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| Thread ID: 64128 | 2005-12-04 07:01:00 | Sunday night Brain Bender - Airplane Physics | miknz (3731) | PC World Chat |
| Post ID | Timestamp | Content | User | ||
| 410002 | 2005-12-04 20:32:00 | science.howstuffworks.com | Scouse (83) | ||
| 410003 | 2005-12-04 21:00:00 | Lol internets. Right, it's all good fun :D Ok lets get back to some basics, but first drcspy's fixed point problem. The aircraft is initially sitting on a non moving runway with it's engines shut down and it's sitting there under the influence of gravity. The surrounding air is still, the question says so, there is no wind to confuse the issue. So the frame of reference for the plane is the earth, and aerodynamically, the frame of reference is the non-moving air around it. Now for some basic physics. There are two principal pressures to measure in a moving stream of fluid. The static pressure measured at right angles to the the flow, and the dynamic head pressure measured facing into the flow. The dynamic head is the static pressure plus a velocity squared component, hence that is how the pitot tube can measure air speed. When air passes over an aerofoil, or an aerofoil passes through air, the velocity over the top of the airfoil is higher than the flow under the airfoil, due to angle of attack and shaping of the aerofoil section. The higher velocity has a lower static pressure than the slower velocity under the aerofoil. Hence because of Fred Bernoulli's theorem, there is a resultant lift. Now for a moving runway to have any, whatsoever, influence on the air flow over a planes wings it would have to drag the atmosphere with it. Now it does, but only to a very small extent, I dont know how many times I have to repeat this. It is called a boundary layer. Wheel bearing friction came into it initially as I said wheel speed was the only difference to a normal take off. Wheel speed increases if the runway moves against the planes direction, and decreases if it moves with the plane. Since when the planes engines start up and the pilot opens the throttle, thrust is produced, the plane will start to move, and the runway starts to move also according to the question. As others have also said the wheels are free spinning, the friction is not that high and the brakes are off, so the plane itself doesn't know the runway is moving underneath it. But it does know it is moving through the surrounding air which is stationary relative to the earth, it knows because it feels lift on the wings. And the pitot tube begins to register. This process will continue as the plane accelerates, and until the pilot calls out "rotate" at round about 140 knots ;) The wheels will have left the runway, and except for the wheel bearings, the plane would not have known the runway was moving under it. I dont really need to add any more :dogeye: |
Terry Porritt (14) | ||
| 410004 | 2005-12-04 21:11:00 | Let’s try another approach . (we’ll go round one more time and then crash land) :groan: If the plane is capable of flying with the conveyor turned off, then it is capable of flying with it turned on . Why, because the conveyor has nothing to do with whether the plane will fly or not . Now, can I please be credited NZCEA or whatever it’s called these days? :D |
B.M. (505) | ||
| 410005 | 2005-12-04 21:46:00 | Okay, that was a bit of a trick one. Yes, the plane will take off. Even though the wheels are matching the speed of the conveyer, its the thrust (the reaction between the engines/prop/jets/whatever and the air) that will be propelling the plane forward. Those wheels would be rotating at a hell of a speed by the time the plane got enough lift to take off though. The wheels/conveyer thing is a bit of a red herring. If the conveyer was moving in the takeoff direction, at the right speed and rate of acceleration, it is conceivable that the plane could lift off without the wheels revolving at all. I think... Let's think for a while how does a plane fly. Forward thrust - wheel accelerate relative to the stationary ground- air flow across the wings - upward thrust induced - up it goes. You can argue that the forward thrust will be still there to push the plane forward, but let's not forget that the wheel is needed to act as a mechanism for the plane to do so. For the air flow to pass by the wing, the wheel need to move in order to bring the aeroplane into movement. When the motion of it is constantly being canceled out by the conveyor belt, its relative acceleration to the ground would be nil. Don't you think so? Cheers :) P.S. Who is on my side besides JJJJJ? |
Renmoo (66) | ||
| 410006 | 2005-12-04 22:01:00 | The wheels and convor belt have nothing to do with it. If you didnt mind scratching the bottom of the plane you could take off with the wheels in the up position. If a plane could fly without moving forward, why do we need long runways?!? |
Rob99 (151) | ||
| 410007 | 2005-12-04 22:02:00 | Let's think for a while how does a plane fly. Forward thrust - wheel accelerate relative to the stationary ground- air flow across the wings - upward thrust induced - up it goes. You can argue that the forward thrust will be still there to push the plane forward, but let's not forget that the wheel is needed to act as a mechanism for the plane to do so. For the air flow to pass by the wing, the wheel need to move in order to bring the aeroplane into movement. When the motion of it is constantly being canceled out by the conveyor belt, its relative acceleration to the ground would be nil. Don't you think so? Cheers :) P.S. Who is on my side besides JJJJJ? Look the wheeks do sod all (during takeoff), apart from keeping the belly of the plane off the runway. The wheels in this problem will be "freewheeling", if they are locked then it would get messy. The plane can accelerate, with regards to its airspeed - the belt can't do anything to stop it. |
gibler (49) | ||
| 410008 | 2005-12-04 22:03:00 | James, James, James, many planes don't have wheels they have floats for use on water or skis for use on ice :eek: Some planes used to be catapaulted off ships, so they didn't have any runway! I'm surprised no one brought up the red herring of aircraft carriers :) |
Terry Porritt (14) | ||
| 410009 | 2005-12-04 22:07:00 | Initially, I read the problem as being only the conveyor was moving with the thrust not being applied by the plane. In this case, it will not take off. Rereading the problem, the plane is applying thrust, so it will be able to move relative to the air and generate uplift. James, wheels have nothing to do with the plane being able to move forward along the conveyor once thrust is applied. Seaplanes have no wheels, the thrust of the plane overcomes any friction from the skids and water until it is moving fast enough to generate lift and take off. Think of the wheels as simply being a source of friction which the thrust must overcome for the plane to move forward relative to the ground and air. Snap, Terry |
user (1404) | ||
| 410010 | 2005-12-04 22:19:00 | The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. There is no wind. Can the plane take off? Note that the conveyer belt is NOT moving in the opposite direction relative to the speed of the plane. In this case the conveyer belt is moving in the opposite direction relative to the speed of the wheels The conveyor belt will accelerate to match the opposite direction of the wheels untill either, something breaks. Or the friction in the wheels equal that of the wind force created by the plane engines. Therefor it is my opinion that the plane will not takeoff.. Unless the plane is able to overcome the friction in the wheels. |
J_Joyce (6569) | ||
| 410011 | 2005-12-04 22:23:00 | Now either we get Macgyver to rig up a makeshift plane to test or we actually test it in real life to prove any theories. | bob_doe_nz (92) | ||
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