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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 | ||
| 410022 | 2005-12-04 23:50:00 | Another way to look at it is if on a normal runway the pilot applies sufficiently strong brakes to stop the plane moving forward then no matter what engine speed he applies he will not take off With enough forward thrust from the propellors or jets the locked wheels will just slide. Similarly my neighbour has a driveway steep enough that you can lock your wheels and gravity will keep you moving. The wheels just hold the plane off the ground and get used for low speed braking and steering. The engines act on the air. If this was a discussion about a submarine in water would there be the same difficulty in grasping the concept. |
PaulD (232) | ||
| 410023 | 2005-12-04 23:54:00 | Imagine a plane is sat on the beginning of a massive conveyor belt/travelator type arrangement, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation. 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 J_Joyce has a valid comment on the interpretation of the question. But "speed of the wheels" is also ambiguous as they have 2 speeds. The translational speed of the wheel axle in a straight line, and the surface speed of the tyre. They are not necessarily the same. I must confess I couldn't understand the proposition, and so I don't really know what the conveyer is trying to do, I interpreted it as the conveyer moving opposite to the direction of the plane. But consider this; the wheels are free spinning, and since the bearing friction is the only mechanical coupling between conveyer and plane, the conveyer can do what ever it likes without affecting the motion of the plane. Instead of wheels imagine the undercarriage replaced with frictionless hover pads, then the conveyer can have any weird speed without affecting the plane. |
Terry Porritt (14) | ||
| 410024 | 2005-12-04 23:55:00 | the engines provide thrust........but if the plane is tied down or the brakes appllied then thrust is NOT going to get you off the ground, unless it's directional thrust such as applied in a downward direction as in a VTOL aircraft. | drcspy (146) | ||
| 410025 | 2005-12-04 23:58:00 | the easiest/simplest way to put this to rest is for *someone* to please assure myself and any others who are confused that yes the plane will be travelling relative to a fixed point.........thus creating airflow over the wings and thus creating lift..............otherwise no matter how much thrust its got its NOT going to lift.......horizontal thrust does not and cannot supply lift it's air travelling over the wings that provides lift....... | drcspy (146) | ||
| 410026 | 2005-12-05 00:04:00 | There is no mention of brakes or tying down in the problem as posed, so it doesn't arise. The reference point or points is the planet earth, and the surrounding air is still relative to planet earth. Interpreting the problem literally suggests to me a feed back mechanism, whereby the faster the wheels turn due to forward motion of the plane, the faster the conveyer tries to go, it just doesn't make sense as proposed unless I'm missing something. The problem lies in interpretation of the phrase "...moving in the opposite direction of rotation". |
Terry Porritt (14) | ||
| 410027 | 2005-12-05 00:18:00 | OK D Maybe this will put you out of your misery. The plane is not bonded to the conveyor, it could stand still if tethered, go backwards if not. Dynamically it is referenced to the mass of the atmosphere, not the conveyor or the ground. When the pilot applies take-off power, that thrust will move the aircraft forwards "for every action there is an equal and opposite reaction". The wheels are just passengers, disinterested bystanders if you like. Application of take-off power will move the plane forward in relation to your "fixed point" and the wheels will roll freely on the surface of the conveyor. The conveyor cannot move the plane backwards to maintain its position because the wheels are free to rotate at any speed required, and nor can it counter the take-off thrust, which is relative to the mass of the atmosphere. The conveyor cannot inhibit forward movement, it can only affect the wheels' rotational speed because they are not driven. On thinking about it, wheel rotation speed could be "interesting" during take-off because of the notional feedback to the conveyor, but this will not prevent the plane from accelerating away from your fixed point, reaching a velocity sufficient to provide lift off, and disappearing skywards, at which point the conveyor will give up the unequal struggle and grind to a halt. Landing would be something entirely different, because braking the wheels would tell the conveyor to slow down. Cheers Billy 8-{) |
Billy T (70) | ||
| 410028 | 2005-12-05 00:39:00 | Wow, what a thought provoker. My understanding from reading all this and the links is that the question makes us think that the plane will never move forward relative to say someone standing beside it, but in reality it will, the wheels will just turn faster. The counteraction (is that a word?) of the treadmill won't hold it in one place, as thrust is coming from a source independant to the wheels. As it increases the rate of the air passing around the wings it will ascend. Jack and other FS people. Try a flight starting at CYLT. Select daytime, in Summer or it will be dark. Try and take off. If you cant get fast enough, slew up (y then F4) then resume flight, take a look behind you in spot view and watch the runway .... |
Jester (13) | ||
| 410029 | 2005-12-05 01:05:00 | www.imagef1.net.nz | JJJJJ (528) | ||
| 410030 | 2005-12-05 01:35:00 | My revised opinion is that NO, it won't take off. Much of the hugely in-depth theorising above has absolutley bugger all to do with the issue. Also far too much waffling! Given the original question assumes zero affects from friction, and no wind etc, then there's no way that gravity is going to fail to keep the aircraft pretty well attached to the conveyor. Sure the pilot could zoom his engines to full throttle and suddenly up the wheel-carriage - the plane will momentarily move forward, but then flop onto it's belly, and then face the same dilemna as it did when it had wheels on the ground. [edit] but this of course depends on how much power the plane has - if it's like got amazing power-to-weight ratio then it'll manage to stay airborne (even if just a few feet) for long enough for the engines to give enough forward thrust before the belly touches the ground, to allow the wings to grip the air |
Greg (193) | ||
| 410031 | 2005-12-05 01:52:00 | As Safari said: "The spinning wheels are not producing any force on the aircraft." ....and from the URL he gave... www.avweb.com While the speed of the conveyor belt in the opposite direction is superficially attractive in saying the airplane cannot accelerate, it truly is irrelevant to what is happening with the airplane, because the medium on which it is acting is the air. The only time it could be a problem is if the wheel speed got so high that the tires blew out. |
Strommer (42) | ||
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