[LawnGnome]

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). Can the plane take off?


Problem I came across a while back. I will tell people the correct anwser in a few days.

If you have done this problem before, don't ruin it for others.

 

[GSurge]

Is it correct to assume that the plane, from the perspective of an observer standing a few feet away, is not moving?

 

[colonelkernel8]

No. There is no air moving over the wings. There is no lift.

Edit after the braincells kicked in:
Though upon further consideration, the plane could indeed move, because the wheels are in no way given any "drive" and are free rolling. So thrust would be able to move the plane forward regardless of how fast the wheels were spinning on the conveyor. The answer is undoubtably, YES. What is my prize?

 

[GlendaleViper]

I would like to usurp The Colonel's first answer.

 

[Davesrose]

OK, I'll play along...even though this question is all over the internet and is a red herring because of all the "correct answers"

I vote yes, it does take off because lift is created by friction of the engine's thrust of the air over the wing. A plane has wheels to alleviate friction on the ground. Now starting up, the plane won't go forward because the conveyer belt is matching the plane's forward velocity. However, when the engines have enough thrust, they are driving air over the wings....thereby creating lift. Think of it this way....an aircraft carrier throws a jet airplane off its deck because there's not enough distance for the airplane to go forward and generate thrust on its own. The plane doesn't suddenly fall out of the sky or start to decelerate because the wheels have stopped moving and it's now in still air. No, it keeps right on...as it's engines are pushing the air along.

 

[marvin]

Quote:

Originally Posted by colonelkernel8 /img/forum/go_quote.gif Edit after the braincells kicked in: Though upon further consideration, the plane could indeed move, because the wheels are in no way given any "drive" and are free rolling. So thrust would be able to move the plane forward regardless of how fast the wheels were spinning on the conveyor. The answer is undoubtably, YES. What is my prize?

Heh.

Yep. The wheels of the plane would be spinning twice as fast as they would be under normal circumstances.

Also, the air dragged along by the conveyor belt contraption would increase the airspeed over the wings, thereby increasing lift while on or near the conveyor belt. By how much, well, who knows. Modeling would work. Course, the amount of lift added by this effect would precipitously drop after the plane rises above the artificial wind created by the conveyor belt... This may make things a bit dicy for the pilot if they did not take this into account and plan accordingly.

 

[Happy Camper]

Quote:

Originally Posted by Davesrose /img/forum/go_quote.gif OK, I'll play along...even though this question is all over the internet and is a red herring because of all the "correct answers" I vote yes, it does take off because lift is created by friction of the engine's thrust of the air over the wing. A plane has wheels to alleviate friction on the ground. Now starting up, the plane won't go forward because the conveyer belt is matching the plane's forward velocity. However, when the engines have enough thrust, they are driving air over the wings....thereby creating lift. Think of it this way....an aircraft carrier throws a jet airplane off its deck because there's not enough distance for the airplane to go forward and generate thrust on its own. The plane doesn't suddenly fall out of the sky or start to decelerate because the wheels have stopped moving and it's now in still air. No, it keeps right on...as it's engines are pushing the air along.

The engine does not create the air movement over the wing. The thrust moves the body through the air until the speed creates the difference in pressure to force lift on the bottom of the wing. Carriers have to gain speed into the wind to generate easier lift. It is alot harder for a plane to take off on a windless day. They can though because of the thrust of the catapult in addition to the engine. I have seen planes drop on initial departure from the deck. The plane's speed will eventually gain lift.

I do agree with the final assessment though.

 

[GlendaleViper]

Quote:

Originally Posted by marvin /img/forum/go_quote.gif Also, the air dragged along by the conveyor belt contraption would increase the airspeed over the wings, thereby increasing lift while on or near the conveyor belt. By how much, well, who knows. Modeling would work. Course, the amount of lift added by this effect would precipitously drop after the plane rises above the artificial wind created by the conveyor belt... This may make things a bit dicy for the pilot if they did not take this into account and plan accordingly.

Ah, see, you're thinking quite outside of the box! I thought the same thing initially, but I questioned how much usable air the belt would actually provide the wings.

As far as the jets providing enough thrust/wind on their own, is that really true? Or, at least, enough to lift a plane? Since the wheels and conveyor are perfectly mated, it effectively negates any forward momentum and therefore lift, no?

Might be an interesting and exorbitantly expensive motorsport, though. The 747 Rodeo. Harness that conveyor wind like Marvin mentioned and see if you can ride it for 8 seconds.

 

[omegaman]

If the plane was a Harrier jump jet then it would take off

otherwise No !!!!!!!!!!

 

[LawnGnome]

Quote:

Originally Posted by omegaman /img/forum/go_quote.gif If the plane was a Harrier jump jet then it would take off otherwise No !!!!!!!!!!

Elaborate.

 

[GlendaleViper]

Oh, don't tell me it was such a simple trick!

 

[3x331m]

Air speed is irrelevant to Conveyor speed. As long as the plane has air speed to gain enough lift, it would take of, regardless of the conveyor/runway speed.

If the conveyor can move in such speed and direction to cancel all air speed of the plane, then we have a very nice duck on the runway...

VTOL like Harrier is cheating. The jet propulsion is "perpendicular" to the conveyor.

 

[marvin]

Quote:

Originally Posted by GlendaleViper /img/forum/go_quote.gif Ah, see, you're thinking quite outside of the box! I thought the same thing initially, but I questioned how much usable air the belt would actually provide the wings. Since the wheels and conveyor are perfectly mated, it effectively negates any forward momentum and therefore lift, no?

Well, depends on the width of the belt. If it was just big enough to cover the wheel span, then no, not much wind to the wings at all. If it was as long and wide as a typical airstrip, that's a lot of air it's moving...

Re: Wheels/Conveyor mating

Doesn't matter. Airplane wheels are pretty much free spinning. The wheels aren't driving the plane, the thrust from the engines are, so all the conveyor belt will do is spin the wheels.

 

[bellsprout]

my god not this again

yes the plane will take off

if ur convinced it won't, think again until u change ur mind

 

[GlendaleViper]

Oh, I recognize that. That's what I was trying to say is that there would be no forward movement at all. The wheels in this case act as a bearing of sorts.