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MythBusters tackles "plane on a conveyor belt problem"


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#286 David Scaife

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Posted 30 January 2008 - 07:24

If you have a car on a conveyor belt which is going backwards as fast as the car is going forwards, it will not move. Some people here, the ones who think it will not lift off, are working on the premise that the plane will remain stationary, like if a car was on the conveyor belt. Indeed, if the plane was stationary, it would not take off. However, what the rest of us are arguing is that the plane does not stay stationary.

Example: If you had two African swallows with a piece of string joining them, with a free-rolling coconut on the string, would "the system" be forced to stay stationary? No, because the conveyor belt only makes that coconut roll faster, which has no bearing at all on whether or not the birds can fly forwards because the birds exert their force on the air, not on the conveyor belt.

Replace those birds with jet engines. Replace the coconut with a wheel. Put an aeroplane chassis between them. Is the plane going to stay stationary? No, it's going to move forwards.


#287 Marcel T

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Posted 30 January 2008 - 08:18

Example: If you had two African swallows with a piece of string joining them, with a free-rolling coconut on the string, would "the system" be forced to stay stationary? No, because the conveyor belt only makes that coconut roll faster, which has no bearing at all on whether or not the birds can fly forwards because the birds exert their force on the air, not on the conveyor belt.

Replace those birds with jet engines. Replace the coconut with a wheel. Put an aeroplane chassis between them. Is the plane going to stay stationary? No, it's going to move forwards.

Totally get it now. I didn't google this one but I'm sure something like this would have set it straight for me elsewhere.

#288 Slugsie

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Posted 30 January 2008 - 09:07

Well I can't sleep and I've been thinking about this again. Damn it!

My theory is the plane will not take-off, regardless of if it can move down the conveyor. Here's why.

There's been a lot of talk about the wheels having no effect. This is impossible. For the wheels to have no effect the wheel bearings holding the weight of the plane would need to be entirely frictionless (impossible). The force required to compress the wheels as they rotate would have to be nil (impossible). Another way to think of it is; If the plane had these magic wheels and was in a vacuum (i.e no air resistance) the plane would move with any forward force applied to it (e.g. a small child could push it along with one hand) Another way to think of it is that the wheels behave the same regardless of if the plane is there or not. Ridiculous.There is a resistance which is why the engines have to be revved to get the pane moving when taxiing.

The next thing I was thinking about is; If the plane's engines were turned off and the conveyor belt it's on accelerated would the plane remain still? The answer is no. The forces, as mentioned above, that act through the wheels would still act moving the plane backwards. Therefore a force is required just to keep the plane in the same place. Meaning that more force is required to accelerate the plane on the conveyor than on the ground. Regardless of if the plane can move along the conveyor.

Now the question is; Does the plane have enough extra power to overcome these additional forces and move along the conveyor to reach take-off speed?

I'm going with no.


The problem is that the friction forces you claim will prevent the aircraft from taking off are still present (albeit halved under normal circumstances) during a normal everyday take-off. Do you honestly think that an aircraft on a normal runway, fitted with bearings that have twice the friction of normal ones will have any issues taking off? Do you really think that normal aircraft are running so close to their limits?

Think of it this way, if an aircraft needs 50mph of airspeed to achieve take-off, and is taking off with a 50mph tail wind, then it will need to achieve 100mph of ground speed. It'll do that with no problems, so it'll fly.

#289 OP The_Decryptor

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Posted 30 January 2008 - 09:52

The plane is not doing any WORK against the air, so there will not be any lift, its as simple as that, some people are making this way to confusing.

The engines act on the body of the plane, moving it forward (which creates lift).

#290 vincent

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Posted 30 January 2008 - 15:32

The plane is not doing any WORK against the air, so there will not be any lift, its as simple as that, some people are making this way to confusing.




If that was the case then no planes would have the need for engines, one can simply place them in a windy area (good luck with that by the way) and get them to lift off.

#291 Lt-DavidW

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Posted 30 January 2008 - 15:40

For those who say 'yes':

1. Where is the plane going to get the momentum from to suddenly propel itself forward off the treadmill and into the air?

2. If you think instead that the plane will lift vertically above the treadmill, then once it has lifted off, why not remove the treadmill and.. ta-dah! a hovercraft. :huh:

This would only work if the treadmill was a "treadmill of moving air".

#292 OPaul

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Posted 30 January 2008 - 15:43

For those who say 'yes':

1. Where is the plane going to get the momentum from to suddenly propel itself forward off the treadmill and into the air?

The thrust generated from the engine or propeller.

#293 Lt-DavidW

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Posted 30 January 2008 - 16:12

The thrust generated from the engine or propeller.

Oh, so the plane's engines will be on then?

That explains why I found the idea of a conveyor-belt-powered plane so ridiculous. :laugh:

In that case, the engines will move the plane forward regardless of how the wheels are turning in response to the treadmill. Wheels on planes are only there to reduce friction between the plane and surface.

The effect the treadmill will have on the plane is it will make the wheels spin more, increasing friction. But this should not stop the plane from taking off, assuming it has enough power to overcome the friction and build up the required speed...

#294 Fish

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Posted 30 January 2008 - 23:31

The answer is obvious. If you could, in fact, make an aeroplane take off from a stationary treadmill, then in 100 years of flight, surely we'd be doing it by now.

An aeroplane's wings create lift when air is passed over them. To do this, some form of thrust is required to move the plane forwards through the air. If the plane was standing on a treadmill, as soon as the wheels started to move forward, the treadmill would turn, effectively giving the plane no velocity. The plane might appear to "rock" gently forwards and backwards as the engines create more thrust and the treadmill compensates by speeding up, but this forward velocity would never be enough to create enough airflow over the wings to lift the plane into the air.

#295 sundayx

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Posted 30 January 2008 - 23:50

Obviously you're wrong. Read this thread again from page 1.

#296 Fish

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Posted 31 January 2008 - 00:16

Wings require airflow over them to generate lift. If you run on a treadmill, do you feel air blowing in you face?

(Note: By treadmill, I refer to a free-rolling conveyor belt, not a powered device)

#297 sundayx

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Posted 31 January 2008 - 00:27

If I had wings that propel me forward rather than my legs, then I probably would. My legs just compensate me from falling on the floor. (My last chance to convert others.)

#298 kernatch

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Posted 31 January 2008 - 02:50

OK, I didn't bother reading all the posts, but a quick glance shows there are still people who don't think the plane can take off. I'm also aware that it's likely that many others have done what I'm about to do (which is explain how it can), but perhaps an alternative angle will help the others.

The fundamental mistake most people make is thinking that a plane works like a car, so let's talk a bit about cars first.

OK. How do cars move forward? They have engines which turn their wheels. Their wheels grip the surface of the road, which makes the car move forward.

Let's stick wings on a car, so that when the relative speed between the car and the air reaches a certain speed, the car can take off. Let's also presume it's a windless day. We put it on this magical runway, and the driver starts to put his foot on the accelerator. Oh no! The car just stays still and there's no air going over the wings; the car doesn't take off.

If cars have wheels that grip the ground, what do planes have? Planes have engines that 'grip' the air. What about plane wheels? Plane wheels are completely un-powered. Why are they there? To reduce the friction between the plane and the ground to something that's negligible. Let's put the plane on the magical runway. But first, let's think about what would happen if the runway was moving backwards at, say, 50mph, but the plane's engines were off (although its wheels were unlocked). Although it might not feel intuitive, the plane would in fact stay more or less still. Once again, because of those magical low-friction wheels. You can try this at home: put some paper on the floor and then put a cylinder (deodorant spray, bottle, whatever you can find) on top. Pull the paper from underneath it, and you should see that the cylinder stays roughly in the same place!

OK, so we have a plane that can stay (more of less) still even with a runway moving backwards underneath it. So let's go back to how it moves forward. Those wonderful engines! Turn them on, and the plane continues to move forward as normal. The difference is that the wheels will be spinning twice as fast as they normally would. Some people have pointed out that this might cause the wheels to explode - a fair point, but I don't think is why most people object to the idea that the plane can still take off.

#299 XDViPeR

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Posted 31 January 2008 - 02:59

Alright, I don't agree with what they did in Mythbusters!. When they pull that curtain with the truck the other way, the plane is still going and moving, it's gaining forwardness, it has to pick up! but when the belt is going the same speed with the plane, it wont allow it to move so no air on the wings, this isn't possible, mythbusters did this wrong, what do you guys think.

#300 OPaul

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Posted 31 January 2008 - 03:00

Episode just aired, plane took off no problem. What was sad was that the pilot appeared not to understand how his plane functioned and believed his plane would remain stationary.