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Post by edinburghstokie on Jul 13, 2009 12:18:57 GMT
I know this has been on the internet for a while, but haven't seen it on the oatie, and thought it would make a change to the conspiracy theories and how wank britain is.
A plane is standing on a runway that is made of a large conveyor belt. The plane fires up its engines, but as it moves forward, the conveyor belt senses the speed of the plane's forward motion and instantaneously moves at exactly the same speed in the opposite direction. Can the plane take off?
Discuss.
I say yes.
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Post by mermaidsal on Jul 13, 2009 12:21:31 GMT
Doesn't it have to be displacing a certain amount of air though, unless it's a VTOL?
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Post by edinburghstokie on Jul 13, 2009 12:23:56 GMT
no, it isn't a vertical take off plane.
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Post by mermaidsal on Jul 13, 2009 12:24:49 GMT
Ok, well doesn't that mean it has to be moving forwards and displacing air with its wings rather than just moving at x or y velocity?
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Post by jonesinamillion on Jul 13, 2009 12:36:06 GMT
No speed = no pressure difference = no lift = no take off, sorry!
You're not flying from the new treadmill #4 @ Heathrow i hope?
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Post by Funky on Jul 13, 2009 12:51:56 GMT
No
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Post by edinburghstokie on Jul 13, 2009 13:00:41 GMT
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Post by beagriestache on Jul 13, 2009 14:21:51 GMT
I say no. The speed of the plane through the air is what creates lift beneath the wings.
The plane is moving fast relative to the treadmill even though it's going nowhere (does that make sense) but the plane is stationary relative to the air - therefore no lift.
If the air was moving as well then yes. It's like when you see sea-birds about to fledge from a cliff, they quite often spread their wings and hover a little. This is because the wind is blowing towards them, providing lift as it passes under the wing.
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Post by Olgrligm on Jul 13, 2009 14:22:54 GMT
I say no. The speed of the plane through the air is what creates lift beneath the wings. The plane is moving fast relative to the treadmill even though it's going nowhere (does that make sense) but the plane is stationary relative to the air - therefore no lift. If the air was moving as well then yes. It's like when you see sea-birds about to fledge from a cliff, they quite often spread their wings and hover a little. This is because the wind is blowing towards them, providing lift as it passes under the wing. That's what I was going to say. |
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Post by edinburghstokie on Jul 13, 2009 14:53:05 GMT
But why isn't the plane moving forward? The wheels don't provide the thrust. The thrust comes from the engines, therefore it should move forward (the friction in the wheels will be minimal compared to the thrust).
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Post by beagriestache on Jul 13, 2009 14:55:59 GMT
Cos the treadmill is moving in the other direction so the two movements cancel each other out, with the result that the plane is effectively stationary.
Do you move forward when you run on a treadmill at the gym?
(Are you fishing, if so, I bit ;D)
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Post by edinburghstokie on Jul 13, 2009 15:01:37 GMT
what drives the plane forward are the engines though. the treadmill will then go at the same speed backwards. However the link between the two are a set of wheels, which to all intents and purposes can be considered frictionless. Therefore they will just spin.
The plane will go forward at x metres per second.
The treadmill will go at -x metres per second.
The wheels will turn twice as fast (as if travelling at 2x meters per second when not on a treadmill).
Therefore the plane will go forward and gain lift and take off.
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Post by beagriestache on Jul 13, 2009 15:06:07 GMT
No, fundamental error is the frictionless part. If there was no friction how would the wheels be turning?
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Post by Funky on Jul 13, 2009 15:06:35 GMT
Still no
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Post by edinburghstokie on Jul 13, 2009 15:08:14 GMT
No, fundamental error is the frictionless part. If there was no friction how would the wheels be turning? Ok, almost frictionless. The forces involved ar far outweighed by the thrust forward from the engines. |
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Post by wembley4372 on Jul 13, 2009 15:11:06 GMT
Ah frictionless wheels - would the plane stand still on the treadmill if the engines were off but the treadmill was running? Or is it impossible for that to happen?  |
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Post by beagriestache on Jul 13, 2009 15:11:20 GMT
No, imagine a treadmill made of ice.
There genuinely would be no friction in that case.
You could rev those engines to destruction and the plane would go nowhere because the air is not moving under the wing, therefore no lift.
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Post by edinburghstokie on Jul 13, 2009 15:13:11 GMT
How does a plane take off on water or ice then? The wheels aren't connected to the engines.
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Post by edinburghstokie on Jul 13, 2009 15:15:05 GMT
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Post by beagriestache on Jul 13, 2009 15:20:17 GMT
Well because ice is not truly frictionless, I used that to try to make it easier to picture the plane going nowhere on the treadmill.
The important bit is the fact that the plane is relatively stationary. If the speed of the treadmill itself equals the speed of the wheels on the treadmill which is a direct result of the thrust from the engines and friction between the wheels and the treadmill, then the plane remains stationary, there is no lift and the huge thrust force from the engines is cancelled out.
My brain hurts
Just seen your diag - yes the thrust of the engines is 40000, but that translates to lets say 100 at the wheels to get them turning (the plane is heavy remember), the rest is wasted as heat. The treadmill is also say 100 in the opposite direction, therefore plane stays stationary, no lift.
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Post by edinburghstokie on Jul 13, 2009 15:25:04 GMT
Hold on. The speed of the treadmill is going at the exact opposite of the plane's speed. The wheels will turn twice as fast so it will go forward.
Treadmills cancel out forward force that pushes off the ground (e.g., cars, legs). The planes engines have nothing to do with the ground. They just push the plane forward.
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Post by jonesinamillion on Jul 13, 2009 15:26:01 GMT
What a great quandry, i must admitt i didn't give it sufficient thought first time around.
I'm gonna go for yes now as the wheels will not even come into the equation.
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Post by beagriestache on Jul 13, 2009 15:27:10 GMT
You may have missed my edit. Yes they push the plane forward but only by translating engine thrust into wheel impetus which is a result of friction at the treadmill.
The engine thrust is a bit irrelevant.
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Post by edinburghstokie on Jul 13, 2009 15:27:31 GMT
Consider this, take a toy car and tie a string to the front. Place it on a treadmill and pull it forward. pulling on the string represents the propulsion from the propeller, the wheels will spin as fast as they have to, but the toy car will move forward because there is a force pulling it in that direction. that is why the plane will take off. Now take a toy car with a motor and put it on the treadmill with the speed of the car and the treadmill equal, it will not move because the speed is powered by the force supplied to the wheels.
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Post by edinburghstokie on Jul 13, 2009 15:28:33 GMT
What a great quandry, i must admitt i didn't give it sufficient thought first time around. I'm gonna go for yes now as the wheels will not even come into the equation. Wahey, I have one converted! |
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Post by Deleted on Jul 13, 2009 15:28:38 GMT
What about the fog?
With an owl….up a tree.
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Post by jonesinamillion on Jul 13, 2009 15:29:45 GMT
What about a fog-tree up an owl?
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Post by beagriestache on Jul 13, 2009 15:30:07 GMT
OK, so what you're basically saying is the plane is moving faster than the treadmill?
Yes, deffo it'll take off. Like at airports!
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Post by Dr. Doofenshmirtz on Jul 13, 2009 15:30:25 GMT
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Post by beagriestache on Jul 13, 2009 15:30:39 GMT
What about the fog? With an owl….up a tree. PMSL ;D |
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