Brain Teaser

[Matty Norwood]

jump jet, enough said

[bennymacca]

Ok, i think there has been enough debate on this, so ill give you the answer.

Should have stuck you your first answer garth, the plane WILL be able to take off.

A plane generates its thrust by using propellors/jet turbines/whatever to push against the air. once the plane is moving, air moving over the wings generates lift, and the plane can take off.

in the case of the conveyor belt, the plane's jets push against air, which is still stationary relative to it, and so the jets will work as normal and the plane will take off as normal. the effect on the wheels is that they will spin faster, but will not stop the plane from moving forward.

a planes wheels are only used to reduce friction between the plane and the tarmac, allowing it to take off - nothing to do with the propulsion of the plane, and so spinning the wheels a little faster has no effect on the plane's ability to take off.

this is the main source of confusion with this one. if it was a car on the same conveyor belt, then the car would not move anywhere. but then imagine going up to the car, standing just off the conveyor belt, and giving the car a push. you would be able to do this right? that is because your force is decoupled from the wheels/conveyor setup, in much the same way that the jet pushing on the air is decoupled from setup.

i hope this explains it well enough, no doubt there will still be differint opinions.

my engineering mates and i discussed this once, and most of us came to the conclusion that the plane would take off, but there were still a couple that were adamant that the plane would stay there.

mythbusters did this a while ago, and found that the plane took off as normal, but then mythbusters aren't always right

[David]

a planes wheels are only used to reduce friction between the plane and the tarmac, allowing it to take off - nothing to do with the propulsion of the plane, and so spinning the wheels a little faster has no effect on the plane's ability to take off

Basically what I was trying to say!
The thought of the plane standing still is only true if the wheels are the driving force, which they aren't.

[Garth Kay]

So the point of the myth is that the plane cannot stay stationary on the conveyor belt?

That's why I did state "theoretically". But in your op the conveyor always matches the forward ground speed of the plane.
Hypothetically it remains stationary; it is almost impossible to do but yes it would remain stationary and thus no air over the wings.

Now if the conveyor belt did not completely match the forward ground speed of the plane and we bring friction into the equation, then yes the plane does eventually have forward momentum and it is possible to take off.

In regards to your OP, I could remove the wheels of the plane, sit it on the ground and grab a metal cable, tie it to the tail and then to the other end to some huge building. Start the plane up and go full throttle. Same thing as in your op, would the plane take off??

If it really is this easy for a plane to take off, why don't aircraft carriers have conveyor belts instead of propulsion "steam?" cannons.

[bennymacca]

So the point of the myth is that the plane cannot stay stationary on the conveyor belt?

That's why I did state "theoretically". But in your op the conveyor always matches the forward ground speed of the plane.
Hypothetically it remains stationary; it is almost impossible to do but yes it would remain stationary and thus no air over the wings.

Now if the conveyor belt did not completely match the forward ground speed of the plane and we bring friction into the equation, then yes the plane does eventually have forward momentum and it is possible to take off.

In regards to your OP, I could remove the wheels of the plane, sit it on the ground and grab a metal cable, tie it to the tail and then to the other end to some huge building. Start the plane up and go full throttle. Same thing as in your op, would the plane take off??

garth you are missing the point here

lets make the conveyor move really fast and then put the plane on it.

at this point in time, the wheels are spinning forward to match the conveyor spinning backward, correct? the plane will be stationary.

now, turn the jet engines on.

the jet engines create a thrust on the air, which is stationary relative to the plane, and so the plane will move forward.

this will make the wheels spin faster, but the plane will still move forward, and eventually gain enough speed to take off. the conveyor has no effect on the distance or speed required to take off.

does this make sense?

If it really is this easy for a plane to take off, why don't aircraft carriers have conveyor belts instead of propulsion "steam?" cannons.

the limiting factor in aircfart carriers is the length of the runway - it is not long enough for planes to gain enough speed.

putting a conveyor belt is going to do nothing - the plane on the conveyor takes off as normal, it doesn't take off in any shorter distance.

trust me, this is not a trivial problem and we argued about this for ages before i came up with this reasoning

[David]

But which door is the plane behind?

I don't want no d0nkey.

[Garth Kay]

I'm not denying anything you have stated benny the cunt, but in your op you states the conveyor belt always matches the speed of the plane.

Hence why I said impossible to do. I just read the mythbusters stuff and yes they did get it but I think they missed the point of the myth in this one.
Which I find fascinating, as I love Mythbusters. So hence the clarification; is this Myth about a plane remaining stationary and being able to take off, or about a plane on a treadmill where the treadmill is going take off speed when the plane is on it and the plane throttling up to take off speed.

I think you need to clarify a few points in you op if you want this to get really technical.

[David]

I'm not denying anything you have stated benny the cunt, but in your op you states the conveyor belt always matches the speed of the plane.

Yes, it does.

Hence why I said impossible to do. I just read the mythbusters stuff and yes they did get it but I think they missed the point of the myth in this one.
Which I find fascinating, as I love Mythbusters. So hence the clarification; is this Myth about a plane remaining stationary and being able to take off, or about a plane on a treadmill where the treadmill is going take off speed when the plane is on it and the plane throttling up to take off speed.

I think you need to clarify a few points in you op if you want this to get really technical.

I think most people assume, or imagine, a plane that's dead still because of the negativing factor of the treadmill.

I don't think there's anything to clarify in the OP, speed of treadmill one way, speed plane can travel the other way, what happens?

[muzzington]

<Grows beard and puts on togo>

So anyway.... what if said aircraft builds up maximum speed and then, once attained, starts to 'slow down' at a rate comparable to what it would to take off normarlly whilst the conveyor belt remains at the previous maximum speed of the aircraft.

Would said aircraft then be able to take off backwards? No one said anything about actually flying.

[bennymacca]

I'm not denying anything you have stated benny the cunt, but in your op you states the conveyor belt always matches the speed of the plane.

Hence why I said impossible to do. I just read the mythbusters stuff and yes they did get it but I think they missed the point of the myth in this one.
Which I find fascinating, as I love Mythbusters. So hence the clarification; is this Myth about a plane remaining stationary and being able to take off, or about a plane on a treadmill where the treadmill is going take off speed when the plane is on it and the plane throttling up to take off speed.

I think you need to clarify a few points in you op if you want this to get really technical.

ok, forget about mythbusters thing about the conveyor to go at the takeoff speed of the aircraft, i also think there was some flaws in their arguments.

the salient point here is that it doesnt matter what happens with the conveyor belt. because the plane is stationary with relative to the air, then it will generate thrust against the air.

whatever the conveyor does is decoupled from the motion of the rest of the plane by the fact that the wheel axles are (essentially) frictionless and so the effect of the conveyor is just to spin the wheels, not affect the plane at all.