Here is the problem:

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 planes speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?

Think about this for a minute.

Source

The Straight Dope Answer

O_o, There's no air passing over the wings, so there's no lift.. It doesn't matter what speed the plane is moving. If it hasn't got any lift it's a vehicle, and not an aircraft.

Did you check the Answer, alot of people seem to disagree with it as well

Yes - the planes engines put the thrust to the air, not the ground - the wheels just hold the plane up.

The plane will accelerate as normal (although sligthly slower due to wheel friction) and take off fine.

This is one of those riddles I can't fathom how anyone can get wrong, but so many still do. Mythbusters actually featured this thing, had the plane take off like expected, and STILL had people denying a plane on a conveyor belt can take off.

Glad to see Jakg has some common sense too. Give us a proper riddle! Like.... Where does god live?

Think of it this way, if you were riding a bycicle on a treadmill, apart from looking like an utter pillock, you wouldn't move - as fast as you went the treadmill would match you.

Imagine someone is NEXT to you giving you a push while you freewheel on your bike. You'd move, right?

Same thing applies to a plane.

(I've seen threads hundreds of pages long with proofs posted without a resolution appearing!)

It depends if you are referring to the wheels traveling at the exact opposite velocity as the conveyor or the plane traveling at the exact opposite velocity as the conveyor.

"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)"

(I am referring to it as a treadmill here)

If you are talking about the wheels, then they would be rolling forward at 5mph while the treadmill goes in reverse at 5mph. (The plane doesn't move)

If you are talking about the plane moving forward at 5 mph while the treadmill goes in reverse at 5mph, than the plane would eventually run off the end of a limited treadmill. This would mean the wheels were actually spinning at 10mph.

If the first is true, than there would be no lift, if the second where true (at higher speeds of course), than there would be lift.

Here we go!

You can't measure the plane's speed relative to the conveyor. Relative to the conveyor the conveyor itself will stand still and as such can't go in the opposite direction of the plane as presented in the riddle.

So you measure relative to the ground the conveyor is sitting on. The plane will be moving forward at speed X and the conveyor is moving backward at speed X. The wheels will move at speed 2X relative to the conveyor. If speed X is sufficient to give lift, the plane will take off. Q.E.D.

its a badly worded question..

if you think about it.. the plane "CAN" take off assuming that the planes engines are on and throttled up for take off...."

although i only realised how blind i was after i read jakg's answer.

forget about the conveyor travelling at the same speed of the plane. the conveyor is only interested in the wheels, not the engines.

Another flawed riddle. You missed pasting the fact that the conveyor belt is moving in the opposite direction matching the rotational speed of the wheels.

Search for it on RCGroups.com, an RC flight forum. The discussion has been going on for years. A search will give you thousands of threads started on this topic. There is even threads created asking "How many airplane/conveyor threads do we need..." And Mythbusters did not prove it because they didn't match the wheel speed, only the plane speed.

The first thread on RCGroups that I saw: Something to think about.... will it fly? Started on Nov. 28, 2005 and was closed on Aug. 2, 2006 after 2825 posts.

The 2nd thread started after the first one was closed: Something to think about.... will it fly? Part II Started on August 2, 2006 and after 6744 posts, the final one was only back on Feb. 17, 2008.

Of course, a lot of that was off-topic arguing. In searching the RCGroups forum, I found 27 of the 38 listed threads that was directly relevant to the "plane on a conveyor" riddle. Within those 27 threads are 12,250 posts.

Let's not make this forum turn out like that one.

Quote from mrodgers :

You missed pasting the fact that the conveyor belt is moving in the opposite direction matching the rotational speed of the wheels.

That's a logical impossibility as long as the plane moves relative to the ground (which it will once the engine is running), the conveyor's speed is measured relative to the gound and the wheels are on the conveyor. Only when the plane is standing still (relative to the ground) can this condition be fulfilled, and a such the plane obviously won't take off.

This is all theoretical bull though. In real life the conveyor and the wheels have no bearing on the plane's movement whatsoever (ignoring rolling friction) and all the flawed preconditions in the world won't stop the plane from taking to the air with the wheels happily spinning faster than the conveyor is moving relative to the ground.

Seems a tricky question but easy if you make concepts clear. First think of why airplanes fly.

So if the conveyor makes the plane stationary relative to the ground then there will be no lifting cause air is not flowing and so no forces aplly on the wings

I've watched the mythbusters episode and they didn't prove anything because the plane actually did move relative to the ground (and relative to the air that flows and lift on the wings ). If you look at their car and conveyor example, the car does not move relative to the observers. A plane in that situation will not lift off ever

Ah, very clever. I had to think twice about this one (30 seconds of thought), but you're right, it can take off!!! Nice one

It won't take off if it doesn't move. No movement means no help of air to lift the wings. And it is not possible to make an experiment like this because the plane will push itself by moving the air backwards, not pushing the ground backwards like cars.So the conveyor will do no effect holding the plane still... But take it on a different look, if the all runway was a conveyor the plane will take off like normal but maybe with a little bit more effort because of the extra friction of tyres.

The plane moves mate, generates an airspeed, thus lift, thus takeoff.

Think some more. It's REALLY obvious.

yep i know i just mean it won't lift if it doesn't move evet if the engines running full throttle or tyres spinning on 200 mph

Quote from RocksGt :

If you look at their car and conveyor example, the car does not move relative to the observers. A plane in that situation will not lift off ever

A car wouldn't move as it puts the force to the ground via wheels - a plane WOULD move as the engine exerts force on the air, not the ground.

The plane would move, but the wheels would move twice as fast because of the conveyer belt. You could reverse the conveyer belt, so the wheels don't move at all, and it would still take off.

Hint: Ignore wheel speed, as it is not important on a plane.

Quote from spurri :

This conveyer has a control system that tracks the planes speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

Quote from Jakg :

A car wouldn't move as it puts the force to the ground via wheels - a plane WOULD move as the engine exerts force on the air, not the ground.

So I understand the plane should be stationary for an external observer

No, the plane moves completely as normal relative to the observer, the control tower, and the Earth, but moves twice as fast relative to the conveyer belt

It is not possible to give a definite answer without more details about the setup. If made with some assumptions then it would take off.

Planes' forward force (lift) comes from the engine pushing against surrounding air.
Planes' upward force (lift) comes from wings passing through the same air.
Planes' rearward force (drag) comes from air resistance and the friction of wheel bearings on the runway.

The engine pulls the plane forward, as its' thrust is far greater than drag.
The wheels contribute to the drag when it's on the ground - doubly so because they're turning twice as fast as normal,
since the runway matches the plane's speed in the negative direction -
but they're on bearings, so it's still a tiny amount, much less than air resistance as the plane's speed builds.
(Note: The original problem does not factor in wheel bearing resistance)

As the plane's forward speed builds (thrust > drag = acceleration),
the wings move through the air faster, and lift increases,
and eventually equals and surpasses the weight of the aircraft.

Takeoff.


Every forum this damn thing pops up.

What details Kegetys?

Quote from tristancliffe :

No, the plane moves completely as normal relative to the observer, the control tower, and the Earth, but moves twice as fast relative to the conveyer belt

Hmmm... maybe I'm misunderstoonding something

If the conveyor matches the plane speed but in opposite direction the plane would remain stationary for an external observer. Of course the engines push the plane cause of the air that they move, not by delivering power to the wheels, but so long the wheels are in contact with the conveyor the plane will not move forward if both (the wheels and the conveyor) have the same speed in opposite directions. So if there are not forward movement there will not be air flow to lift on the wings .

If the plane moves forward (for an external observer, as you can see in the mythbusters video) then it must be one of those:
1.- The wheels have more speed than the conveyor
2.- The wheels are spinning or sliding over the conveyor