.

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I've got a question. Why do you have to be a dick?

The friction caused by the axles of the plane's wheels is not a function of the planes velocity. It is a function of the normal force of the plane on the wheel axles and the coefficient of friction. So the friction in the axles is a constant force and it is very small compared to the thrust exerted by the plane at take off.

SOOOOO, the plane takes off.

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The answer is so intuitively obvious that no answer should be required.

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I've got a question. Why do you have to be a dick?

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The answer is so intuitively obvious that no answer should be required.

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...and as they both sink beneath the waves, the frog cries out, "Why did you sting me, Mr. Scorpion? For now we both will drown!" Scorpion replies, "I can't help it. It's in my nature!"

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I concur: it would be very easy for the jet engine to counter the trivial amount of force in the negative direction by the friction in the wheels. That's the reason it can take off; the engine can do a lot more than counter that trivial force.

Thank you, Eric, for agreeing that you can supply JUST ENOUGH thrust to keep the plane in place.

Now, please follow.

1. We are at a point where the plane is stationary, but the wheels are spinning. No net velocity through the air.

2. We increase the speed of the conveyor and the pilot adjusts the thrust JUST ENOUGH so that the plane is STILL stationary.

3. Go to 2.

Please, now, tell me at what point the net velocity is positive.

The answer is when the force from the thrust is greater than that of the axle. However, it is conceivable though perhaps not PRACTICAL that you could build a conveyor that could accelerate its speed to infinity, and as long as you have an engine that could produce infinite thrust, the pilot could ALWAYS apply JUST ENOUGH thrust to keep the plane stationary.

So if you can concede that this is indeed how it would happen given the problem constraints, why couldn't a conveyor be built to do just the opposite, i.e. keep the plane in place no matter the thrust?

Again, for all you other engineers, this problem is ASININE and for all PRACTICAL purposes, the plane will fly. I am not, however, discussing practicality.

[This message has been edited by Big_Russ (edited 12/7/2006 7:15p).]

What you are talking about assumes that as the conveyor belt increases its speed, the friction force increases in the axles which causes the pilot to have to increase the thrust to keep the plane stationary.

This does not happen because the friction in the axles is NOT a function of the speed that the wheels are turning. The friction force is constant and it is certainly too small to counter-act the thrust from the plane when it is taking off.

I don't understand why you are arguing about this because it is really not going anywhere.

OneManArmy,

Good point, and you may have answered my question. At that point you have overcome the friction any additional forward force will propel the plane instead of keeping it on the conveyor.

And to think you raised that without being a dick.

Correct, once that small force is overcome, the plane will continue to move forward.