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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 | ||
| 410032 | 2005-12-05 01:57:00 | The problem with this in my opinion is the *magic* conveyor. It will exactly match the speed of the wheels. Now with friction in effect the wheels will create drag on the plane. (same as a seaplane taking off into current for those who like the seaplane reference) My belief is that the plane would take off.. BUT only when the wheels slid which would eventually happen. If our magic conveyor held traction on the wheels I think we would get some impressive RPM and heat on the wheels and no take off. -Qyiet |
qyiet (6730) | ||
| 410033 | 2005-12-05 02:00:00 | 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.Which of course is just stuff and nonsense. What people who believe that argument are suggesting, is that regular aircraft can just hop up and fly without wheels, a la Harrier or helicopters. :stare: |
Greg (193) | ||
| 410034 | 2005-12-05 02:00:00 | I don’t believe all this. :groan: Anyone who thinks “God Defend New Zealand” is not an appropriate anthem will need to do a rethink! :D :D :D |
B.M. (505) | ||
| 410035 | 2005-12-05 02:12:00 | My revised opinion is that NO, it won't take off. You are so far wrong Greg, that you are into -10 out of 10 territory. You are making too many assumptions and overlooking simple facts of mechanical and aeronautical behaviour. The opposite of this absurd conundrum is a plane on a zero-friction surface. Will it take off twice as fast? Of course not! Reverse the "runway" direction and the wheels wil go backwards, but will the plane take off faster? It is the speed at which the jet thrust is expelled, or the combined aerofoil effect of the propellors as they pull (not push) the plane forward (by the same principle as the lift of the wings) that determines its movement, and that relates to the mass of the air, mildly influenced by the rolling friction of the undercarriage. You have been bamboozled by irrelevant information, intended to have just that effect. Think about the plane taking off from an ordinary runway, the wheels do nothing except keep it from flopping to the ground. So what difference does it make how fast those wheels turn, even if the runway suddenly decided to roll backwards? The two effects are quite separate, the thrust wins, the plane takes off. Cheers Billy 8-{) |
Billy T (70) | ||
| 410036 | 2005-12-05 02:19:00 | Lock the wheels into a sort of conveyor (steam Catapult) and then we're talking! | PaulD (232) | ||
| 410037 | 2005-12-05 02:25:00 | Greg, all the wheels do is keep a space between the plane and the ground . They aren't driven by the engines . IT'S NOT A CAR . Friction exists between the wheels and the ground . It keeps the area of each wheel in contact with the "ground" stationary with respect to the "ground" . If the axle moves wrt the ground, the wheel rotates . If the ground moves wrt the axle, the wheel rotates . If the "ground" somehow "knows" the direction and magnitude of the wheel's rotation around the axle, and moves to provide a "counter" effect, the wheel will just rotate twice as fast . I think you'll run out of power to drive this several-mile-long conveyor belt with this positive feedback mechanism . Thrust produced by the engines (whether jet or propellors) causes the plane to move relative to the air . All the "flight speeds" V1, V2, etc, are air speeds . The plane doesn't care whether the "ground" it's standing on is moving . Aircraft carriers turn into the wind if possible to help their planes take off, by adding the ship's speed to the air speed produced by the plan's engines . The plane's speed relative to the air matters . The speed relative to the deck doesn't . There have been many cases of planes flying at 150-200 mph, while not moving at all relative to the ground below, or even "backwards", because of head winds . Tail winds have the opposite effect . |
Graham L (2) | ||
| 410038 | 2005-12-05 02:34:00 | Glad to see someone agree with me :) | JJJJJ (528) | ||
| 410039 | 2005-12-05 03:04:00 | Imagine if you will that on this perfectly calm day there is a slab of still air in front of the plane . In order to get lift the plane's wings have to cut through this air with some going over the wings and some going under . In the example in question the plane never gets to this slab of air which remains a constant distance from the wings . Therefore the plane cannot take off . 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 Good post . I've switched sides . :D I had another look at the original question: 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 . There is no wind . Can the plane take off? Gravity holds the plane on the ground . The premise in the question is that the wheels never move forward while in contact with the runway . If they try to, the runway speeds up in the other direction . Thus the plane can never move forward while the wheels remain touching the runway . Its position remains stationary . Without air crossing the wings, takeoff is impossible . |
Winston001 (3612) | ||
| 410040 | 2005-12-05 03:07:00 | Oh c'mon Billy and Graham - all you've managed to point out is that the plane needs air-speed to get airbourne . That's already an established fact . But it actually needs to get that airspeed first, remember? It ain't going very far forwards nor upwards with it's wheels like skates on a reverse-motion surface! :D |
Greg (193) | ||
| 410041 | 2005-12-05 03:26:00 | Greg: Forget the engines for a moment . Imagine a glider, with a tow rope going to a winch at the other end of the runway . It's supported on the conveyor belt with its wheels . When the winch starts, the glider will accelerate towards it . It doesn't matter what the conveyor belt does . As long as the wheels have decent bearings, they will spin . But they don't affect the speed of the glider . It's moving as a result of the pull of the tow rope . The plane's engines work against the air causing the plane to move in the direction defined by the thrust . Any rotation of the wheels while it's on the "ground" is just a byproduct of the plane's motion . As the centre of rotation of a wheel (the axle) moves relative to the surface the wheel is touching, the wheel will rotate to keep the contacting area stationary wrt the surface . It has to because of the friction . That's all there is to it . If you now cover the impossible conveyor with some impossible coating which has zero friction against the rubber of the wheels . The wheels won't rotate at all . The plane will still move in the direction defined by the thrust of the engines, until it reaches take off speed . (I know this works with skis on ice (and the skis don't rotate at all) . I've been in Hercules and Starlifters which do this often . |
Graham L (2) | ||
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