| Forum Home | ||||
| PC World Chat | ||||
| 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 | ||
| 410012 | 2005-12-04 22:25:00 | Good fun. I greatly respect Terry's knowledge of physics but Drspy made more sense to me. However I looked at Safari's link where the issue is better explained. I now agree that the plane would take off. The fundamental error, which is really the trick, is the assumption that the wheels drag the plane backwards on the conveyor belt. Thus the engine needs to push forward to keep the plane in one spot. But all the wheels are doing is holding the plane off the ground. So if they were frictionless then the conveyor could move at any speed you like and the wheels would just turn on the spot. Imagine the conveyor is made of ice. Same result. The plane doesn't move backward. Obviously in the real world there is some friction between the wheels and the ground but this is a thought experiment which can ignore that. And it only takes a little bit of thrust to overcome the friction anyway. So we have an aeroplane sitting on a fastmoving conveyor belt with it's engines roaring. Now here is the error. We assume that the plane will be dragged backwards if the engines are turned off. But in the absence of wheel friction, there is no drag. The plane will silently sit on one spot, wheels spinning madly. Turn the engines on and there is nothing to prevent the plane being thrust forward - and it will take off. So the trick is that the question appears to imply that the plane will not move forward - but there is no such implication. The plane is free to move itself through the air. |
Winston001 (3612) | ||
| 410013 | 2005-12-04 22:37:00 | hmmmmmm........lets add one more factor............beside the plane as it is initially parked there is a man standing still.......he wont move at all throughout the exercise........now......when the 'action' happens will the plane, regardless of the conveyor belt/thrust, or whatever move relative to the man ..........if so , if it moves 'away' from the man in a forward direction then after a while once speed has built up it will take off........if it doesn't move relative to the man it wont take off............. so...........which is it ? |
drcspy (146) | ||
| 410014 | 2005-12-04 23:01:00 | there is a man standing still.......he wont move at all throughout the exercise........ Haven't you ever seen Billy Connolly's travelator sketch? The plane will as usual fly through the AIR with the greatest of ease and rather than stand still the guy will be flung off the conveyor belt. |
PaulD (232) | ||
| 410015 | 2005-12-04 23:04:00 | nope the man is standing BESIDE the conveyor ..........he's in a fixed position..... | drcspy (146) | ||
| 410016 | 2005-12-04 23:11:00 | 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 :) I know that you are right, Terry, but I still don't get it. The purpose of floats and skis is just a mechanism to enable the planes to move along the ground smoothly so that the air flow across the wings would be greater. :confused: :blush: Cheers :) |
Renmoo (66) | ||
| 410017 | 2005-12-04 23:17:00 | 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 |
Dally (6292) | ||
| 410018 | 2005-12-04 23:20:00 | yes what I am trying to ascertain and have asked several times is simple if there is a fixed point of reference......in my example a man standing beside the conveyor..... does the plane move relative to this point cause if it doesn't then it CANT take off... |
drcspy (146) | ||
| 410019 | 2005-12-04 23:24:00 | hmmmmmm........lets add one more factor............beside the plane as it is initially parked there is a man standing still.......he wont move at all throughout the exercise........now......when the 'action' happens will the plane, regardless of the conveyor belt/thrust, or whatever move relative to the man ..........if so , if it moves 'away' from the man in a forward direction then after a while once speed has built up it will take off........if it doesn't move relative to the man it wont take off............. so...........which is it ? Once it has cleared the ground i.e. got lift, then it will move away from the man in a forward direction. One day it will dawn on you .. only the airspeed matters. :D |
gibler (49) | ||
| 410020 | 2005-12-04 23:26:00 | Of course the plane will move backwards if the power is turned off.The suggestion that it will sit still with it's wheels spinning is ludicrous. Then as power is applied the backward movement will slow untill the backward speed ant the forward thrust is equal. Then what happens? In real life my guess is the wheel struts would break. |
JJJJJ (528) | ||
| 410021 | 2005-12-04 23:29:00 | jeezus..................that plane CANT get LIFT unless it has got air travellign over the wings............how the hell is it going to lift off if it's NOT travelling away from the man, (reference point).......as someone else said......if its sitting still, (brakes on or tied down or whatever) it CANNOT lift unless there is air travellign over the wings...........and it dont matter wether its got propellors or jets there will be NO air travelling over the wings untill and unless the plane MOVES relative to some fixed point............. | drcspy (146) | ||
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 | |||||