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


Randazzo

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The conveyor spins back to match the speed of anything on it, trying to keep it at the same spot. In case of a car, conveyor doesn't have to do anything, it just has to spin freely, yes. In case of an airplane, conveyor actively tries to stop the airplane and fails.

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The conveyor spins back to match the speed of anything on it, trying to keep it at the same spot. In case of a car, conveyor doesn't have to do anything, it just has to spin freely, yes. In case of an airplane, conveyor actively tries to stop the airplane and fails.

This part. The bolded part. Where does it say it in the thought experiment? Is that somehow the function of every conveyor?

Even if we DID say it matches the car's speed, that doesn't mean it will stay in the same spot because then for the conveyor to move back, the car has to move forward. Car moves forward, belt moves back = speeds are matched. Car stays still, belt moves back = speeds are NOT matched.

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For [any deity of choice]'s sake, it's a well known thought experiment, and yes, it states that there's some sort of a control system which speeds up the conveyor belt if needed, matching airplane's speed in the opposite direction. In terms of control theory, it's a perfect negative feedback loop. It won't work against airplane because airplane doesn't push against the conveyor, it pushes against air which isn't controlled by the feedback. It will work against the car because any torque the car applies will speed up its wheels -> conveyor goes faster -> car stays on the spot. And if we're talking about zero internal friction, both for the car and conveyor, and perfect wheel grip on conveyor belt - there's no need in any control system. Any torque that engine applies will speed up the wheels and conveyor anyway - and the car will STILL stay at the same spot. End of story.

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For [any deity of choice]'s sake, it's a well known thought experiment, and yes, it states that there's some sort of a control system which speeds up the conveyor belt if needed, matching airplane's speed in the opposite direction. In terms of control theory, it's a perfect negative feedback loop. It won't work against airplane because airplane doesn't push against the conveyor, it pushes against air which isn't controlled by the feedback. It will work against the car because any torque the car applies will speed up its wheels -> conveyor goes faster -> car stays on the spot. And if we're talking about zero internal friction, both for the car and conveyor, and perfect wheel grip on conveyor belt - there's no need in any control system. Any torque that engine applies will speed up the wheels and conveyor anyway - and the car will STILL stay at the same spot. End of story.

Ok so let's go with that requirement. Does it really make sense to take the car's speed relative to the conveyor, and the conveyor's speed relative to some other spot? They are both variables in the problem, they should both be measured against something else, and not to themselves. If we did that, we would find that both the car/plane and conveyor MUST move to satisfy your conditions. This artificial requirement of the belt existing soley to stop the car/plane is neither stated in this thread's nor in your own requirement above.

It just kind of materializes on its own, I imagine from the niche function that treadmills have found at gyms.

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A car moves by having its propulsion interact directly with the ground. An aircraft moves by having its propulsion interact directly with the air. The ground is almost entirely irrelevant.

If the wheels had enough friction to prevent an aircraft from taking off on a conveyor belt, then the aircraft wouldn't be able to take-off on a normal runway. Simple as that.

This is like asking if a car going 80mph without brakes will stop before that very dangerous-looking cliff ahead. Spoiler: it won't. Because a speeding car in neutral has about as much friction with the ground as an aircraft moving down a runway for take-off.

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I don't understand. How can this even be a question?

If the airstream created by forward movement and propulsion is strong enough to allow the lifting surfaces to lift the plane, then it'll take off.

The conveyor belt only speeds up the rotation of the wheels, so they roll faster and create a bit more friction. But technically you'll never actually be able to completely stop a plane with functional wheels. If you'd be able to stop a plane, then it won't lift off, because most planes won't just fly in the air when being stationary (or by propeller/etc-airstream alone). Duh.

Idk. Am I stupid or ist the question stupid? I'd assume the latter.

Edited by Temeter
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The answer is no.

The whole principle behind flight is lift via air flowing over the wings.

The mythbuster's test was invalidated do to air going over the wings.

If you have no air air flow then the plane will not take off.

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The answer is no.

The whole principle behind flight is lift via air flowing over the wings.

The mythbuster's test was invalidated do to air going over the wings.

If you have no air air flow then the plane will not take off.

I think you're misunderstanding the question. The conveyor belt can't really stop the plane because the wheels will counteract its backwards movement (as said, they just roll faster). So it'll move forward and create lift.

E.g., if the planes moves forwards 20kmh and the conveyor moves backwards 30kmh, then the wheels will just roll (and create some small friction) as they would normally at a speed of 50kmh.

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The answer is no.

The whole principle behind flight is lift via air flowing over the wings.

The mythbuster's test was invalidated do to air going over the wings.

If you have no air air flow then the plane will not take off.

You may have mistaken an aircraft taking off on a conveyor with an aircraft taking off in a vacuum.

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I think you're misunderstanding the question. The conveyor belt can't really stop the plane because the wheels will counteract its backwards movement (as said, they just roll faster). So it'll move forward and create lift.

E.g., if the planes moves forwards 20kmh and the conveyor moves backwards 30kmh, then the wheels will just roll (and create some small friction) as they would normally at a speed of 50kmh.

Now here I was thinking about a conveyor belt that was only a few feet longer then the plane, where you applied enough thrust to keep the plane in the same place (ie no forward motion) and hence no lift.

As soon as you get forward motion from the plane, lift comes into play, and the plane takes off.

So it all depends on how you define your experiment.

Edited by Korizan
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The conveyor spins back to match the speed of anything on it, trying to keep it at the same spot. In case of a car, conveyor doesn't have to do anything, it just has to spin freely, yes. In case of an airplane, conveyor actively tries to stop the airplane and fails.

If you actually want to look at it as a control problem, matching wheel rotation speed, then the conveyor belt does stop the plane. It's a simple constraint problem. You just need to remember to include torque on the wheel, its moment of inertia, and the fact that the belt is accelerating.

Resulting accelerations end up being impractically high, but again, as a pure theoretical control problem, it has solution for both car and plane, and in both cases, the vehicle stays put. It's a fun little problem to solve, but if you'll need me to run through the equations, I will.

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Now here I was thinking about a conveyor belt that was only a few feet longer then the plane, where you applied enough thrust to keep the plane in the same place (ie no forward motion) and hence no lift.

As soon as you get forward motion from the plane, lift comes into play, and the plane takes off.

So it all depends on how you define your experiment.

I think the point of the excercise is, that the conveyor belt doesn't apply any backwards thrust* that could stop a plane. Plane-wheels are in the first place only supposed to let the plane freely roll in any directon, it doesn't really matter in which direction they roll at what speed, as long as the plane moves forward.

*the only backwards motion is created by friction caused by the weight of the plane lasting on wheels/ground. Which is very small, compared to engine thrust.

Edited by Temeter
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Now here I was thinking about a conveyor belt that was only a few feet longer then the plane, where you applied enough thrust to keep the plane in the same place (ie no forward motion) and hence no lift.

Almost any amount of thrust will move the plane. It is the whole point of the experiment.

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Let's remove airlift from the equation then :) We should place a car in neutral gear on a sloped conveyor belt - this time, gravity pulling down on the car replace the airplane's propeller :) the car will naturally go down the slope while the belt tries to keep moving the car to the top.

As long as the gravity pull is stronger than the resulting friction from the wheel's bearings, the car will roll down the slope - regardless of the wheel spin speed !

If you increase the belt's speed while keeping the slope angle constant, sure there will be a moment where the friction coming from the bearings will be greater than the strength from the gravity pull - and in this case the belt will move the car upwards. But as bearings are built to minimize friction, you're going to keed quite a meaningful belt speed to overcome the gravity pull :)

(Of course, don't forget to get the car in neutral gear and disable parking brakes ;) else the bearings become irrelevant and this time we have gravity fighting against the wheels' friction instead of bearings' friction - and wheels are built to have great friction with the ground.)

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If you actually want to look at it as a control problem, matching wheel rotation speed, then the conveyor belt does stop the plane. It's a simple constraint problem. You just need to remember to include torque on the wheel, its moment of inertia, and the fact that the belt is accelerating.

Resulting accelerations end up being impractically high, but again, as a pure theoretical control problem, it has solution for both car and plane, and in both cases, the vehicle stays put. It's a fun little problem to solve, but if you'll need me to run through the equations, I will.

No it doesn't.

If you increase the belt's speed while keeping the slope angle constant, sure there will be a moment where the friction coming from the bearings will be greater than the strength from the gravity pull - and in this case the belt will move the car upwards.

If I remember school right, kinetic friction doesn't depend on object's velocity.

Edited by J.Random
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  • 2 years later...

You guys don't 

2 hours ago, LordFerret said:

No. You need air passing over the wings to generate lift.

 

2 hours ago, razark said:

Yes, because the thrust from the engines will give the plane positive airspeed, regardless of the speed of the ground.

 

1 hour ago, YNM said:

Yes. Wheels are just rollers init. Airplanes can move without wheels anyway.

images?q=tbn:ANd9GcSZyHMdvYG4p6WVZH4Hjlk

You guys don't now what you're getting into.  This question was actually banned from xkcd's forums.  You'll see why.

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