So, you have a plane on a conveyor belt...

[Kerbart]

How do you propose to increase the velocity of air passing over the lift surfaces without forward movement?

Wheels count.

Forward movement is created "against" the air (propellors, jets, etc) not by the wheels. Wheels don't count in the sense that what happens underneath the wheels is irrelevant.

[KerikBalm]

This whole thing is stupid... I just can't understand why its even an issue for people.

#1) A powered plane's maximum flying speed is significantly greater than its takeoff speed, it would still go forward

#2) The force of air resistance is at least an order of magntiude larger than the wheel friction.... this is true even for a car driving down a highway... a plane... even more so

I can't imagine how people can think about this for any length of time, and not conclude that the aircraft will easily move forward...

[Cydonian Monk]

Of course it'll take off. The propulsion unit is operating against the fluid (air), not against the ground. Besides - if the conveyor was moving fast enough it would eventually cause its own changes to the fluid (air), imparting motion that would help the aircraft.

http://i.kinja-img.com/gawker-media/image/upload/s--4sEe50y_--/1838b145gffppjpg.jpg

No, the frame will clip the wings.

Ha! That's a bit like saying it's faster to drive from Houston to Dallas than it is to take a plane because the plane can't fit through the underpasses on the interstate.

[Bill Phil]

Short answer: Yes, it takes off.

Long answer: Yes, it takes off. Because physics demand it.

[dharak1]

It took me a while to wrap my head around but now I think I get it. The wheels don't even matter because they have a minimal effect on the speed of the plane. It'll move forward no matter the speed of the belt because the speed of the plane comes from the props or jets.

[Randazzo]

I'd like to add this to help people visualize the situation.

http://i.kinja-img.com/gawker-media/image/upload/s--4sEe50y_--/1838b145gffppjpg.jpg

HAH! That is the best thing ever.

This is exactly what I mean. People make all kinds of fundamentally faulty assumptions. Wheels do not count, as they are not powered.

Since this question cannot be answered without assumptions, that is inevitable. You are both right and wrong. While I am tempted to let the trolling carry on, this thread got put in a serious area instead of residing in "games". Really I suppose it is more of an experiment that a game.

The fact is that the question cannot be answered with the information given. You must make assumptions to answer it. People from both camps (though I am not entirely surprised to find the inhabitants of this forum are of a higher grade than the usual bait-takers) instantly make assumptions that suit their desired outcome. It is possible both for the plane to take off (in all likelihood) and it is also possible (theoretically) for it not to take off, given certain conditions. The actual correct answer came up on page two.

Assuming that the engines are able to push the plane at the same speed the conveyor belt is moving (both being a constant), the result is a function of the wheel bearing friction and plane weight.

If the wheel bearing friction is small and the plane is light, you get the results that the Mythbusters got. Increase the weight of the plane and/or the friction coefficient of the wheel bearings, and the belt will start to drag the plane with it, slowing the forward motion. At a certain point, the plane will not be able to accelerate, and therefore to lift off.

You can conduct a simple experiment dis/proving this, just start braking to increase wheel bearing friction and/or add something heavy for additional weight. If you have the belt and the plane, that is.

Did I amuse you enough with my post?

The assertion that the wheels do not matter relies on them being able to spin at speeds exceeding the belt speed. Since they are mechanical devices, they have limits that can be exceeded. Obviously in reality it would be highly unlikely that the plane would be on such a powerful belt (or on a belt at all), and would take off. Regardless, anyone arguing over it has made assumptions that suit their argument.

Except me, of course, I'm just an evil troll.

Edited March 26, 2015 by Randazzo

[Camacha]

Since this question cannot be answered without assumptions, that is inevitable. You are both right and wrong.

You could semantically argue this, but in any other instance it would be false.

[Randazzo]

You could semantically argue this, but in any other instance it would be false.

I ninja-edited with a more worthy explanation.

[Camacha]

I ninja-edited with a more worthy explanation.

I think you said it right. You would need to make assumptions, but most of those assumptions are so logical that not assuming them could be considered a troll. The plane might be under water. Made out of antimatter. Have upside down wings. All true. All irrelevant.

There might be some grey areas that could be discussed, but I have not seen too many of those yet. Most arguments are either based on a flawed understanding of the physics, or intentional frustration of the discussion.

[Superluminaut]

It's just a trick question because we are use to cars.

The plane on the belt moves at take off speed.

The wheels spin at twice the take off speed.

Because the wheels are free spinning, no engine attached to them.

[Camacha]

The plane on the belt moves at take off speed.

Airspeed, not ground speed. Very relevant

[Meecrob]

Are we seriously doing this?

I'll just add this to the mix then

[Camacha]

Are we seriously doing this?

I'll just add this to the mix then

https://youtu.be/pqGhO49xF0Y

Jup

[Iskierka]

Given that the coefficient of friction of a wheel is largely independent of its rotational velocity, there is no velocity at which you could draw the conveyor belt such as to pull the plane back as hard as its engines pulled it forwards, given reasonable assumptions that the brakes are not being applied. Thus, the only case where the belt's own movement directly prevents an aircraft's takeoff is not possible.

On the other hand, there's mechanical limits to how fast those wheels could rotate before breaking off. If you determine those limits, you may find a point at which a conveyor belt would prevent takeoff by breaking the plane - but the belt cannot prevent takeoff simply by pulling a plane back, only by causing physical destruction of required components. Moving the belt does not actually provide noticeably more mechanical resistance than the plane rolling forward under its own power on a runway, so the fringe condition people are suggesting, where the belt is moving fast enough to pull back on the plane, is a non-existant condition.

In fact, the belt would sooner help the plane takeoff, as its movement would pull air along with it, and if it is moving contrary to the attempted takeoff direction, then that is simply increasing air velocity over the wings, meaning a lower ground-relative velocity is needed, even if the conveyor-relative velocity is considerably higher.

[Camacha]

In fact, the belt would sooner help the plane takeoff, as its movement would pull air along with it, and if it is moving contrary to the attempted takeoff direction, then that is simply increasing air velocity over the wings, meaning a lower ground-relative velocity is needed, even if the conveyor-relative velocity is considerably higher.

That is a good approach. Quite a bit of air could be carried along that way, especially at higher speeds.

[Meecrob]

Jup

Crap, I can't even claim to be ninja'd!

Oh well, in before page 732.

Anyone have that .gif of the stick plane attempting this and crashing on the conveyor belt with the caption: "This Thread"?

[pincushionman]

I have to agree with the others saying that the aircraft will take off with a much higher "ground speed" than normal.

With a caveat. As long as the friction force between the ground and the gear (rolling friction for wheels, normal friction for skids) is small compared to the other forces (thrust and drag) in play.

This is usually the case. However, as others have said, there are cases that can be different. Floats or skids on the wrong surfaces, for instance. But if this happens, you're not going anywhere whether it's a conveyor or not.

Wheels also aren't perfect and will have a maximum speed. But you'd need to have one hell of a conveyor to reach it, and it would be less a case of Doesn't Take Off and more Spectacular Failure because it will either be wheels or bearings exploding or the aircraft being bounced out of alignment and tumbling on the ground.

tl;dr: plane takes off, unless Bad Things.

EDIT: Me ninja too. But I can't complain because it took me more than a hour and a half to finish the post (choir rehearsal). But we agreed about what we said, so we must be right.

Right?

Edited March 26, 2015 by pincushionman

[Randazzo]

I wasn't expecting to see people still arguing discussing.

Anyway, I found what I think is a PERFECT analysis for Kerbal. Includes KSP-worthy diagram.

http://blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/

[Camacha]

This is usually the case. However, as others have said, there are cases that can be different. Floats or skids on the wrong surfaces, for instance. But if this happens, you're not going anywhere whether it's a conveyor or not.

Case in point:

[Meecrob]

Signed,

Me

Certified Flight Instructor and owner of two airplanes.

Q: How do you figure out if there are any pilots in the room?

A: Wait for them to tell you.

-Signed-

A pilot who just played into his own joke. It's ok, I'm also an A&P so technically I'm exempt.

[Deutherius]

The actual correct answer came up on page two.

Assuming you mean my comment, that would be page one, not two

[Randazzo]

Assuming you mean my comment, that would be page one, not two

Well played, Sir.

[magnemoe]

Case in point:

Don't do this, yes I have done it in KSP, was supposed to turn of brakes but raised wheels instead, luckily I was not moving.

One on Youtube had an good explanation, outside of KSP planes has an sensor who disabling raising gears while its weight on them.

Some pilots use this to flip the gear up switch before takeoff so they redraw at takeoff, mostly to show off but also to don't have to do it during liftoff.

Here the sensor was not sensitive enough so the wheels redrew before the plane was in the air, it had some weight on the wheels still, or the plane bumped up in the air before going down again and this was enough for the wheel redraw to happen.

[Camacha]

One on Youtube had an good explanation, outside of KSP planes has an sensor who disabling raising gears while its weight on them.

Some pilots use this to flip the gear up switch before takeoff so they redraw at takeoff, mostly to show off but also to don't have to do it during liftoff.

Here the sensor was not sensitive enough so the wheels redrew before the plane was in the air, it had some weight on the wheels still, or the plane bumped up in the air before going down again and this was enough for the wheel redraw to happen.

I know for a fact that a lot of aircraft do not have these kinds of sensors, and maybe even very few or none at all have them. If other people have more on this, I would love to hear it. Most information I am able to dig up about this pertains sensors to monitor load and structural damage, not whether the craft is airborne or not.

In any case, these kinds of sensors sound like a complicated way of dealing with an otherwise simple problem.

https://www.youtube.com/watch?v=edJxjPxYByM

Edited March 26, 2015 by Camacha

[Leszek]

If after all of this discussion there is still someone out there unable to understand why the plane flies just fine, consider:

If the plane was just sitting there, with the engine off, parking break off, and you started the treadmill, the plane will not move. Well not at first. Eventually bearing friction will make it move but ignore that for now. That treadmill will speed up and the plane will just sit there with its wheels spinning.

Planes do not push off the ground to get speed. They push off the air. The engine doesn't care how fast the wheels are spinning.

The wings only care about airflow. Once the engine pulls the plane forward it is only a matter of throttle.