A plane on a conveyor belt

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Blinkz said:
Well I'll be first admit that taking off on a moving conveyor belt isn't the standard operating procedure for the 737, and I'm pretty sure that boeing didn't include it as part of their testing schedule :D so I cannot be certain on what would happen lol
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In the unlikely event that Mr Ryan allows this novel testing method for his airline, could you give me a shout. I've always quite fancied the idea of being part of an experiment to prove something like this theory about conveyor belts wrong, ta. :)
 
Steedie said:
Call me an idiot, I believe you lot when you say it will take off

I just can't get my head round how when you say its moving even though it isnt? I mean surely even if the tyres are going at silly speeds and the engines are on, if the conveyor belt is keeping it still, there isnt any air moving over the wings. Or is it that because the conveyor belt will always be playing catch up with the plane, the plane is theoretically moving, even though physically it hasnt moved an inch?

This post isnt trying to say you're wrong, I just want to try and understand but can't :(
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The conveyer belt can't keep the plane still though, that's the thing.

The conveyer belt matches the wheel's speed, but that won't stop the plane.

Think of it this way.

I'm on a skateboard, on a treadmill. If you stand to the side of the treadmill and hold on to me, I won't go backwards even if the tread mill is going against me.

Now, if you push me, I will roll forwards, even though I'm on a treadmill. Now think about it with the plane - I'm the plane, you're my engine. The wheels are irrelevant, it's YOU that's doing the pushing, and the treadmill can't affect you. The only small effect it can have is more friction in the wheels from spinning so fast, but it won't stop me from going along.
 
Capn' $qu!ff said:
Yeah but the water isn't generating motive force in the opposite direction to the plane therefore it glides forward over the top of the water and is not at a standstill I know technically the plane is moving forward however it's covering no distance therefore the wind pressure moving past the plane is not changing and so it won't be able to create the lift required!
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Nor are the wheels generating motive force. They are a free flowing entity and can spin at a faster speed than the plane is actually moving. If the plane increases its trust so it moves forwards at 10knots, the conveyor belt adjusts its speed to move in the opposite direction at 10knots. The wheels are actually spinning at 20knots because of the counteraction of the conveyor belt. So the plane is actually moving forwards, which generates the wind pressure which equals lift.

It just takes the plane longer to take off.
 
Mohinder said:
The thing you're missing (and the thing I missed at first, the first time round), is that the wheels have no effect on the speed of the plane. They're just spinning freely. So once you get that in your head, you can basically take the treadmill, the wheels, everything out of the equation.

Literally this simple - stuff comes out of the engines

that stuff pushes the plane forward

That the wheels will be spinning twice as fast is irrelevant - because the treadmill can only act on the plane's wheels, and the wheels do not affect the plane's speed
The points in bold is what just refused to sink in with me last time, once it went in I got it.
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Basically, it is impossible to hold a plane stationary on a treadmill.

If the engines deliver enoug power to move the plane forward at 30mph, and the conveyor belt is moving at 30mph in the opposite direction, the wheels will spin at 60mph and the plane will still move forwards at 30mph.
 
In that case I totally understand

I thought the whole point of the arguement was if the plane was being kept still by the belt

But now I understand that to be impossible I finally accept it :p
 
Steedie said:
Or is it that because the conveyor belt will always be playing catch up with the plane, the plane is theoretically moving, even though physically it hasnt moved an inch?
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There is no theoretical movement, it's actually movement.

Think about these two snapshot scenarios:

1) The plane is doing 400 mph, with it's wheels on the belt, which is doing 400 mph in the opposite direction.

2) The plane doing 400mph above belt, with it's land gear up, whilst the belt 10m below is doing 400 mph in the opposite direction?

In the scond scenario, will the belt slow down or affect the thrust of the plan in any way? No, because there is no interation between the plane and the belt and the planes engines thrust against the air, not against the belt.

So what's the difference with the first scenario? The engines still push against the air, not the belt and the wheels will spin freely regardless of the speed of the belt or the plane and the plane will move forward.
 
qwerty said:
Basically, it is impossible to hold a plane stationary on a treadmill.

If the engines deliver enoug power to move the plane forward at 30mph, and the conveyor belt is moving at 30mph in the opposite direction, the wheels will spin at 60mph and the plane will still move forwards at 30mph.
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Yep. It makes perfect sense once it goes in - it's the 'matches the plane's speed' thing that people can't get. I couldn't get my head round it last time, that's why forgetting the whole plane thing and thinking about it in other terms helped it go in :D
 
Cruiser said:
it will take off but just take a lot longer to get off the ground because the wheels are spinning madly?
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Shouldn't really take any longer at all, maybe a little because of the friction in the wheels. The wheels are really entirely irrelevant.

The only way the treadmill might stop it is if the grease in the wheels got too hot and the whole kit and kaboodle burst into flames, but I should hope planes are built to a better standard than that :D
 
Mohinder said:
Shouldn't really take any longer at all, maybe a little because of the friction in the wheels. The wheels are really entirely irrelevant.

The only way the treadmill might stop it is if the grease in the wheels got too hot and the whole kit and kaboodle burst into flames, but I should hope planes are built to a better standard than that :D
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Well actually the wheels (actually the tires) are rated for a certain speed and in certain circumstances this can mean we have the limit the weight which we take off at, usually at hot and high airports.
 
I actually like this problem as it seperates the wheat from the chaff in terms of basic intelligence. :)

The rotation of the wheels are irrelevant to the speed or direction of the plane as they are not driven wheels, the engines thrust the plane forward (with the wheels spinning backwards) in exactly the same manner as if the plane was standing on tarmac (the resistance from the wheels spinning backwards would be inconsequential vs the lb/sec thrust force) - result: plane takes off. Simple!

To further complicate this problem in a real World situation I would presume that the wheels wouldn't be able to withstand the speeds involved in spinning backwards against the motion of the plane and would initially lock up before shearing off completely. That being said, the plane would still technically move (although probably not fast enough to take off) if it was lying flat on the runway/conveyor on its belly.
 
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when both conveyor and plane initially start moving it will require slightly more engery than normal just to initially push the plane foward but after that the conveyor will have no effect, so it will probably be a little longer to take off but then again the plane is likely to move back slightly at first depending on friction so I say it would take off in the same distance.
 
I love it when this comes up :)

There's always those who argue until their face is the colour of these forums that the plane will not take off, because they say the plane won't move, because the conveyor is matching the plane's speed, so the plane can't go anywhere.

Of course, then there's always the "well the wheels/bearings could break and won't spin any more because of the speed" argument.

And then there's those who come in quoting the first thing they've found in Google about pressure differentials across the wing.


This subject is great - enough to bring the often sane population of a forum to it's knees! :D
 
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