hmmm.... physics question for you guys....

[wulfe13]

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 opposite direction).

The question is:

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

[Carbuff]

Re: hmmm.... physics question for you guys....

No, its like running on a treadmill, the plane will go no where. You need air to pass over the wing for lift to occur. Since the plane is not moving there is no air moving over the wing. I dont understand the second part?

[Accept2]

Re: hmmm.... physics question for you guys....

The plane will take off because it has VTOL like a Harrier.........

[Stinkbug]

Re: hmmm.... physics question for you guys....

I agree with Carbuff's statements. No lift will be generated by the wings and so it will not leave the ground. The exeption would be a plane that can do vertical take off like the Harrier. [img]/images/graemlins/grin.gif[/img]

[Carbuff]

Re: hmmm.... physics question for you guys....

Oh, sure, a harrier, trick question!!! [img]/images/graemlins/laugh.gif[/img] I still dont understand the second part of the question??

[VitaminG]

Re: hmmm.... physics question for you guys....

the plane will take off. Since the power of the plane is generated by its jets or props, the conveyer belt will have no effect on the forward thrust of the plane (although I wonder if the drag on the wheels caused by them spinning twice as fast as normal may slow things down a little)

If a plane derived its forward push from its wheels, your dilemma would bear discussion. But then, if planes were wheel driven, how would they accelerate after they lifted off?

[Number Of The Priest]

Re: hmmm.... physics question for you guys....

Sorry VitG, lift is generated according to Bernoulli's Principle, stating that air MUST pass over and under the wings to generate lift. The top of the wing is curved and there is more distance the air must travel in relation to the bottom of the wing. So, the air on top must travel faster to meet the air on the bottom by the end, resulting in lower air pressure on top and higher air pressure relatively on the bottom. The higher air pressure on the bottom pushes the wing (and the plane) up, allowing lift to occur.

Since the plane cannot move forward when on the conveyor belt, no air can pass over and under the wings to even generate lift.

[SeventhSon]

Re: hmmm.... physics question for you guys....

No it won't. It's like trying to take off with a tail wind equal to your ground speed. you have an effective air speed of 0. no lift.

[VitaminG]

Re: hmmm.... physics question for you guys....

I understand how lift is generated.

But explain to me WHY the plane would remain stationery on a conveyor belt? If you put a car on a belt and matched the speed of the belt to the speed of the car, the car would remain in the same spot. The drive of the car's wheels would be offset by the speed of the belt travelling in the opposite direction.

How does that work with a plane, considering that props or jets (not wheels) are what push a plane forward.

[zeegler]

Re: hmmm.... physics question for you guys....

Vitamin G is right. The plane is not propelled by its wheels. The speed of the plane relative to the ground is inconsequential. The speed of the air travelling over the wings is what matters. Let's say you put a giant fan in front of the plane, the plane could actually be stationary relative to the ground, but still take off.

[Newc]

Re: hmmm.... physics question for you guys....

Air flow dictates lift on a standard (non-VTOL) airplane, so no, the plane will not rise. If you put a wind generator in front of it providing enough air flow, it would rise, but then the plane would also move forward and crash into the wind generator.

Carbuff - the "second part" of the question is the same as the first - Will it be able to gain altitude. It's just phrased differently.

Vit - The question assumes that the conveyor will always turn at an equal speed to the thrust of the plane, so the plane cannot "outrun" the conveyor, and the conveyor cannot "outrun" the plane.

[Carbuff]

Re: hmmm.... physics question for you guys....

VG, I see what youre saying now. I was thinking that you were holding the plane in place. Thats why I didnt understand the second part. But if the runway is a normal distance it might crash before taking off. As you said the drag on the wheels might not let it get fast enough to take off before the runway ends.

[Number Of The Priest]

Re: hmmm.... physics question for you guys....

It then becomes a question of relativity. The car travels at +100mph and the conveyor belt goes at -100mph. The net speed of the car is zero and so the car goes nowhere.

Same thing with the plane. The plane's wheels are attached to the plane which holds the engines, does it not? [img]/images/graemlins/wink.gif[/img] The plane thrusts forward with a speed of +300mph and the conveyor belt goes at -300mph. The plane goes nowhere and its wings cannot even slice the air to generate lift.

[VitaminG]

Re: hmmm.... physics question for you guys....

zing! Thanks Zeegs. I figured someone else would get it.

Newc, read Zeegler's post

[Newc]

Re: hmmm.... physics question for you guys....

Yeah, I was typing mine when everyone else was typing theirs [img]/images/graemlins/laugh.gif[/img]

However, as soon as the plane gained lift, it would hit the fan.