think about this...

[LAGmaniac]

[ QUOTE ]
(puff)............................... (exhale)

If it were possible for a plane to take off from a conveyor type runway, how come one doesn't exist?

[/ QUOTE ]

Because there is no intrinsic benefit to lift off imparted by the conveyor belt.

[elwoodblues]

[ QUOTE ]
Patrick, all I want to hear from you is an explanation of how the plane will move forward with respect to an observor GIVEN that the wheel speed is the same as the runway speed with respect to an observor.

[/ QUOTE ]

The only thing I can think of is that the wheel has to skid to some degree.

[ QUOTE ]
OK, let's put the plane at the end of the runway at rest, just like it normally is. However, we're on a magic runway that can move at whatever speed necessary to meet the condition of OP. So the runway moves at the same speed as the wheels. What part of the wheels? Well, the obvious point is the contact point of the wheels, but I say to hell with that. Let's say the whole [censored] wheel can't have any velocity relative to the runway. So, we fire up the engines. The enines propel air backwards and the plane forwards. But, holy [censored]! That means the wheels are moving! We gotta get that runway going! So the runway accelerates too, and the wheels remain motionless. Mean while, the plane is still accelerating. Holy [censored]! We can't have the wheels moving! Heap some more magic dust into the runway motor so it speeds up! And the wheels don't move. And now the plane speeds up again. Holy [censored]! But this time, the plane has reached 88 miles an hour (relative to the air traffic control tower) and goes back in time. Wait til [censored] OOT hears about this!

[/ QUOTE ]

I fully understand what you are saying. However, think about this-- I mean really think it over.

It is possible to apply a force via the wheels and have the wheels move at the same speed as the runway. (think of getting a smog check and how they test your car)

It is not theoretically possible to apply a force from the jet engine and have the wheels move at the same speed as the runway. The wheels will speed up, no MATTER how fast you make the runway, the wheels will move at that speed plus more, thanks to the force from the jet.

[ QUOTE ]
I'm no physics expert, but I think I have an example that might satisfy Nasty/Crazy/goofball.

Imagine that there is no friction at all involved with the wheels and the conveyor belt. The wheels and the belt are moving at the same rate, but this is not necessarily because of friction.

Imagine that the conveyor belt and the wheels are both completely stationary. You expect the wheels to spin faster as the plane moves forward because of the effects of FRICTION BETWEEN the wheels and the conveyor belt. If there is no friction, then the wheels will remain stationary - regardless of the speed of the plane (and, incidentally, of the conveyor belt).

This is a specific hypothetical situation in which the plane clearly accelerates under the conditions outlined in the OP (there is no suggestion that the identical speeds of the wheels/belt are due to friction).

I'm not 100% sure how this situation works if the speed of the runway and wheels scale linearly due to friction, but I don't think it's relevant. Even if we imagine the speed of the wheels and the speed of the conveyor belt to be completely independent (they just happen to be identical), the principle of the solution stands.

[/ QUOTE ]

We are assuming "no slip" between the wheels and the runway. If you are stating that a plane on skis on ice will take off, I fully agree.

[Voltron87]

[The_Bends]

Heh, I just posted this on another forum and the car crash is starting all over again.

[Voltron87]

it got a little crowded, so i just ignored the vector forces and the free body diagram. that stuff is soooo simple anyway. the important stuff is there. the thing in my hand is either the olympic torch, an ice cream cone, or a giant blunt.

[Patrick del Poker Grande]

[ QUOTE ]
[ QUOTE ]
OK, let's put the plane at the end of the runway at rest, just like it normally is. However, we're on a magic runway that can move at whatever speed necessary to meet the condition of OP. So the runway moves at the same speed as the wheels. What part of the wheels? Well, the obvious point is the contact point of the wheels, but I say to hell with that. Let's say the whole [censored] wheel can't have any velocity relative to the runway. So, we fire up the engines. The enines propel air backwards and the plane forwards. But, holy [censored]! That means the wheels are moving! We gotta get that runway going! So the runway accelerates too, and the wheels remain motionless. Mean while, the plane is still accelerating. Holy [censored]! We can't have the wheels moving! Heap some more magic dust into the runway motor so it speeds up! And the wheels don't move. And now the plane speeds up again. Holy [censored]! But this time, the plane has reached 88 miles an hour (relative to the air traffic control tower) and goes back in time. Wait til [censored] OOT hears about this!

[/ QUOTE ]

I fully understand what you are saying. However, think about this-- I mean really think it over.

It is possible to apply a force via the wheels and have the wheels move at the same speed as the runway. (think of getting a smog check and how they test your car)

It is not theoretically possible to apply a force from the jet engine and have the wheels move at the same speed as the runway. The wheels will speed up, no MATTER how fast you make the runway, the wheels will move at that speed plus more, thanks to the force from the jet.

[/ QUOTE ]
THIS ISN'T A [censored] CAR! The jet engines have no connection to the wheels whatsoever. They do not drive the wheels, the wheels just spin. The jets provide a thrust force. That's it. There's no driveshaft down to the wheels. The thing that spins the wheels is friction with the ground and relative motion between the plane and the runway. How [censored] stupid are you?

[SomethingClever]

Well, this assumes the plane is in neutral.

And the pilot didn't put the e-brake on.

[LAGmaniac]

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
I'm sad.

I saw this thread a few hours ago, and thought it would be a 10 reply thread, then die in obscurity. I'm saddened greatly that there can be any sort of debate about this.

It's WINDSPEED that enables lift. If the plane isn't moving relative to the wind, it ain't going up.

As a very correlate aside, planes take off into the wind. They also land into the wind.

So saddened.

Josh

[/ QUOTE ]

I thought it was that simple when I first looked at it but it really is not. Nobody in this thread disputes that you need air flowing over the wings (forward movement) to create lift, (thank god).

The issues are whether the forward movement of the plane is prohibited by the conveyor belt, the original conditions of the problem, or neither.

[/ QUOTE ]

Doesn't:

[ QUOTE ]

The runway moves in the opposite direction of the plane at the exact same speed as the plane's wheels.


[/ QUOTE ]
mean no net movement? Same speed, opposite direction.

Bah, nm. I guess I could read the thread if I wanted to see what was causing the confusion.

So sad [img]/images/graemlins/frown.gif[/img]

[/ QUOTE ]

Well that's the crux of the problem, the obvious answer is that since the wheels spin freely, the wheels' movement have no effect on the plane, like wearing roller skates on a treadmill. Therefore the conveyor belt can't stop the plane from moving forward and taking off.

But if you get into the semantics of the problem, the condition that the wheels' speed = belt speed implies that the wheel can't move forward with respect to the runway without violating that condition.