WolstonFlyer Posted October 13, 2016 Author Share Posted October 13, 2016 Posted by Biggles' Elder Brother - Moderator on 13/10/2016 00:22:25: 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 Of course an aircraft can take off from a sheet of ice, the surface of the ice is not moving in the opposite direction to the rotation of the wheels, so the wheels roll forwards relative to the ice, the wings have air speed and then lift off. If the plane is on the huge conveyor belt and the belt started to move backwards slowly and the plane did nothing at all then the plane would travel backwards sat on the belt at the same speed as the belt - it is carried along on the belt just like shopping in a supermarket. So the plane starts its engines to provide thrust and now the wheels start to turn in relation to the surface of the conveyor belt, the engines provide enough forward thrust to turn the wheels fast enough so that the plane is no longer moving backwards sat on top of the belt, it is stationary, held in place by the thrust of the engines pushing it forward in relation to the belt. Now the plane increases its thrust to move forwards but instantly the conveyor belt increases its backwards speed to match the forward speed of the wheels, the plane is still not moving forwards in relation to the surface of the belt. If the belt is designed so that it can always match the speed of the rotation of the wheels but in the opposite direction does the jet plane ever get any forwards air speed to generate lift and take off? It's enough to make your brain melt ! Quote Link to comment Share on other sites More sharing options...
Rich too Posted October 13, 2016 Share Posted October 13, 2016 no Quote Link to comment Share on other sites More sharing options...
Don Fry Posted October 13, 2016 Share Posted October 13, 2016 No, the plane breaks. The belt travels at the rotational speed of the wheels. With jet engines trying to move the aircraft forward, and failing because the belt constantly speeds up, the rotational speed of the wheels becomes enormous, exceeding design conceptions and limits, and undercarriage failure is inevitable. The belt survives, its designers built for this scenario. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted October 13, 2016 Share Posted October 13, 2016 It's not a void or trick question. As your sub-heading says, it's a physics question. Those who apply the laws of physics will see that the plane will take off. Those that don't apply the laws of physics will argue all day that it won't, but they would be wrong. That's what the question is really about - getting people to argue. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 13, 2016 Share Posted October 13, 2016 How can you apply the laws of physics to a flawed scenario? The laws of physics would also say that neither the wheels nor the belt could achieve infinite speed, which would need to happen instantaneously - which is what the question's conditions would require. The wheels will also generate friction even though they are rotating in harmony with the belt surface - tyre cling, deformation, bearing friction, even friction between the air and the rotating periphery... The physicist will tell you that it will take off whereas the engineer will tell you that something must break. Edited By Martin Harris on 13/10/2016 07:53:33 Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted October 13, 2016 Share Posted October 13, 2016 OK, here's my interpretation of the laws of physics to this scenario. The thrust will push the plane forwards, overcoming drag until a point where the wing is generating enough lift for it to overcome the mass of the plane. At this point the it will be capable of flight. Let's say this speed is 150mph. The wheel will accellerate at the same speed as the plane. i.e. from 0 to 150mph. The conveyor will accelerate BACKWARDS matching the speed of the wheel. i.e. from 0 to 150mph. The only effect the conveyer has is to make the wheels rotate at twice the speed that they would do on a stationary runway, i.e. 0 to 300mph as measured on a wheel mounted speedometer, but the plane would still take off at 150 mph ground speed. Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted October 13, 2016 Share Posted October 13, 2016 Oh - and as an Engineer, heres how I'd build a working system. The high tensile steel rope attaches to the wheel on the plane, passes around a pully at the rear end of the conveyor, runs the full length of the conveyor ans attaches to an anchor on the top surface of the belt. The belt will match the speed of the wheels but in the opposite direction. Quote Link to comment Share on other sites More sharing options...
Andy Meade Posted October 13, 2016 Share Posted October 13, 2016 Posted by Phil Cooke on 12/10/2016 21:11:50: It really doesn't matter how fast the wheels go round, no forward motion of the airframe = no aerodynamic lift = no take off. I'm with Phil! Quote Link to comment Share on other sites More sharing options...
Bryan Anderson 1 Posted October 13, 2016 Share Posted October 13, 2016 Posted by WolstonFlyer on 12/10/2016 21:06:43: This simple little question has caused a lot of healthy discussion on Facebook today. Edited By WolstonFlyer on 12/10/2016 21:07:49 This is extremely badly phrased. "Exactly match the speed of the wheels". What is the speed of a rotating wheel? A wheel typically has two speeds: one translational and the other rotational. I think it means there is no slip between the tyre contact patch and the belt. Short answer: the plane will take off if the engines provide normal take-off thrust. Long answer. Start with no motion and no engine thrust. The situation is stable and nothing will move, ever. There is no problem. If, somehow, the initial conditions are such that the wheels are spinning and the belt moving such that the plane as a whole is stationary, then. in the absence of friction, that state will persist for ever. With friction, what happens depends on the inertia in the wheels /plane and the conveyor belt systems. Unless matched magically, I would expect some movement. Now begin again and start the engines. If there is no friction between the contact area of the tyres and the belt, the wheels will not rotate and just slide contradicting the conditions implied in the question. The plane will accelerate and take off. With friction, there will be a force in the backwards direction trying to rotate the wheel. This force must be supplied by the belt. As the plane accelerates, the wheel rotational speed has to increase so a force must persist. The belt system has to supply rotational energy to the wheel. Normally, the Earth does that. Now consider a very large wheel. If large enough, the contact area becomes almost indistinguishable from a caterpillar track in contact with the ground and with no relative motions. (Alternatively, zoom in on this area). The backwards velocity of the tyre at the contact patch relative to the hub exactly matches the forward velocity of the wheel hub, the plane velocity, So there is no problem. The belt does not need to move at all though it must be constrained. The Earth doesn't when a plane takes off (at least to an observer with poor sensors standing on the Earth). Quote Link to comment Share on other sites More sharing options...
John F Posted October 13, 2016 Share Posted October 13, 2016 Posted by WolstonFlyer on 13/10/2016 01:16:24: If the belt is designed so that it can always match the speed of the rotation of the wheels but in the opposite direction does the jet plane ever get any forwards air speed to generate lift and take off? It's enough to make your brain melt ! Not really. This reminds me of a discussion that ended up with a few members of a motorbike forum being sin binned for a few days as they pedantically argued whether a piston actually stops at TDC. Pedantic, and inconsequential, stuff about friction, roller bearings not taking the strain, the tyre not the wheels is moving, etc, etc, ad inifinitum et nauseum, to one side, the question is simple. The forward speed of the aircraft is negated by the reverse speed of the treadmill. Therefore, if there is no forward movement can the aircraft take off? Take off energy is supplied by the lift coefficient of air passing over the wing. No air moving over the wing = no lift. (VTOL capable aircraft are obviously excluded from this scenario!) Edited By John F on 13/10/2016 10:10:52 Quote Link to comment Share on other sites More sharing options...
Andy48 Posted October 13, 2016 Share Posted October 13, 2016 The forward speed of the aircraft is NOT negated by the reverse effect of the treadmill. The forward speed is provided by the jet engines and the plane will take off. Actually this is not a theoretical problem but one that happens every day any plane takes off. Let's not forget the speed of rotation of the Earth. If John F's theory is right, then every plane taking off against the direction of rotation of the Earth would get nowhere. The Earth's rotation is doing exactly what the conveyor belt is doing. The only difference is that the plane's wheels are not rotating to match the speed of the Earth. Another way to look at it is this. Imagine a plane where the power to take off was actually provided by the wheels. The plane would accelerate enough to take off, but once contact is lost with the ground, the plane would slow down and return to the ground. The plane would in effect stay on the runway at take-off speed. Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted October 13, 2016 Share Posted October 13, 2016 The simple empirical answer is yes, it will take off. Mythbusters proved this on tv, admittedly not with a 747 but with a lightplane. Unless the brakes are left on the plane doesn't care about the wheel speed, it only cares about airspeed, which the conveyor cannot affect. Quote Link to comment Share on other sites More sharing options...
Barrie Dav 2 Posted October 13, 2016 Share Posted October 13, 2016 I think that I'll go back to bed............... Quote Link to comment Share on other sites More sharing options...
Robert Welford Posted October 13, 2016 Share Posted October 13, 2016 Mention this on the RCMF forum and there will be a riot! I dare you ... Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 13, 2016 Author Share Posted October 13, 2016 Ahh Bob found the Mythbusters video, does that answer this specific question, they used a prop driven plane, not a jet, and does the speed of the 'belt' exactly match the speed of the wheels all the time, do they measure the speed of ether? Go on Robert, you cannot say a dare like that and then not do it. Of course this is a question that is designed to cause contradiction. Back in the real world the plane would simply power forwards with thrust from the engines, the wheels would rotate forwards faster than the belt is moving backwards (they freewheel) and the plane would take off . The illogical part is when you try and explain how the belt speed can "always equal" the speed of the wheels, the result would be the treadmill quickly spinning up to infinity. If you missed the earlier link, have a look here It is a bit of fun to get you thinking, there is no correct answer that matches the rules of the specific question as it is written. Edited By WolstonFlyer on 13/10/2016 11:45:16 Quote Link to comment Share on other sites More sharing options...
FlyinBrian Posted October 13, 2016 Share Posted October 13, 2016 Posted by Robert Welford on 13/10/2016 11:16:20: Mention this on the RCMF forum and there will be a riot! I dare you ... They argued about this one several years ago! Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 13, 2016 Share Posted October 13, 2016 Yes. In an equally stupid modification to this ridiculous scenario, my 747 is facing into a 200mph hurricane. So it doesn't need to move anywhere. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted October 13, 2016 Share Posted October 13, 2016 Newton's third law! Jet engines work by creating a reactive force against the air - not the ground!! This resultant force means they will accelerate relative to the air. (F=ma) The aeroplane is firmly attached to the engines so it will accelerate through the air as well. The airspeed will increase over the wings, lift will result and the aircraft will take off!!!!! Its all about the air - what the ground is doing is utterly immaterial! It could be still, it could be moving, it could be doing the bosa-nove - it matters not one jot! BEB PS Got to stop here - about to suffer a serious, frustration driven, sense of humour failure!!! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 13, 2016 Share Posted October 13, 2016 Sorry BEB - just take deep breaths! I'm sure this is a real world versus theoretical poser. You and I know that if the aircraft was on a moving road, the only effect would be that the wheels would spin faster than the road is moving but the terms of the question don't allow this. Quote Link to comment Share on other sites More sharing options...
Richard Wills 2 Posted October 13, 2016 Share Posted October 13, 2016 Speed is always relative to something, in this case a wheels speed is relative to the aircraft. If you assume that the belt is stationary at the start this speed is zero. So the belt is designed to match this speed relative to the wheels and aircraft of zero. What would happen is the thrust moves the aircraft forward, and to maintain zero speed relative to the wheel, the belt moves forward relative to the ground at the same rate as the aircraft, The wheels never actually turn, and the aircraft takes off normally. Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted October 13, 2016 Share Posted October 13, 2016 It seems to me there is not very much trickery about the question, is it not a case of just considering the drive medium, flexible or solid? If the brakes were full on and the engines run up to full power the plane wouldn’t move, the braking force exceeding that of the engines but all the ‘motion slip’, being taken up in moving the air, or ‘flexible drive medium’, through the engines rather than moving the plane forwards. If the brakes were then slightly released the plane starts to move forward and instantly pushes against a pressure switch which applies enough power to start the belt moving backwards sufficiently fast enough to match the force moving the plane forward; the whole mass of the plane is now being moved backward at exactly the same speed as the wheels are taking it forward; nett result: the pane remains stationary. The brakes are released further and the same thing happens until the power applied to the belt ultimately equals that applied by the full power of the engines. Two forces acting in opposition just moving vast quantities of air. The power required to drive the belt backwards has to be in excess of 4 jet engine thrusts in parallel. It is after all effectively holding the dead weight of the plane against the engines. If the drive medium were solid, i.e. an externally driven winch rope pulling the plane forward against the pressure switch the rearwards moving belt would again prevent the plane moving forward and if the belt drive power exceeded the winch motor power unless there were some form of slipping clutch the winch motor would not be able to move and thus stall. Or something else would break! If the wheels were being driven by a solid prop shaft from the engines the plane would still not move, two power sources again acting in opposition, but with the ‘ flexible slip’ now being the relatively increased movement between the belt and the wheels; the power requirements very considerable again. I would hazard a guess after all this and say that because the plane is never going to move forward it’s never going to take off. Maybe the real question should be is what happens to the wheels in amongst all of this? The force required to overcome the natural inertia of the wheels must increase on some sort of exponential curve and in the end they simply refuse to turn any faster. I suspect they will then cry ‘enough is enough’. In practise it might be difficult to try this, but with enough determination it might be possible to scale it down sufficiently to try it. I certainly won’t be doing it though… I agree with entirely with John F, if you consider anything other than the question exactly as stated then it does become meaningless and any answer might cover any situation. Rehashing the question slightly, let’s say we have an object, on wheels, standing stationary on a road with a casual observer to one side. Then the object is moved forward with any means of propulsion, the observer sees this at say 10 mph. Then the road moves in a rearward direction at the same 10 mph. What does the observer see then? It surely cannot be the same 10 mph forward, I think it would be zero mph forward, the two motions are now balancing each other out. The forward motion is zero but the power is translated into increased motion between the wheels and the road. If the forward motion were created by means of a winch the coefficient of friction might play a part and the wheels simply dragged across the surface; but if the mass downward of the object and the power applied to the rolling road were more than sufficient I think the winch would eventually be stalled again. I’ll stick with this one until I change my mind… PB Quote Link to comment Share on other sites More sharing options...
Don Fry Posted October 13, 2016 Share Posted October 13, 2016 BEB, yes you are right, but the belt is moving at a speed causing the wheels not to move. The energy being put into the system by the engines is negated by losses in the wheel system, and hence failure occurs in the undercarriage, It is designed for a couple of hundred kph, not the many thousands that belt has to travel at to cause so much energy to be put into destroying the undercarriage. Irrelevant whether a prop or a turbine doing the energy thansfer. Someone said above that rotation of the earth replaced this hyperthetical belt, not so, the earth moves the air at the same pace, allowing the plane to move through the air, and fly. This plane don't move, hence don't fly. The undercarriage fails. Edited By Donald Fry on 13/10/2016 13:01:57 Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted October 13, 2016 Share Posted October 13, 2016 This thread will go round in circles even faster than the wheels! That's what the question is designed to do. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 13, 2016 Share Posted October 13, 2016 speed of wheels exceeds maximum design rating of conveyor belt conveyor belt stops takeoff occurs as normal Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted October 13, 2016 Author Share Posted October 13, 2016 Everybody having fun? It is a brain twister after all Quote Link to comment Share on other sites More sharing options...
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