Flyer Posted October 12, 2016 Share Posted October 12, 2016 Posted by WolstonFlyer on 12/10/2016 21:54:12: Posted by Flyer on 12/10/2016 21:52:22: The question was though ' can it fly'........ The question says 'can it take off' , not can it fly. I apologise Wolston, it's the merlot talking ! But same applies, aircarft CAN take off. That answers the question. Now where did I leave those thinners.............. Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 12, 2016 Author Share Posted October 12, 2016 Posted by Pete B - Moderator on 12/10/2016 21:48:06: Once it reaches take-off speed, it will fly..... But will it reach take-off speed? Please be aware that this is not my question, I am just being naughty posting it here for you good people to have a go at. Quote Link to comment Share on other sites More sharing options...
Flyer Posted October 12, 2016 Share Posted October 12, 2016 You are answering a different question BEB; the question was can it take off, not will it Now awaits the wrath of the moderator............ Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 12, 2016 Author Share Posted October 12, 2016 You need some more wine Flyer, Quote Link to comment Share on other sites More sharing options...
Flyer Posted October 12, 2016 Share Posted October 12, 2016 Posted by WolstonFlyer on 12/10/2016 22:04:51: You need some more wine Flyer, Always . It's just the glass that gets smaller....... now maybe that's another thread.......... Quote Link to comment Share on other sites More sharing options...
Harry Twist Posted October 12, 2016 Share Posted October 12, 2016 If the wheel rotational speed always matches the conveyor speed but in the opposite direction- then - whatever the thrust from the engines, the aircraft will remain totally stationary. If it is stationary the wings cannot generate any lift at all - so no it cant take off and wouldnt even move forwards or backwards. My analogy would be - imagine being on a running machine and as you constantly increase your pace you balance that by constantly speeding up the conveyor that you are running on - you will stay in the same place on the machine. I reckon.......!! Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted October 12, 2016 Share Posted October 12, 2016 I’d have also thought no, simply by the fact that if the engines were not running but the belt was going backwards the plane would move backwards. If the engines then were then started and the thrust resulting in the wheels turning forward to exactly match the belt under all conditions the plane would simply stand still. Thus to me it’s quite difficult to see how it could take off. How far does a person on a running machine actually move forward? All the forward movement is transformed into rearward movement, resulting in the runner staying in one place. PB Edit. Pipped at the post, Harry… Great minds and all that! Edited By Peter Beeney on 12/10/2016 22:45:41 Quote Link to comment Share on other sites More sharing options...
David perry 1 Posted October 12, 2016 Share Posted October 12, 2016 If theres air flowing over the wings, yes. If not, absolutely no. From the information in this question it seems to be suggested rhat the aircraft is stationary with respect to the base of the conveyor belt, ie the wings arent going forwards. In such a case tjat bird aint gonna fly. Quote Link to comment Share on other sites More sharing options...
David perry 1 Posted October 12, 2016 Share Posted October 12, 2016 A trickeir conundrum is rhis, imagine youre driving a car with the windows open and your passenger starts flying his quad inaide the car, holding it stationary. He then slides it sideways rhrough the window. Wjat bhappens?Conversely, he flies the quad outside, pacing your car. Then slides it sideways in thriugh a window...what happens?D Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 12, 2016 Author Share Posted October 12, 2016 Tricky question David Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted October 12, 2016 Share Posted October 12, 2016 It will take off. It's not a car - it isn't driven through its wheels!! The wheels are therefore irrelevant. They are just casters! The drive (thrust) is being supplied by the jet engines, which require no contact with the ground. If you want to visualise what would happen imagine the exact equivalent sitution (physically) of four ropes tied to the engine nacelles and the aircraft being pulled along by them via some mechanism. If you pulled hard enough and fast enough (which the 747's engine would in practice of course) it will take off. BEB Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 12, 2016 Author Share Posted October 12, 2016 Another vote for Yes.... are you sure? The belt exactly matches the speed of the wheels rotating but the opposite direction, so does the plane ever move forward? Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted October 12, 2016 Share Posted October 12, 2016 yes - I'm sure! Try a free-body-diagram on it and you'll be sure too! BEB Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted October 12, 2016 Share Posted October 12, 2016 Hands up if you googled "free body diagrams" Interesting stuff Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 12, 2016 Share Posted October 12, 2016 My immediate thought was as BEB but I was assuming this was in the real world. I somehow suspect that such a conveyer belt could not be built with any materials/technology available today but assuming the conditions stated could be matched, it's impossible for the aircraft to be moving forward through the air if the conveyer belt is moving backwards at the same rate that the aircraft's wheels are rotating. However, if the brakes could stop the wheels rotating and sufficient lubricant was applied to the belt (stupid, but just as likely as building such a conveyer - or teflon pads were strapped to the tyres?) within the conditions specified, there's no reason why the aircraft couldn't then move forward relative to the belt, so I will stick with my original judgement that it could take off! Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 12, 2016 Author Share Posted October 12, 2016 Interesting isn't it! Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 13, 2016 Author Share Posted October 13, 2016 OK because it is late and I am not cruel: I agree that the problem is stupid and has no true solution, it is a trick question. Statement A: if the treadmill moves in the opposite direction of the wheels, always at the exact same speed of the wheels, the wheels cannot ever move in reference to the treadmill. Statement B: the engines generate an enormous force in the opposite direction of the plane, therefor, due to the law of equal and opposite reaction, the plane has an equally enormous force pushing it forwards. Statement C: in a free body diagram of a frictionless system, there is no force opposing the force of the engines Combining these statements shows that it is impossible for the plane to move in relation to the treadmill; however, there is no force stopping it from doing so. These statements are contradictory and therefor the problem is void. If you really want to have a read go here Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted October 13, 2016 Share Posted October 13, 2016 Sorry no. The conveyer is merely a frictionless support. So a direct analogy would be imagine the aircraft was on a sheet of ice. Could it take off? Of course it could!Second one - all the conveyer does is remove the effect of traction. But if an aircraft required traction to move forward it couldn't fly could it because when it's flying it's not in contact with the ground! BEB Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 13, 2016 Share Posted October 13, 2016 As there is no mention of the belt's coefficient of friction, why can't we simply assume the material used has a negligible value and take off with the wheels locked? Works for me! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 13, 2016 Share Posted October 13, 2016 Great minds (in your case BEB!) ? Edited By Martin Harris on 13/10/2016 00:24:12 Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted October 13, 2016 Share Posted October 13, 2016 If there is no slip between wheels and belt then dynamically it is frictionless Martin Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted October 13, 2016 Share Posted October 13, 2016 Indeed Martin! Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted October 13, 2016 Share Posted October 13, 2016 Would it be possible for you to send me the two envelopes containing money W.F i promise not to open first and send one back Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 13, 2016 Share Posted October 13, 2016 There must be more energy being applied to the rolling road (allowing for mechanical losses) than is being generated by the aircraft in order for the conditions specified to be met and maintain equilibrium - does this impact on your free body explanation (I don't know - just wondering). There can be no doubt that if the wheel's rotation is matching the belt speed and there's no slip as per the original question, the aircraft isn't moving relative to the surrounding air - disregarding air movement generated by friction along the length of the belt. As stated by WF - a trick question in reality! Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 13, 2016 Author Share Posted October 13, 2016 OK The engines apply thrust and the plane would roll forwards with the tyres touching the belt, we all get that, perfectly normal. At the same exact instant the belt moves in the opposite direction because as the question states, it is "designed to exactly match the speed of the wheels, moving in the opposite direction". Is the plane moving forwards? How can it be, the wheels are rolling forwards at the same speed that the belt is moving backwards. Then the engines power up more to push the plane forwards, the wheels are still in contact with the belt and the belt instantly matches that change in speed, the plane does not move forwards, it is sitting still with the wheels turning forwards at the same speed as the belt running backwards. For the plane to move forwards and get any airspeed to generate lift the wheels have to turn forwards faster than the belt is moving backwards. The question is a paradox designed to create discussion Edited By WolstonFlyer on 13/10/2016 00:49:31 Quote Link to comment Share on other sites More sharing options...
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