A jet plane on a large treadmill

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Originally Posted by focus

This is still WRONG. This will only be true if the treadmill is hooked up to the car's speedometer. If the treadmill is hooked up the FORWARD SPEED OF THE CAR then the car will still move, albeit at half the speed registered on the speedometer.

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[Ed McMahon] You are correct sir! [/Ed McMahon]

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Originally Posted by The AD

It was never stated in the problem which speed the treadmill is reacting to.

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Yes it did

"This conveyor has a control system that tracks the plane's speed" As in, the physical tube of aluminum with wings and shit that you climb into.

copy&paste the orginal question into Word and search for "wheel" or "wheelspeed" Let me know how it turns out.

The only resistive force that the treadmill will place on the plane is rolling friction from the plane's free-spinning wheels which is so little that it can be assumed as zero (Don't believe me? Just think about how long it would take the same plane to come to a complete stop after a normal landing with no brakes and no reverse thrust). Secondly, the treadmill will not accelerate towards infinity, only up to and including the velocity of the plane. It will spin the wheels twice as fast as normal and that's it.

The treadmill is irrelevant, there is no spoon, end of fucking debate.

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Originally Posted by The AD

which is utterly and completey wrong. Ever heard of wind resistance or drag? Do you believe that is dependent on velocity? Guess what, that's friction.

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rolling friction vs. wind resitance = apples vs. oranges

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Originally Posted by The AD

It was never stated in the problem which speed the treadmill is reacting to.

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That is correct, however, you cannot switch frames of reference while solving a problem. Keep the treadmill speed equal to airspeed and you get an answer without violating any laws of physics or mathematics.

AD and all other non-fly arguers, please fill in the following so we can see your viewpoint

speed of plane relative to air = ???
speed of the plane's tire (car speedometer) = ???
treadmillspeed = ???

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Originally Posted by Dantheman

rolling friction vs. wind resitance = apples vs. oranges

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or more to the point: Fluid friction (drag is an example) is velocity dependent. Surface friction (what I am talking about) not velocity dependent.

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Originally Posted by DJSapp

ok

Incorrect, the treadmill counters by moving backward at an equal rate. The airplane's wheels do not transfer the treadmills speed to the aircraft. The aircraft continues moving forward relative to the air.

But then the wheels would never begin moving, and the treadmill would never start.

Airspeed = 1 to begin moving
conveyor = 1 to match
Wheelspeed = airspeed + conveyorspeed

wheelspeed = 1 + 1 = 2

according to your argument:

wheelspeed = conveyorspeed

2 does not equal 1

Your argument is incorrect.

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Ah, but the wheel can spin without having the axle move. :)

I should have said:
"At the same moment any plane forward movement (speed) starts, the treadmill counters by rolling backwards at the enough speed to keep the plane stationary. "

Your math neglected polarity (+/-)
Plane speed = 1, conveyor speed = -1, so plane speed + conveyor speed = 0 (not 2)

I still hold that the wheel's rotation (and tread ) speed will match the conveyors speed, but the wheel's axel (and the rest of the plane) will not change from zero.


In your example, the plane's thrust has overcome the counteracting forces from the treadmill, BUT the problem places no limits on the treadmill at all.
The problem does constrain the plane's limits by stating it's a 747 which has a fixed maximum thrust & knowable friction in the wheels.
This makes the conveyor an "unstoppable force" acting to counter a stoppable object with limits to the forces it can exert.



That said,
My argument for conveyor drag induced airflow getting to be high enough so that the plane's airspeed gets to be high enough for lift and take off still stands. The plane will take off, not by moving forward on the belt, but by the belt moving the air in the opposite direction. :cool:

And then it will stall and crash as soon as it leaves the slipstream. :frown:

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Originally Posted by TomK

That said,
My argument for conveyor drag induced airflow getting to be high enough so that the plane's airspeed gets to be high enough for lift and take off still stands. The plane will take off, not by moving forward on the belt, but by the belt moving the air in the opposite direction. :cool:

And then it will stall and crash as soon as it leaves the slipstream. :frown:

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Yes, that's possible, but the conveyor would have to be 50 miles long to build up a boundary layer big enough such that the airplane's wings are within in.:FIREdevil

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Originally Posted by TomK

In your example, the plane's thrust has overcome the counteracting forces from the treadmill, BUT the problem places no limits on the treadmill at all.
The problem does constrain the plane's limits by stating it's a 747 which has a fixed maximum thrust & knowable friction in the wheels.
This makes the conveyor an "unstoppable force" acting to counter a stoppable object with limits to the forces it can exert.

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There is a definate limit on the treadmill. All it can do is go faster as the plane goes faster. This does nothing to increase the frictional forces counteracting the jet's engines.

Holy shit. 10 pages and still at it. In a weirdly nerdy alternate Universe I'm prolly impressed as hell.

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Originally Posted by TomK

Ah, but the wheel can spin without having the axle move.

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No. The axle is connected to the airplane, and hence, airspeed.

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I should have said:
"At the same moment any plane forward movement (speed) starts, the treadmill counters by rolling backwards at the enough speed to keep the plane stationary. "

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That is not what the question stated. The treadmill matches the planes speed.

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Your math neglected polarity (+/-)
Plane speed = 1, conveyor speed = -1, so plane speed + conveyor speed = 0 (not 2)

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Ok, you caught me in a tecbnical error, but my answer is still correct.

Wheel speed is the difference between the object the wheel is attached to (airplane) and the surface it is contacting (treadmill), unless the wheels are not rolling (i.e. skidding)

Airspeed - Conveyor Speed = Wheel Speed

1 - -1 = 2

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I still hold that the wheel's rotation (and tread ) speed will match the conveyors speed, but the wheel's axel (and the rest of the plane) will not change from zero.

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If the axle does not move, how does the wheel begin to rotate? No power is transmitted to airplane wheels, they are free spinning.

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In your example, the plane's thrust has overcome the counteracting forces from the treadmill, BUT the problem places no limits on the treadmill at all.
The problem does constrain the plane's limits by stating it's a 747 which has a fixed maximum thrust & knowable friction in the wheels.
This makes the conveyor an "unstoppable force" acting to counter a stoppable object with limits to the forces it can exert.

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You are switching frames of reference. You begin by setting treadmill speed equal to airspeed, once the plane moves, the treadmill speed becomes equal to wheel speed.

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That said,
My argument for conveyor drag induced airflow getting to be high enough so that the plane's airspeed gets to be high enough for lift and take off still stands. The plane will take off, not by moving forward on the belt, but by the belt moving the air in the opposite direction. :cool:

And then it will stall and crash as soon as it leaves the slipstream. :frown:

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Sure, given a belt long and wide enough.

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Originally Posted by The AD

Yes, that's possible, but the conveyor would have to be 50 miles long to build up a boundary layer big enough such that the airplane's wings are within in.:FIREdevil

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Don't you dare bring practicality into it. The treadmill has NO limits. :D

Beaver and all you guys saying the plane will take off is dumb. Jet engines provide thrust, not lift... The air moving under the wing creates lift....and if air is just being sucked through the engine how is there air moving under the wings...HOW!!???

Dont you think the military would just create giant treadmills for their planes if this were possible...no instaed they would spend millions trying to design a vertical lift jet.

i just cant see how you morons would say it would take off....A plane needs air moving under its wings to take off, if its essentiely just sitting on a treadmill the air around the jet is not moving....just the ground and wheels. How do you fools not get that? hahah simple thought

If the plane is essentially sitting still relative to the runway around it all that is happening is the wheels and treadmill are spinning...The planes wings arent actually moving by the air any faster....I mean a plane requires air to be moving under the wings to create lift correct? Where are you getting this moving air from?? The treadmill is keeping the plane still relative to the surrounding air no matter how much air is forced through the jet engines.

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Originally Posted by altaski

Beaver and all you guys saying the plane will take off is dumb. Jet engines provide thrust, not lift... The air moving under the wing creates lift....and if air is just being sucked through the engine how is there air moving under the wings...HOW!!???

Dont you think the military would just create giant treadmills for their planes if this were possible...no instaed they would spend millions trying to design a vertical lift jet.

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Keep reading the thread. Everybody and their dog knows that a plane needs airflow to create lift. Nobody is saying otherwise. People who say it will take off, believe that the plane will move forward regardless of the treadmill thus creating airflow.

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Originally Posted by dbp

There is a definate limit on the treadmill. All it can do is go faster as the plane goes faster. This does nothing to increase the frictional forces counteracting the jet's engines.

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There is no limit on how fast the treadmill can go, the limit to the "speed" is how much force the 747 can muster. Since the "speed of the treadmill" is always 100% of the "speed of the airframe", as soon as the plane tries to move forward, the treadmill accelerates to match.
Nothing I said talks about increasing the friction, that stays constant (albeit very low, but NOT zero).
This means that the treadmill will be constantly accelerating untill the force being transfered to the plane counteracts the plane''s forces trying to push it forward. Otherwise, the plane's speed would overcome the counteracting forces that the treadmill would induce and therefor be unable to match the speed of the plane. This would violate the question.

I am as yet unconvinced that the plane's groundspeed would change from 0.



Oh snap! - in coming up with the drag induced airflow angle, I neglected to consider that air is a fluid not a solid. Since it IS a fluid, the air below the wings (closer to the treadmill) will always be higher speed than the air above the wings. So, only negative lift.

Back to no takeoff.

this reminds me of nomograms. some fire behavior analyst will spend hours, weeks, untold amounts of money to figure out how a hillside's going to burn. Cranked out rednecks on contract crew arrive and come to the same conclusion on first sight.

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Originally Posted by TomK

Nothing I said talks about increasing the friction, that stays constant (albeit very low, but NOT zero).
This means that the treadmill will be constantly accelerating untill the force being transfered to the plane counteracts the plane''s forces trying to push it forward.

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What kind of forces are being transfered to the plane by the treadmill? I see forces acting on the free-spinning wheels. The engine pushing on air above all this could care less how fast the treadmill moves or tires rotate. Again this is not a car.

Ok, lets approach this another way....

You've got a plane already flying.

How much does it not having it's landing gear on the ground effect it's ability to accelerate by increasing thrust?

Answer: none. though air density and friction from the air is different from a plane on the ground.

Can the plane accelerate using the same mechanism (jet engines) on a stationary runway?

Answer: Yes, that's how it works normally.

Assuming that the friction of the wheels rotating on the runway at any speed is negligible compared to the thrust provided by the jets. Does the engine's ability to provide thrust change significantly?

Answer: No, the jets privide thrust to accelerate the plane by the same mechanism. The thrust has not changed.


If the plane is accelrating based on it's jets alone, and the wheels are not providing a significant amount of retarding force due to friction, does the speed of the surface that the plane is resting upon make a difference in the plane's ability to accelerate.

Answer: if there is no significant retarding force due to the wheels on the conveyor (and there isn't because they're rotating nearly freely), then any force not absorbed by friction in the wheels, and air friction, is applied towards acceleration of the plane.

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Originally Posted by TomK

There is no limit on how fast the treadmill can go, the limit to the "speed" is how much force the 747 can muster. Since the "speed of the treadmill" is always 100% of the "speed of the airframe", as soon as the plane tries to move forward, the treadmill accelerates to match.

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YES!!! GODYES!!!! HE CAN BE TAUGHT!!!

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This means that the treadmill will be constantly accelerating untill the force being transfered to the plane counteracts the plane''s forces trying to push it forward. Otherwise, the plane's speed would overcome the counteracting forces that the treadmill would induce and therefor be unable to match the speed of the plane. This would violate the question.

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And he throws it!!

The treadmill would be accelerating to match the speed of the airframe. For the treadmill to counteract the planes thrust, the treadmill would need to move much much much faster than the airframe, violating the rule treadmill speed = airframe speed.

Altaski: take some physics classes. Learn Newtonian motion. Then debate.

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Originally Posted by dbp

What kind of forces are being transfered to the plane by the treadmill? I see forces acting on the free-spinning wheels. The engine pushing on air above all this could care less how fast the treadmill moves or tires rotate. Again this is not a car.

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I don't see how "not a car" factors into this. We know force is tranferred between the plane and the ground because the tires do spin, freely or otherwise.

What would happen if the plane served only peanuts, and not pretzels for snacks? Would that have any bearing on the problem?

Unrelated and stupid, but have any of you guys seen Flight of the Phoenix?

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Originally Posted by The AD

I don't see how "not a car" factors into this. We know force is tranferred between the plane and the ground because the tires do spin, freely or otherwise.

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BZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZT

We have a wrong answer.

A car has an engine. That engine turns wheels. The wheels push the ground. The car goes forward. If the ground is moving at exactly the same speed backwards as the wheels rotation would dictate, the car goes nowhere.

A plane has engines. That engine turns fanblades, The fan blades push the air. the plane goes forwards. Because the wheels on the plane only match the net speed differential between the plane and the ground and have no propulsive effect on the plane the plane can accelerate feely.


And another way of looking at it....
Consider the landinggear have very low friction with the ground. No engines and no brakes involved in the landing gear. the plane would roll down hill if it weren't a level runway.

Now, consider a hover craft. It's floating on a cushion of air that behaves identically to the landing gear at least in one axis.

Now, put that hover craft on the same treadmill system that the plane will take off from. Can the hovercraft accelerate forward?

Yes, because the ground under it has only as much impact on it's accleration as the air cushion transmits back to it.

I still think it won't fly.

But what happens if we try to land our 747 on the tread mill with its magic speed matching apparatus? A complete dead stop with wheels rotating or does it keep moving off the conveyor?

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Originally Posted by DJSapp

YES!!! GODYES!!!! HE CAN BE TAUGHT!!!



And he throws it!!

The treadmill would be accelerating to match the speed of the airframe. For the treadmill to counteract the planes thrust, the treadmill would need to move much much much faster than the airframe, violating the rule treadmill speed = airframe speed.
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It's a bullshit question I think we all can agree.
In real life, the force from the jet engines would easily overcome any resistance and allow the plane to accelerate off the treadmill, with the wheel tread speed being much higher than either groundspeed or airspeed.


Is this a theoretical exersise, or is there a real problem to be solved?

"In theory, there is no difference between theory and practical application, but in practice, there is." :biggrin: