Mythbusters to take on: PLANE ON A TREADMILL!!

[Kycntryboy]

NERDS

I think the guy with the plane he was going to try to keep it in one spot, so it he adjusts the power thats almost as good as instantaneous. I'm guessing along the same power adjustments with flying close forms.

 

[CWJones411]

I thought the point of the conveyor belt was that it would move at the same speed but opposite to the wheels so the plane stays in the same place. Is this it or did I read it wrong?

 

[nugget81]

Just because the airplane's wheels are turning does not mean that it is moving through a mass of air. Does the airplane's TAS increase, ever? NO. Flying is achieved by lift overcoming weight, not thrust overcoming drag.

 

[raptor10]

Everybody, put your hands together for me, like you're praying. Now move your right hand away from your body, and your left hand towards your body... did your hands move? Yes, because the "thrust" provided by your right arm was enough to overcome the friction between your hands...

 

[CaptainRon]

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[FUPaladin]

If the conveyor isn't motorized, and is free-wheeling and frictionless, and moves rearward in conjunction with the speed of the tires (not the aircraft), then you won't take off.

This case is purely a physics problem, because it couldn't remotely happen in real life. If the conveyor always matches the speed of the wheels, then even the tiniest amount of thrust makes the tires speed up to infinity, and since there's no time in that equation, it happens in an instant. Like you said, it has to be the first case.

I thought the point of the conveyor belt was that it would move at the same speed but opposite to the wheels so the plane stays in the same place. Is this it or did I read it wrong?

In order for the plane to stay stationary in this case (relative to the tower), all it needs is enough power to overcome the friction in it's bearings. That means just a little bit of thrust to overcome static friction then even less thrust to hold the plane stationary against its rolling friction. If you put a car on a treadmill moving 100 mph backwards, the car would have to accelerate to 100 mph to stay stationary. But a plane, with free-spinning wheels, just needs enough power to counter the amount of friction that 100 mph puts on its wheel bearings. That's why if the plane actually goes to full power for a takeoff, the effect of the conveyor if negligible and the plane takes off virtually normally.

 

[phrogpilot73]

Just because the airplane's wheels are turning does not mean that it is moving through a mass of air. Does the airplane's TAS increase, ever? NO. Flying is achieved by lift overcoming weight, not thrust overcoming drag.

Correct. However, because the conveyor belt is moving in the opposite direction at the same speed of the AIRPLANE, not the wheels, than it's TAS will increase.

Think about it this way. Thrust from prop/jet begins. Conveyor is stopped (because the airplane's forward velocity is zero), thrust overcomes friction, starts rolling aircraft forward. Conveyor belt starts and matches aircraft speed, but it is now accelerating and moving forward. If the aircraft is not moving forward through the air mass, than the conveyor is not moving.

 

[squeeze]

Just because the airplane's wheels are turning does not mean that it is moving through a mass of air. Does the airplane's TAS increase, ever? NO. Flying is achieved by lift overcoming weight, not thrust overcoming drag.

Refer to my previous post about "stupid." Arguing that it won't fly, then trying to phrase a suitable argument to support that pushes through the wrong-stupid threshold.

 

[HokiePilot]

Just because the airplane's wheels are turning does not mean that it is moving through a mass of air. Does the airplane's TAS increase, ever? NO. Flying is achieved by lift overcoming weight, not thrust overcoming drag.

Why doesn't the TAS ever change? The propeller exerts a force relative to the airmass pulling the plane forward. The only force holding the plane back is the rolling friction. We easily overcome that everyday. The plane will move forward through the air and takeoff.

Break

After rewatching the clip I believe that they may not actually answer the question. The pilot not using all available power to get airborne and instead trying to stay stationary with the ground is not the question. The question is can the conveyor keep the plane from accelerating if the plane is pilot is trying to go flying.

Break

I sure wouldn't want to be the pilot the first time that they try this. Could you imagine the wind catching the end of the tarp and pulling it up into the propeller or the tires instantaneously changing speeds as they came off the end of the tarp and possibly skidding. Bad Times

 

[BigIron]

...... But as a physics problem, the plane takes off just like it would on a normal runway.

and then falls out of the sky because it is going 6 knots through the air.

 

[CWJones411]

...so it would be impossible for a conveyor belt going any speed to hold the plane in the same place?

 

[xj220]

Friction in the wheel bearings is going to really decide what happens. If you have high enough friction, then the plane won't take off since it will hinder the aircraft's ability to roll. However, if there is little friction then the aircraft can easily overcome any resistance and accelerate up to speed. Here's a way to think of things:
A car would not go anywhere since it's motive force is the wheels itself and needs to directly overcome the conveyor (which it can't). However, an aircraft's motive force is independent of the ground and only needs to push off the surrounding air mass (and any extra friction associated with the wheel bearings and tires).

 

[HokiePilot]

...so it would be impossible for a conveyor belt going any speed to hold the plane in the same place?

As long as the tires don't blow, YES. Rolling friction does not increase with an increase in wheel speed.

 

[bert]

Normally I would take the high road and stay out of this, but the Calc professor in me (which leans me towards calc-based physics) can't resist:

This case is purely a physics problem, because it couldn't remotely happen in real life. If the conveyor always matches the speed of the wheels, then even the tiniest amount of thrust makes the tires speed up to infinity, and since there's no time in that equation, it happens in an instant. Like you said, it has to be the first case.

You are correct that it couldn't happen in real life. It got ugly after that. Make a graph of what you just described (always liked making students do that...) and you'll see why.

Leave it to the phrog driver to get it right earlier: you guys are arguing two different problems and anybody who applies the solution of one to the other is going to be wrong. You can define the problem to make either answer correct. And if that doesn't make somebody happy, you can just change the assumptions.

 

[scoober78]

Game, set and match...Scoober takes the point.