chilli Posted October 11, 2008 Share Posted October 11, 2008 lol I kept out of this last time, but it's pretty obvious to realise the treadmill and spinning of the wheels are entirely irrelevant, it's just red herring to even worry about it. Unless, someone left the handbrake on Quote Link to comment Share on other sites More sharing options...
edge Posted October 11, 2008 Share Posted October 11, 2008 Would be nice to see a real plane on a tread mill. Quote Link to comment Share on other sites More sharing options...
chilli Posted October 11, 2008 Share Posted October 11, 2008 at least with a treadmill the plane can get up to speed without covering any ground, so it could take of without a runway, and runways do take up a lot of space Quote Link to comment Share on other sites More sharing options...
edge Posted October 11, 2008 Share Posted October 11, 2008 at least with a treadmill the plane can get up to speed without covering any ground, so it could take of without a runway, and runways do take up a lot of space Or just tie them down to the floor and let them spring off. Or put them in a big cannon and fire them off. Would make airports so much more interesting. Quote Link to comment Share on other sites More sharing options...
extendor Posted October 11, 2008 Share Posted October 11, 2008 Source - Mythbusters site - "In order for the conveyor belt to prevent the plane from accelerating, it must apply Drag equal to or greater than the Thrust generated by the plane's engines. Since the only point of contact between the conveyor and the plane is the plane's wheels, this force must be applied through the wheels. However, since the wheels of the plane are specifically designed to minimize the application of force from the ground to the plane, the conveyor is unable to apply enough Drag to prevent the plane from accelerating, and so the plane accelerates forward, gains airspeed, and takes off." Quote Link to comment Share on other sites More sharing options...
chilli Posted October 11, 2008 Share Posted October 11, 2008 Or just tie them down to the floor and let them spring off. hey isn't that almost what they do on aircraft carriers, a giant piston fires them forward to launch the aircraft and a cable to stop it when it lands, the g-force must be bonkers here's one for you guys though. Anyone know for sure of the brakes on an aircraft can hold it against full thrust? When we were going to Prague earlier in the year we had an aborted take off (moments before the point of no return) and they taxied it off and did some tests by thrusting up the engines whist stationary and the whole plane was shaking. I'm so glad they asked us to change planes later, wouldn't have exactly felt confident in it after that incident! Quote Link to comment Share on other sites More sharing options...
edge Posted October 11, 2008 Share Posted October 11, 2008 hey isn't that almost what they do on aircraft carriers, a giant piston fires them forward to launch the aircraft and a cable to stop it when it lands, the g-force must be bonkers here's one for you guys though. Anyone know for sure of the brakes on an aircraft can hold it against full thrust? When we were going to Prague earlier in the year we had an aborted take off (moments before the point of no return) and they taxied it off and did some tests by thrusting up the engines whist stationary and the whole plane was shaking. I'm so glad they asked us to change planes later, wouldn't have exactly felt confident in it after that incident! On a uk plane the brakes should be ok but i feel sorry for anyone going a j-spec. Quote Link to comment Share on other sites More sharing options...
Scott Posted October 11, 2008 Share Posted October 11, 2008 Yes but the original problem states that - to continue your boat analogy - as the wind picks up, the current also picks up to compensate, so the boat remains stationary. It doesn't matter how hard the wind blows, the current matches speed to counter any forward movement of the boat. The problem was not about whether a plane could take off from a treadmill running at a constant speed - which is why the oft quoted Mythbusters episode is moot; it can, it'll just need to generate more thrust than usual to counter the reverse action of the treadmill. So back to the plane; as the engines produce thrust the treadmill increases speed so that - via friction or whatever - the reverse forces on the plane are the same as produced by the engine. As such, the plane remains stationary as the treadmill and wheels spin at silly speeds until the wheels overheat and fall apart! I think there was some confusion over whether the treadmill countered the plane speed or the wheel speed. I saw it as the former. In that scenario, you would be as well anchoring the plane to a wall. There is no point in saying if force x = force y would they cancel each other out. Of course they would. The question is whether the same thing would happen with a plane, as what happens with a car. If a car travels at 30mph on a treadmill that is doing 30mph in reverse will it move... no. If a plane is doing 30mph and then u turn on a treadmill to -30mph will the plane move?..... yes!! Only it will be at a slower speed, possibly around 28mph due to friction. In order to stop the plane you would need to have the treadmill at 500mph which is a ridiculous conclusion to come to. Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted October 11, 2008 Share Posted October 11, 2008 Just give up mate, just give up... lol Quote Link to comment Share on other sites More sharing options...
tbourner Posted October 11, 2008 Share Posted October 11, 2008 treadmill goes backwards at a speed of -10, plane produces thrust for a forward speed of +10, plane moves forward at +10 while wheels spin at +20. What if, while keeping everything else the same, the plane drops it's thrust to 7? Forward speed of +7, wheel speed of +17? Now drop the thrust to 3...forward speed of +3, wheel speed of +13? Now drop the thrust to 0, forward speed of +0, wheel speed of 10? See, the "it's not a car, the wheels don't power it/matter" seemed logical until that last step. From the "it would take off" argument you should be able to set the treadmill to any speed and the plane will remain stationary as the wheels free wheel to compensate. No cos the conveyer is tuned to match the speed of the plane, so at +10 the conveyer would do -10, wheels would do +20. At +7 the wheels would do +14, at +3 the wheels would do +6 and at 0 the wheels would do 0 cos the conveyer has stopped. I can't see this being the case, so the treadmill via the wheels does produce a backward force on the plane, meaning you have to produce a little bit more thrust to get to take off speed. But each time you try to do this, the treadmill increases speed to counter. This of course relies on this theoretical treadmill being able to counter the speed instantly of course. Yeah the wheels do produce a backwards force, but as said they are designed to be as frictionless as possible, that's their job after all, so the additional thrust required by the engines to counteract the friction from the wheels would be about 0.0000001%. Quote Link to comment Share on other sites More sharing options...
Ian C Posted October 11, 2008 Share Posted October 11, 2008 I'd just like to point out I was: First non-divvy Thank you. Quote Link to comment Share on other sites More sharing options...
OhGod Posted October 11, 2008 Share Posted October 11, 2008 Hmm, it seems - from your explanations above - that I was giving too much credit to the friction in being a drag force. My thinking was with a plane sat on said treadmill with engines off, the treadmill builds up speed to -10MPH, so the plane is now moving - although the wheels will not be turning - at -10MPH too. Then the plane would have to produce thrust for +10MPH to match the treadmill, and a further +1MPH to gain any forward movement - at which point the treadmill speeds up to -11MPH and the plane remains stationary. Seemed logical to me anyway, but from what you've said above, and with a bit more thinking myself, I'm guessing that once you're over the initial inertia and got the wheels turning that the friction forces - and therefore the drag forces - drop to very little? As such - as Scot says - the treadmill would need to go at -400MPH to counter. So, the plane does take off, and no explosions were caused - well, none by said treadmill anyway. Thanks for explaining where I was going wrong rather than the typical "You're stupid because your answer is different than mine but I'm not going to provide any proof for my stance, or disprove yours, because I *know* I'm right and you should trust me" or the "lolz, it's not a car, the wheels don't provide any thrust therefore I'll just ignore them completely and pretend they don't provide any drag either" - summarising, of course. No cos the conveyer is tuned to match the speed of the plane, so at +10 the conveyer would do -10, wheels would do +20. At +7 the wheels would do +14, at +3 the wheels would do +6 and at 0 the wheels would do 0 cos the conveyer has stopped. Sorry, the bit you're replying to wasn't meant to be governed by the problem's rules, it was an attempt to show that although the wheels aren't powered, and don't supply any thrust, they do supply drag - but as mentioned above I think I was overstating what effect this would have. Now, what about if... Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted October 11, 2008 Share Posted October 11, 2008 Quote Link to comment Share on other sites More sharing options...
Swampy442 Posted October 11, 2008 Share Posted October 11, 2008 Lol Ohgod, take it you've not seen the Mythbusters episode in question As for the slagging front, think you got off lightly Quote Link to comment Share on other sites More sharing options...
Pot Posted October 11, 2008 Share Posted October 11, 2008 I called my Dad today to double check my opinion (that the plane would take off), he's a retired engineer who used to design many parts for planes and so on (Smiths Industries, and Dowty for anyone local)... Anyhow, even if it was a frictionless (And of infite length - corrected below) treadmill, he answered instantly that the plane would take off, the treadmill would move, but the wheels would not rotate, only because of the grip from the tyres pulling the treadmill along... Assuming that it was a treadmill that went forwards, as well as backwards (the 'normal' way) It's also worth considering (a fact I didn't know), that many large aeroplanes have other engines in the wheels, to give that initial bit of inertia for them to get moving... - So, the treadmill would start moving without the plane moving, but then the thrust from the planes wing engines would take over, due to the air that's being pushed... Quote Link to comment Share on other sites More sharing options...
Pot Posted October 11, 2008 Share Posted October 11, 2008 Oh, and he also commented, it'd fall of the end of the treadmill, so his answer's after I told him it's a treadmill of infinite length... Quote Link to comment Share on other sites More sharing options...
RobSheffield Posted October 11, 2008 Share Posted October 11, 2008 If the treadmill is moving, how do the refueling trucks get to it? Eh? Eh? Eh? Eh? Quote Link to comment Share on other sites More sharing options...
Pot Posted October 11, 2008 Share Posted October 11, 2008 They're like, parked next to it... Quote Link to comment Share on other sites More sharing options...
RobSheffield Posted October 11, 2008 Share Posted October 11, 2008 They're like, parked next to it... No, cos the treadmill is moving And what about the passengers? How do they get to the terminal? Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted October 11, 2008 Share Posted October 11, 2008 No, cos the treadmill is moving And what about the passengers? How do they get to the terminal? photocopier! Quote Link to comment Share on other sites More sharing options...
RobSheffield Posted October 11, 2008 Share Posted October 11, 2008 photocopier! I cant see how that will help Gaz.... Quote Link to comment Share on other sites More sharing options...
OhGod Posted October 11, 2008 Share Posted October 11, 2008 And what about the passengers? How do they get to the terminal? They're provided with powered roller skates at check-in. Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted October 11, 2008 Share Posted October 11, 2008 I cant see how that will help Gaz.... well if you factor in the coefficient drag caused by the wheels and the power cord on the copier, and the office mule taking a picture of his ass, and the fact that I just burnt my arm on the bastard cooker, and I've drunk a lot of wine, I think the plane will strawberry off high over the horizon. Quote Link to comment Share on other sites More sharing options...
Pot Posted October 11, 2008 Share Posted October 11, 2008 No, cos the treadmill is moving And what about the passengers? How do they get to the terminal? No, they're on their own treadmill... What sort of cheapskate (no pun intended) unit do you think this is, with only one treadmill!... Quote Link to comment Share on other sites More sharing options...
michael Posted October 11, 2008 Share Posted October 11, 2008 What in-flight film are they showing? Quote Link to comment Share on other sites More sharing options...
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