MythBusters tackles "plane on a conveyor belt problem"

By The_Decryptor
in Science News & Discussion

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Grand Master

Slugsie

Posted
Posted
Without going through and reading all the post here. Isn't a plane taking off more about wind speed then forward momentum. For instance, a strong gust towards a stationary plane would cause the plane to lift because the air is flowing over the wings. So what's really needed is the air to move, or the object to move through the air. Seeing as a plane on a conveyor doesn't move, and the air isn't moving at any considerable speed... the plane will get no lift. Or is that all wrong?

I suggest that if you're going to comment then you really do need to read some of the comments. You're point has been made, and is wrong. It would be right if we were talking about a car or a bike, but we're not.

Grand Master

OPaul

Posted
Posted
I suggest that if you're going to comment then you really do need to read some of the comments. You're point has been made, and is wrong. It would be right if we were talking about a car or a bike, but we're not.

There are around 140 replies to this thread, it would be impractical for me to go through them all. Where abouts was the point made and disproved? Just a page number will do.

- EDIT -

Is this it?

Nope. If the aircraft is moving forward at 100mph, then the point of the scenario is that the conveyor is moving backwards at 100mph. This will result in the wheels spinning at 200mph and thus the forward speed of 100mph of the aircraft is maintained. Thus it is moving through the air, and generates lift, and thus can take off.

If the plane is moving through the air, does that mean if I sit on the wing of this plane on a conveyor my hair will blow?

Mentor

monkey13

Posted
Posted

So this is all completely pointless then Slugsie. The problem has become:

"What happens if you place a plane in a situation (conveyor belt or whatever) that has no effect on its ability to move, accelerate and reach take-off speed"

You might as well say "What happens if the pilot is wearing a wig?"

Grand Master

Fred Derf Veteran

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Most people still have it wrong. Regardless of what the conveyor is doing there still will be forward momentum.

Some people are still picturing a plane hovering still as the conveyor moves backward. It won't. It will move forward normally and takeoff normally.

If the conveyor has any effect on the wheels due to friction then it will be minimal. Assuming the wheels can take double their normal takeoff stress then everything will be fine and everything will work normally. The conveyor will not hold back the plane in any way.

Apprentice

SkyLost1984

Posted
Posted
So this is all completely pointless then Slugsie. The problem has become:

"What happens if you place a plane in a situation (conveyor belt or whatever) that has no effect on its ability to move, accelerate and reach take-off speed"

You might as well say "What happens if the pilot is wearing a wig?"

If the pilot was wearing a wig, you would need a miniscule amount of thrust more to get the plane to take off speed!

Grand Master

Slugsie

Posted
Posted
There are around 140 replies to this thread, it would be impractical for me to go through them all. Where abouts was the point made and disproved? Just a page number will do.

The point has been made on pretty much every page. No-one has yet managed to provide a mechanism whereby a normal conveyor (albeit pretty big) is able to excerpt a force on a normal aircraft that can prevent the aircraft from moving, given that the wheels of a normal aircraft are free to spin in either direction and at any speed (unless the pilot is explicitly applying the brakes, in which case he is explicitly trying not to take off). You must remember that an aircraft moves by pushing the air, not by pushing the ground. You must also remember that if the aircraft is stationary (as everyone seems to think will happen), then so is the conveyor (it automatically matches the speed of the aircraft, just in the opposite direction). If someone can explain how that is done - without relying on additional forces not stated in the original problem, then I may be prepared to admit the aircraft will not take off.

Grand Master

Slugsie

Posted
Posted
So this is all completely pointless then Slugsie. The problem has become:

"What happens if you place a plane in a situation (conveyor belt or whatever) that has no effect on its ability to move, accelerate and reach take-off speed"

You might as well say "What happens if the pilot is wearing a wig?"

Correct. The conveyor belt is simply a distraction, and has no bearing on the outcome.

If the plane is moving through the air, does that mean if I sit on the wing of this plane on a conveyor my hair will blow?

Depends if you're bald or not. :whistle:

Grand Master

spenser.d

Posted
Posted
The point has been made on pretty much every page. No-one has yet managed to provide a mechanism whereby a normal conveyor (albeit pretty big) is able to excerpt a force on a normal aircraft that can prevent the aircraft from moving, given that the wheels of a normal aircraft are free to spin in either direction and at any speed (unless the pilot is explicitly applying the brakes, in which case he is explicitly trying not to take off). You must remember that an aircraft moves by pushing the air, not by pushing the ground. You must also remember that if the aircraft is stationary (as everyone seems to think will happen), then so is the conveyor (it automatically matches the speed of the aircraft, just in the opposite direction). If someone can explain how that is done - without relying on additional forces not stated in the original problem, then I may be prepared to admit the aircraft will not take off.

Well of course the aircraft is moving. If the conveyor belt matches it's forward speed, it just won't look like it's moving, but as far as the aircraft itself could tell, it's moving. That being said, with respect to air and forces outside the aircraft, it would still seem to be standing still and there'd be no air running under it creating lift, despite the fact that it's "moving".

-Spenser

Apprentice

SkyLost1984

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Depends. If all the stewardesses had waxed that morning this may offset the added weight of the syrup :p

very true, next time im on a plane, ill make sure to ask the stewardesses if they have waxed becuase im scared of not reaching the end of the runway in time :p

Grand Master

Fred Derf Veteran

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Well of course the aircraft is moving. If the conveyor belt matches it's forward speed, it just won't look like it's moving, but as far as the aircraft itself could tell, it's moving. That being said, with respect to air and forces outside the aircraft, it would still seem to be standing still and there'd be no air running under it creating lift, despite the fact that it's "moving".

-Spenser

You seem to be missing the point. It will look like it is moving. It will actually be moving up the conveyor belt. It won't just moving on the treadmill/conveyor belt. It will be moving forward compared to stationary objects on the ground. It will not hover in place as some imagine.

Grand Master

OPaul

Posted
Posted
Well of course the aircraft is moving. If the conveyor belt matches it's forward speed, it just won't look like it's moving, but as far as the aircraft itself could tell, it's moving. That being said, with respect to air and forces outside the aircraft, it would still seem to be standing still and there'd be no air running under it creating lift, despite the fact that it's "moving".

-Spenser

Nah, I understand this now. The plane will look like it's moving forward because it will moving forward at the same speed it normally would be. All the conveyor belt does is act on the wheels, but the wheels have nothing to do with moving the plane forward. As long as the wheels are free to move, the plane will be pushed forward by the thrust generated by the engines.

Apprentice

SkyLost1984

Posted
Posted (edited)

Simply put, if a plane moving at for example 200mph, is on a conveyor also moving at 200mph in the opposite direction, it would be as if the conveyor was not there.

If the plane was moving at 200mph, and the conveyor was moving the other way at 500mph, the effect would be the same, as if it wasnt there. The only differernce would be that in the first example, the wheels would be spinning at 400mph, and in the second, they spin at 700mph. The spinning wheels have no effect on the plane at all

However if the wheels did not exist, and the plane way laying on its belly on the same conveyor, providing the same forces as above, then this would cause the plane to be affected by the movement of the conveyor

Nah, I understand this now. The plane will look like it's moving forward because it will moving forward at the same speed it normally would be. All the conveyor belt does is act on the wheels, but the wheels have nothing to do with moving the plane forward. As long as the wheels are free to move, the plane will be pushed forward by the thrust generated by the engines.

thats correct, except the plane will not LOOK like its moving forward, it will actually be moving forward

Edited by SkyLost1984
Grand Master

Fred Derf Veteran

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Nah, I understand this now. The plane will look like it's moving forward because it will moving forward at the same speed it normally would be. All the conveyor belt does is act on the wheels, but the wheels have nothing to do with moving the plane forward. As long as the wheels are free to move, the plane will be pushed forward by the thrust generated by the engines.

The plane will not just "look" like it is moving it really will be moving forward up the treadmill. It will be moving compared to any reference point on the ground.

Grand Master

OPaul

Posted
Posted
thats correct, except the plane will not LOOK like its moving forward, it will actually be moving forward
The plane will not just "look" like it is moving it really will be moving forward up the treadmill. It will be moving compared to any reference point on the ground.

Yes, that's what I said/meant. It will look as if it is moving forward because it will be moving forward, relative to the ground..the conveyor... and anyone watching standing still.

Grand Master

Fred Derf Veteran

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Yes, that's what I said/meant. It will look as if it is moving forward because it will be moving forward.

Your edit was not there when I started my post or I would have just agreed with you. My post ended up being a duplicate to yours. Edit: My post was a duplicate of what SkyLost1984 said.

Yes, I just wanted to clarify that it really would be moving forward and not just be "moving in place" on the treadmill.

Grand Master

Fred Derf Veteran

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... I didn't edit that post. Your quote is what I posted.

Yeah, yeah, a temporary moment of confusion. I edited my post and thought I could get away with it before anyone noticed.

Alas, I was either too slow or you were too fast. :D

Collaborator

RealFduch

Posted
Posted
No-one has yet managed to provide a mechanism whereby a normal conveyor (albeit pretty big) is able to excerpt a force on a normal aircraft that can prevent the aircraft from moving, given that the wheels of a normal aircraft are free to spin in either direction and at any speed

Let's check it.

The conveyor exerts force on the PLANE (Because there is friction between the plane and the wheels). So the conveyor ~ slightly pressed brakes.

So the problem becomes: "Can the plane take off the runaway if the brakes are pressed a little". And the answer depend on the concrete parameters/situation.

Grand Master

Slugsie

Posted
Posted
Let's check it.

The conveyor exerts force on the PLANE (Because there is friction between the plane and the wheels). So the conveyor ~ slightly pressed brakes.

So the problem becomes: "Can the plane take off the runaway if the brakes are pressed a little". And the answer depend on the concrete parameters/situation.

Yup, that is pretty much spot on. The only effect of the conveyor is to increase the friction within the wheels, and is thus analogous to slightly applying the brakes. It'd contend that pretty much any aircraft would be able to do that. If you want to take it to an extreme to demonstrate the point. Imagine a fighter jet sat there with the pilot pushing moderately hard on the brakes, with the jet engines at 100% with full after burners. Anyone seriously believe that the aircraft is not going to move forwards?

Collaborator

RealFduch

Posted
Posted
Yup, that is pretty much spot on. The only effect of the conveyor is to increase the friction within the wheels, and is thus analogous to slightly applying the brakes. It'd contend that pretty much any aircraft would be able to do that. If you want to take it to an extreme to demonstrate the point. Imagine a fighter jet sat there with the pilot pushing moderately hard on the brakes, with the jet engines at 100% with full after burners. Anyone seriously believe that the aircraft is not going to move forwards?

He-he. Moving forward != taking off. The runaway may be not long enough to speed up when additional friction takes energy away.

Veteran

Minimoose

Posted
Posted

Surely there wouldn't be any lift under the wings (unless the volume of air passing under the wings was the same amount as a normal plane speeding up, so u would need very powerful winds for it to take off like this?), and the forward motion would be countered by the treadmill equaling the thrust forward to an infinite speed. This is just my silly thought process, probably completely wrong :yes:

Also i know nothing of aerodynamics so i don't know how the dynamics of air affect the take off of a plane :)

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