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09/16/2011 04:07:32 PM · #76 |
Originally posted by Cory:
From what? |
A fan in FRONT of the plane (pointing astern) would create an airflow over the wing, providing lift; i.e., you have a mini-wind tunnel.
A fan blowing forward -- essentially having engines thrusting forwards and backwards at the same time, would indeed hold the plane still.
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On the original problem ...
The moving conveyor belt only very minimaslly affects the flow of air above it.
Imagine that the conveyor belt is made of that magnetized rubber used to hang refrigerator art, and that the plane's tires are steel-belted (with magnetized metal), with the belts on the outside, so that magnetic levitation lifts the plane a millimeter or so off the belt.
The belt now starts moving backwards at 100 mph, while the plane's engines simultaneously are set to develop a forward thrust calculated to generate an airspeed of 100mph in still air.
Does the plane move forward, stay still, or move backwards? What happens when the engine thrust is increased to 200mph-equivalent?
If the plane's wheels have normal bearings and the brakes are not set, you have exactly the same situation as the frictionless plane above, less a slight correction for the minimal friction in the bearings and between the tire and the road -- you might have to compensate by increasing the engine thrust to 100.01mph or so ...
BTW: I wonder that no one has pointed out that every time a plane takes off, the Earth is moving underneath it at up to 1100mph or so, and it doesn't seem to affect the take-off much* regardless of the direction it is going.
*The situation is different for rockets intended for orbit and beyond, which need all the assistance they can get to achieve speeds of upwards of 5 miles/second. That's one reason the US spaceport is at Cape Canaveral in Florida, rather than at Edwards AFB in California; so they can take off eastwards from a low latitude to take advantage of the Earth's rotation, while also being over the ocean in case of a Challenger-like disaster.
Message edited by author 2011-09-16 16:16:05. |
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09/16/2011 04:42:46 PM · #77 |
Originally posted by NikonJeb:
Am I missing something here, or did Steve  Yo_Spiff already mention the fly in the ointment? No, the wheels don't move the plane, the engines do, but what makes it FLY is the lift from the wings. If the conveyor belt matches the groundspeed, then effectively there's no air moving across the wings. No air moving across the wings, no flight, right? |
The plane's engines push the plane through the air by pushing against the air around the plane. That's what makes the plane move. It's propelled forward the same way it is in the air. If the plane flew over a conveyor belt that matched it's speed relative to the ground, would it then fall from the sky? THE GROUND IS NOT INVOLVED IN THE PROPULSION OF THE PLANE...PERIOD.
If the plane is going 10mph, the belt is moving at -10mph, the plane's wheels simply will turn as if the plane were going 20mph. |
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09/16/2011 04:43:45 PM · #78 |
Originally posted by GeneralE:
BTW: I wonder that no one has pointed out that every time a plane takes off, the Earth is moving underneath it at up to 1100mph or so, and it doesn't seem to affect the take-off much* regardless of the direction it is going. |
Which is the point that makes it clear that the conveyor belt is a red herring. The only thing that matters is the airspeed moving over and under the wings. |
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09/16/2011 05:11:11 PM · #79 |
Relativity, hutch cha cha .... what a show .. The plane moves relative to the air not the ground. A plane could be going a hundred miles and hour relative to air, but be going backwards relative to the ground and still be flying :-)
Message edited by author 2011-09-16 17:11:47. |
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09/16/2011 06:12:38 PM · #80 |
Originally posted by millsa:
Originally posted by HawkinsT:
So essentially what we have here is you assume the plane will take off based on it being able to move forward, which could only come about by the wheels slipping. I on the other hand, am asserting the plane will not take off due to it not being able to move forward as I assume the wheels don't slip... so we are simply interpreting the poorly-defined system differently. Would you agree on this? |
The wheels don't slip, the question is impossible to produce in the real world, the conveyor can never match the speed of the wheels once a thrust is applied. This is because the speed of the wheels is always the speed of the conveyor + the speed from the forward velocity of plane. ie always greater than the conveyor. |
This is wrong. It can be reproduced. The question posits that the conveyor moves at a speed that is the opposite of the speed of the plane relative to the surrounding ground, not opposite to the speed at which the plane's wheels spin.
Assuming that the air is stationary:
The plane moves at 100 mph relative to the ground around it.
The treadmill travels at -100mph relative to the ground around it.
The wheels spin as if the plane were travelling at 200mph on the ground.
The plane is still travelling at 100mph relative to the air around it. And so it takes off.
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09/16/2011 07:26:49 PM · #81 |
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09/16/2011 07:33:59 PM · #82 |
Next question: elliptical machine...
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09/16/2011 08:35:08 PM · #83 |
Originally posted by Matthew:
Originally posted by millsa:
The wheels don't slip, the question is impossible to produce in the real world, the conveyor can never match the speed of the wheels once a thrust is applied. This is because the speed of the wheels is always the speed of the conveyor + the speed from the forward velocity of plane. ie always greater than the conveyor. |
This is wrong. It can be reproduced. The question posits that the conveyor moves at a speed that is the opposite of the speed of the plane relative to the surrounding ground, not opposite to the speed at which the plane's wheels spin. |
I'll give a third approach. The treadmill can never, ever, ever stop the airplane. Ask yourself how you stop a plane through its wheels? (The only place the treadmill can interface with the plane) You can't. The wheels are just spinning on an axle. |
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09/17/2011 12:05:42 PM · #84 |
Originally posted by DrAchoo:
Originally posted by Matthew:
Originally posted by millsa:
The wheels don't slip, the question is impossible to produce in the real world, the conveyor can never match the speed of the wheels once a thrust is applied. This is because the speed of the wheels is always the speed of the conveyor + the speed from the forward velocity of plane. ie always greater than the conveyor. |
This is wrong. It can be reproduced. The question posits that the conveyor moves at a speed that is the opposite of the speed of the plane relative to the surrounding ground, not opposite to the speed at which the plane's wheels spin. |
I'll give a third approach. The treadmill can never, ever, ever stop the airplane. Ask yourself how you stop a plane through its wheels? (The only place the treadmill can interface with the plane) You can't. The wheels are just spinning on an axle. |
I love this formulation best--it makes the mental image quite plane... (haha) |
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