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DPChallenge Forums >> General Discussion >> The Plane on the Conveyor Belt
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09/16/2011 08:26:07 AM · #1
I apologise in advance for even bringing this up, and I hope it's not a duplicate thread, but the 'Plane on the Conveyor Belt' problem reared it's ugly head in conversation last night.

I know the answer to this now, but if you haven't heard it before, it'll mess with your head for a couple of days before you come to the realisation... Anyway, here it is:

A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

Can the plane take off?
09/16/2011 08:33:12 AM · #2
nope. It's the airspeed that's the issue, and the plane isn't moving relative to the air.
09/16/2011 08:35:08 AM · #3
No lift with no forward movement. Hey, my photography's on a conveyor belt!
09/16/2011 08:38:12 AM · #4
They tackled this question on Mythbusters and end result is that the plane could take off (not very stably though)
09/16/2011 08:43:57 AM · #5
Originally posted by gcoulson:

They tackled this question on Mythbusters and end result is that the plane could take off (not very stably though)

How!?? Did the conveyor belt movement create enough airflow to generate some lift?
09/16/2011 08:44:41 AM · #6
Ah! Nice to see at least two people are about to go through the same pain I went through... :)

Actually, the plane does take off. Both in theory and in practise ('the-plane-doesn't-take-off' diehards often complain about the problems with the Mythbusters experiment)

I'll draw out your agony by not putting forward my proof just yet...
09/16/2011 08:49:27 AM · #7
Ah, right. Got it. Well it's a trick question, and a silly one. Everyone knows it's not the wheels that propel the plane, but the engine/props.

I took the conveyor belt aspect to mean that the plane was fixed in place. To be honest I thought the question was just poorly written, but apparently that was the entire gimmick.
09/16/2011 09:08:56 AM · #8
Originally posted by bohemka:

Ah, right. Got it. Well it's a trick question, and a silly one. Everyone knows it's not the wheels that propel the plane, but the engine/props.

That's right. But let's assume jet engines for the moment. From the 'jet engines' point of view, the plane is being propelled forward - the ground (conveyer) is moving along under the plane at 100mph or whatever. So relative to the ground, the plane is moving at 100mph.

Originally posted by bohemka:

I took the conveyor belt aspect to mean that the plane was fixed in place.

But doesn't the conveyor belt fix it in place? - If I was standing on a conveyor belt and moved forward at 2mph, and the conveyor belt moved backwards at 2mph, then I'll be left back where I started, no matter how fast I try to outrun it, it will always match my speed and ensure I stay in my starting position.

(Playing devil's advocate here, if anyone wants to google it, you'll see the pages and pages of forums threads that have been debating this problem since 2006... )
09/16/2011 09:09:02 AM · #9
Actually, I think there is an old (very old) thread here somewhere where this is debated. Any third-year mechanical engineering student can answer this. The fact that the plane is on the conveyor makes absolutely no difference. As 21.gif bohemka points out, the plane is not driven through the wheels, but by the thrust of the (prop or jet) engines. Off she goes!

09/16/2011 09:12:39 AM · #10
Originally posted by kirbic:

Actually, I think there is an old (very old) thread here somewhere where this is debated.

Yes, I vaguely remember it, but I was searching for ages before starting this thread and couldn't find it.

Every so often I hear it brought up again, and the debate starts. My mission is to convert everyone to the 'plane-will-take-off' camp!
09/16/2011 09:21:20 AM · #11
The plane will take off. The conveyor belt will spin the wheels while the engines propel the plane forward.

Message edited by author 2011-09-16 09:23:15.
09/16/2011 09:23:45 AM · #12
If you run on a treadmill at 12mph, do you feel the 12mph breeze in your face? Nah. No airspeed. No ground speed. No cooling action.
09/16/2011 09:24:49 AM · #13
Originally posted by kirbic:

Actually, I think there is an old (very old) thread here somewhere where this is debated. Any third-year mechanical engineering student can answer this. The fact that the plane is on the conveyor makes absolutely no difference. As 21.gif bohemka points out, the plane is not driven through the wheels, but by the thrust of the (prop or jet) engines. Off she goes!


Heh, of course, the real fun question is can you take off with zero airspeed.. enough engine and you could, but damn those first few seconds would be spooky!

Message edited by author 2011-09-16 09:26:48.
09/16/2011 09:25:20 AM · #14
Related question: If you drive a Corvair on a treadmill, are you safe?
09/16/2011 09:26:45 AM · #15
Originally posted by kirbic:

the plane is not driven through the wheels, but by the thrust of the (prop or jet) engines. Off she goes!

The thrust drives it forward, but it's purpose is to move it through the air to provide lift.
09/16/2011 09:28:44 AM · #16
OK I'll destroy my reputation as a practicing aerospace engineer :) Imagine you're in a wheel chair on the conveyor belt and your holding a rope, the rope is being pulled by someone in front of you but not on the conveyor. No matter how fast the conveyor moves your still going to move forwards towards the person since the wheels aren't part of what is moving you and the rope is getting shorter. The thrust from engines on the airplane are the same as that rope.

In fact it is physically impossible under this situation for the conveyor belt to match the speed of the wheels as they will always spin as fast as the conveyor moves them plus the forward movement. Worst case is the wheels disintegrate as the conveyor continually speeds up trying to match the speed of the wheels!

Originally posted by JH:



Originally posted by bohemka:

I took the conveyor belt aspect to mean that the plane was fixed in place.

But doesn't the conveyor belt fix it in place? - If I was standing on a conveyor belt and moved forward at 2mph, and the conveyor belt moved backwards at 2mph, then I'll be left back where I started, no matter how fast I try to outrun it, it will always match my speed and ensure I stay in my starting position.
09/16/2011 09:36:50 AM · #17
Found the video online. I don't think the experiment actually matched the description. The plane did have forward motion to create lift. The whole idea of this was that the conveyor belt was to have cancelled out all the forward motion. It was a poor effort to duplicate the situation, IMO.
09/16/2011 09:41:50 AM · #18
OK now that that is solved, how about a new one:

Imagine you are sitting in a boat swimming in a pool. You have got a big rock in your hand. When you throw the rock into the pool what happens to the water level of the pool?
09/16/2011 09:53:34 AM · #19
That's the problem with the question, it is physically impossible for the conveyor to match the speed of the wheels. You can't match the question with a real world experiment.

Originally posted by Yo_Spiff:

Found the video online. I don't think the experiment actually matched the description. The plane did have forward motion to create lift. The whole idea of this was that the conveyor belt was to have cancelled out all the forward motion. It was a poor effort to duplicate the situation, IMO.
09/16/2011 09:57:47 AM · #20
What's old is new again...

Message edited by author 2011-09-16 09:58:21.
09/16/2011 09:59:40 AM · #21
Originally posted by eqsite:

What's old is new again...

Ah! Thank you! 'treadmill' - no wonder I couldn't find it.

Still, it's nice to introduce a new generation to the problem... :)
09/16/2011 10:00:37 AM · #22
Originally posted by millsa:

That's the problem with the question, it is physically impossible for the conveyor to match the speed of the wheels. You can't match the question with a real world experiment.


That's not the problem with the question. The trick to the question is that the propulsion of the plane is independent of anything that is going on on the ground, and that being the case, the plane can generate enough speed to generate lift.

Edit to fix quote mishap.

Message edited by author 2011-09-16 10:01:08.
09/16/2011 10:01:50 AM · #23
Originally posted by mike_311:

OK now that that is solved, how about a new one:

Imagine you are sitting in a boat swimming in a pool. You have got a big rock in your hand. When you throw the rock into the pool what happens to the water level of the pool?

Easy. You miss the pool because you play for the Knicks.
09/16/2011 10:09:51 AM · #24
http://www.youtube.com/watch?v=0KsdMuhYJPw

That's really bad science right there.
The airplane IS moving.
09/16/2011 10:33:04 AM · #25
The original question is flawed or deliberately twisted.
If the wheels of the airplane are free to roll on the conveyor, then the only difference between the conveyor lift off and a solid runway lift off would be the speed that the aircraft's wheels are rotating when it attains air speed. It would take just a little longer than normal to reach air speed because of the extra energy needed to spin up the wheels to twice the normal lift off speed of the aircraft.
To keep it simple, if liftoff air speed is 100 mph, and surface of the conveyor is moving at 100 mph,, then the wheels would be rotating at 200 mph at liftoff.
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