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09/17/2011 12:05:42 PM · #1 |
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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09/16/2011 08:35:08 PM · #2 |
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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09/16/2011 07:33:59 PM · #3 |
Next question: elliptical machine...
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09/16/2011 07:26:49 PM · #4 |
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09/16/2011 06:12:38 PM · #5 |
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 05:11:11 PM · #6 |
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 04:43:45 PM · #7 |
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 04:42:46 PM · #8 |
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 04:07:32 PM · #9 |
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 03:55:56 PM · #10 |
Originally posted by NikonJeb: 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? |
Originally posted by Cory: So, if you placed a fan on the front of the plane which was able to push enough air to effectively counteract the planes movement through physical space, then you would have no lift from the wing surfaces, |
The function of an airfoil is dependent on the airflow across it.
In your example, you *would* have lift. |
From what? |
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09/16/2011 03:54:20 PM · #11 |
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? |
Originally posted by Cory: So, if you placed a fan on the front of the plane which was able to push enough air to effectively counteract the planes movement through physical space, then you would have no lift from the wing surfaces, |
The function of an airfoil is dependent on the airflow across it.
In your example, you *would* have lift.
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09/16/2011 03:42:44 PM · #12 |
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? |
Fans move air, conveyer belts do not. :)
So, if you placed a fan on the front of the plane which was able to push enough air to effectively counteract the planes movement through physical space, then you would have no lift from the wing surfaces, although, a powerful enough engine (think rocket, not jet) would provide sufficient thrust to make the aircraft fly, but the lack of airflow over the control surfaces would cause a total lack of control.
Message edited by author 2011-09-16 15:47:38. |
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09/16/2011 03:31:01 PM · #13 |
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?
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09/16/2011 02:31:00 PM · #14 |
Originally posted by mike_311: Originally posted by Medoomi:
haha, yes, but in the field of pure extravagant speculation, it's theoretically (though not practically) possible for the conveyor to accelerate infinitely fast
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if you get infinite velocity, i get a friction free bearing :) |
yep, exactly--and I'd hate to calculate the speed anyway at which a normal bearing will heat sufficiently to actually cause some friction. Add in the lever arm of the wheel & to generate any meaningful resistance, it's got to be an astronomical speed... By which time the tire would have blown to smithereens...
Message edited by author 2011-09-16 14:33:33. |
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09/16/2011 02:25:09 PM · #15 |
Originally posted by Medoomi:
haha, yes, but in the field of pure extravagant speculation, it's theoretically (though not practically) possible for the conveyor to accelerate infinitely fast
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if you get infinite velocity, i get a friction free bearing :) |
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09/16/2011 02:15:19 PM · #16 |
Originally posted by mike_311: ... this whole link based on the assumption that the wheels can exert a force on the goddamn plane. The speed at which the wheels spin is irreverent, they exert no force in either direction!!! the wheels just spin and spin and spin, that's why they are F--ing wheels!!!
if the conveyor belt was moving twice, hell 50 times as fast as they needed to, the plane would still take the frig off!
until i see a proof that he conveyor belt gets force transmitted into the plane i will maintain my stance. |
haha, yes, but in the field of pure extravagant speculation, it's theoretically (though not practically) possible for the conveyor to accelerate infinitely fast--and thus for a wheel to rotate at such speed it will generate sufficient friction in the bearing (!) and/or heat fatigue the joint, or the centripetal force cause an explosion & so detain the plane.
But this said... the original forumlation specified the conveyor belt only rotating at the same forward speed as the plane. That speed won't come close to doing this...
Message edited by author 2011-09-16 14:33:05. |
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09/16/2011 01:50:34 PM · #17 |
goddammit im not moving on, this whole link based on the assumption that the wheels can exert a force on the goddamn plane. The speed at which the wheels spin is irreverent, they exert no force in either direction!!! the wheels just spin and spin and spin, that's why they are F--ing wheels!!!
if the conveyor belt was moving twice, hell 50 times as fast as they needed to, the plane would still take the frig off!
until i see a proof that he conveyor belt gets force transmitted into the plane i will maintain my stance.
im making a new shirt, "The F---ing wheels are IRRELEVENT!!!"
Message edited by author 2011-09-16 13:54:53. |
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09/16/2011 01:44:22 PM · #18 |
I read on another website that the Earth is flat. |
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09/16/2011 12:45:30 PM · #19 |
there is a problem in this article.
you can't add Vb and Vw, the units are different, you need to convert one before you can add it.. one is a lateral velocity (distance over time) and the other is a centripetal velocity (rotations over time).
Message edited by author 2011-09-16 12:46:23. |
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09/16/2011 12:35:32 PM · #20 |
lol! - nice link. Well, I did make sure and apologise in advance of starting this thread. It's also caused a few 'debates' here in the office today. |
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09/16/2011 12:32:21 PM · #21 |
Originally posted by HawkinsT: This is essentially the same as a rolling road, just a more awkward way to construct one. I rolling road built large enough for a plane would not allow it to take off. |
No.
The difference is in where the force is applied. The "rolling road" the propulsive force is generated by the tires pushing on the conveyor. With a plane, the engine applies thrust to the air surrounding the plane, pushing it forward.
For planes, the rough equivalent to the "rolling road" example you cite for cars would be a wind tunnel.
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09/16/2011 12:31:45 PM · #22 |
Originally posted by HawkinsT: You can increase the force generated by the planes engines as much as you like so long as you increase the force from the conveyor equally, and the plane will still not move. |
So... the conveyor has worked itself into a 1000mph frenzy, and the plane is still standing there wheels spinning like mad.
I stand behind the plane with a long metal rod and push really really hard. Is the conveyor belt capable of speeding up enough to generate enough friction to counteract that force? If I started pushing it, would appear to me that the plane is immovable? The rod will end up bending under the stress? |
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09/16/2011 12:30:20 PM · #23 |
Originally posted by HawkinsT: Yes, you're all repeating a simple concept that believe me, I am fully aware of, the wheels DO NOT propel the plane forward... however the force required by the engines to move the plane forward is directly related (when the plane is static), to the resistance of the wheels assuming a level runway and engines exerting their force perpendicular to gravity.
For example, if the wheels don't slip and all wheels have a circumference of 1m, and the plane is moving forward at 1m/s, the plane in 1 second will have moved 1m forward and made one full rotation of its wheels. Now if you place a surface under the plane that is moving backwards at 1m/s, after 1 second the plane (retaining the same force as stated before), will have moved neither forwards nor backwards. You can increase the force generated by the planes engines as much as you like so long as you increase the force from the conveyor equally, and the plane will still not move. This is essentially the same as a rolling road, just a more awkward way to construct one. I rolling road built large enough for a plane would not allow it to take off. |
not quite, you are still assuming the force is transferred through the wheel. in your example the force from the conveyor will cause the wheel to complete additional rotation but the force will NOT be exerted on the plane. while the plane still moves forward at 1 m/s from its own engines, while the wheels spin faster than they would normally due to the conveyor.
i keep harping on it but the wheels DO NOT transfer force. they spin freely. Cars move by exerting force on the wheels and the wheels exert a force on the ground, on a plane the spin free freely (at least during take off). |
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09/16/2011 12:28:38 PM · #24 |
Actually I think this would be a good read for everyone, and then hopefully we can all move on =).
//blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/ |
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09/16/2011 12:20:30 PM · #25 |
Yes, you're all repeating a simple concept that believe me, I am fully aware of, the wheels DO NOT propel the plane forward... however the force required by the engines to move the plane forward is directly related (when the plane is static), to the resistance of the wheels assuming a level runway and engines exerting their force perpendicular to gravity.
For example, if the wheels don't slip and all wheels have a circumference of 1m, and the plane is moving forward at 1m/s, the plane in 1 second will have moved 1m forward and made one full rotation of its wheels. Now if you place a surface under the plane that is moving backwards at 1m/s, after 1 second the plane (retaining the same force as stated before), will have moved neither forwards nor backwards. You can increase the force generated by the planes engines as much as you like so long as you increase the force from the conveyor equally, and the plane will still not move. This is essentially the same as a rolling road, just a more awkward way to construct one. I rolling road built large enough for a plane would not allow it to take off. |
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