slartibartfast Posted February 5, 2008 Share Posted February 5, 2008 I told you so What's "Physics: 101?" School was soooo long ago! That was Rog's way of telling us all why we were wrong and he was right earlier on. He was saying it was basic physics. It's an americanism. As I recall Doug, you were a late convert last time.;) Link to comment Share on other sites More sharing options...
Deskpilot Posted February 5, 2008 Share Posted February 5, 2008 Yes indeed Slarti, and now, like all good converts, I have to spread the word ;) palexxxx, "need a demonstration", sounds like you're not converted yet, You're real name isn't Thomas, is it? :devil: Link to comment Share on other sites More sharing options...
Guest brentc Posted February 5, 2008 Share Posted February 5, 2008 Sorry Airsick, I did plaigarise yes, but I did quote the website where is came from following the post. It is hard to believe until you watch the video that the increase in conveyor speed doesn't affect the aircraft as only a small amount of thrust is required to keep it level. Many people have assumed frictionless wheel bearings which is not required as the friction doesn't increase with speed. I should start another thread on playing Roulette. Whilst each event (spin) is mutually exclusive, others out there think the chances of a colour, or odd or even coming up increase........ they don't. Link to comment Share on other sites More sharing options...
Captain Posted February 6, 2008 Share Posted February 6, 2008 However rolling resistance of the tyres should increase with speed. Link to comment Share on other sites More sharing options...
youngmic Posted February 6, 2008 Share Posted February 6, 2008 Rolling friction typically reduces with increasing velocity. Sounds at odds with common sense but that is apparently the case, (Aerodynamics for Naval Aviators, pg. 184-185). However the case is based on a constant AoA take off. Rolling friction being based on the force pressing the wheels onto the runway. A tricycle type would, in reality I guess have an initial increase to Vr then a linearly reduction there after, provided a zero (effective) AoA used in the initial take off roll. A tail wheel type would be closer a constant linear reduction, with a slight momentary increase at the point where the tail is raised. I can only assume that the drag created by the force exerted by weight is of a sufficient magnitude to nullify the energy required to continue accelerating the mass of the tyre to Vu (unstick speed). M Link to comment Share on other sites More sharing options...
Guest RogerRammedJet Posted February 6, 2008 Share Posted February 6, 2008 The only thing the videos prove is that if the aircraft produces enough thrust to overcome the effects of the conveyor - the aircraft will move foreward! That's kinda obvious! Then there is that font of knowledge, "Mythbusters" - yeah, right! Rog Link to comment Share on other sites More sharing options...
Yenn Posted February 6, 2008 Share Posted February 6, 2008 Could we relate this to an aircraft carrier sailing downwind? Link to comment Share on other sites More sharing options...
Guest brentc Posted February 6, 2008 Share Posted February 6, 2008 Could we relate this to an aircraft carrier sailing downwind? Not really, because we're talking about moving the runway, which in the conveyor's case isn't affecting the apparent wind. An aircraft carrier sailing down-wind will affect the wind strength, which assuming is less than the take-off speed of the aircraft won't make a difference other than the aircraft having a much longer takeoff distance. Roger, you still need convincing by the sounds of it. I'm all out of ways to try and explain it. Captain, why do you think the friction would increase with speed? The bearings offer a set resistance that doesn't increase. The same surface area is in contact with the conveyor at all times. Even if it did, slightly, the aircraft will still fly. I have changed the way I think about these things after I purchased a yacht that sails at up to 1.4 times the speed of the wind ! Yes, it's true! Link to comment Share on other sites More sharing options...
Guest airsick Posted February 6, 2008 Share Posted February 6, 2008 I have changed the way I think about these things after I purchased a yacht that sails at up to 1.4 times the speed of the wind ! Yes, it's true! You bolted the Jab motor to the back of it didn't you! :big_grin: Link to comment Share on other sites More sharing options...
Downunder Posted February 7, 2008 Share Posted February 7, 2008 Of course it will NOT fly. No air over the wings, no lift. What is a runway for? To get forward movement. Why? To get airflow over the wings. Why? Airflow over the wings gives lift.Why? Lift counters gravity/weight and hence said craft will fly. Over and out !! Link to comment Share on other sites More sharing options...
crashley Posted February 7, 2008 Share Posted February 7, 2008 If the airplane used a direct drive to the wheels like in a car to gain enough speed and thus airflow over the wings and it was on the now famous conveyer it would not take off but planes use a prop to develop their speed and the conveyer can't stop the prop moving the plane forward so It will TAKE OFF Perhaps someond could demonstrate a model car on a treadmill and then you will see my point Link to comment Share on other sites More sharing options...
Guest airsick Posted February 7, 2008 Share Posted February 7, 2008 I just posted this in another thread but I think it is relevant here too. It is a video of the Top Gear guys firing a rocket powered mini down a ski slope. If the mini was on wheels and tried to propel itself via wheel power it would not go anywhere as it cannot gain any grip, thus the snow is acting much like the conveyor belt in this question - the wheels slipping is akin to the conveyor belt moving backwards. Using rockets to overcome this the mini does move (admittedly there is also gravity at work but if you watch the video you will soon realise that gravity would not propel the mini this fast on its own). The result? There is airflow over the mini and any wings that are attached to it, which is none in this case. Even it doesn't convince you it is still good to watch. :) https://www.youtube.com/watch?v=VirvXgExICM Link to comment Share on other sites More sharing options...
markendee Posted February 8, 2008 Share Posted February 8, 2008 "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)" To those who say no airflow over the wings as the aircraft is sitting still answer me this. Why would the conveyer be moving at all unless the aircraft was moving? So if the aircraft is at full noise but not moving and the conveyer is not moving where is all that thrust going? Fun isn't it? Link to comment Share on other sites More sharing options...
Ultralights Posted February 8, 2008 Share Posted February 8, 2008 What is a runway for? To get forward movement. Why? To get airflow over the wings./QUOTE]so what happens when the aircraft leaves the runway? Link to comment Share on other sites More sharing options...
Guest Baphomet Posted February 8, 2008 Share Posted February 8, 2008 It doesn't matter a jot if the wheels are being driven or if the prop is providing the thrust, the force stopping the forward motion is friction. Friction is dependent upon a few things, (type of surface establishes coefficient of friction) but the main one is mass. Don't believe it? try dragging an empty/full trailer around. read this http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#plo However "Static frictional forces from the interlocking of the irregularities of two surfaces will increase to prevent any relative motion up until some limit where motion occurs. It is that threshold of motion which is characterized by the coefficient of static friction. The coefficient of static friction is typically larger than the coefficient of kinetic friction." What this means is less force is required to keep something moving than is required to initiate movement. In the case of rolling friction about 10 times less force. Therefore I'm going to have to recant - If the aircraft has enough thrust to get the wheels moving, once moving it now has 9/10ths of the thrust required to initiate movement available to accelerate the aircraft to flying speed. The condition imposed by the question i.e This conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction) cannot be achieved, therefore it will fly. On the other hand, if the condition could be met (magic) then all of the available thrust would be used overcoming friction, and it wouldn't fly :-) Link to comment Share on other sites More sharing options...
Ultralights Posted February 8, 2008 Share Posted February 8, 2008 what about a floatplane taking off on a river against the outgoing tide? Link to comment Share on other sites More sharing options...
Downunder Posted February 8, 2008 Share Posted February 8, 2008 so what happens when the aircraft leaves the runway? It flys.......... Link to comment Share on other sites More sharing options...
Guest Redair Posted February 9, 2008 Share Posted February 9, 2008 What is a runway for? To get forward movement. Why? To get airflow over the wings./QUOTE]so what happens when the aircraft leaves the runway? Answer: It runs onto the grass. Oh come on, that was an easy question, the one about the conveyor was far more taxing, (or was that taxiing, or should it be taxying, oh I'll just go with the Welsh tacsiing). Redair. Need another lie down, although the doctor says my brain IS normal!!! Link to comment Share on other sites More sharing options...
Ultralights Posted February 9, 2008 Share Posted February 9, 2008 i was thinking, that if the aircraft is using the runway for fwd speed, once it leaves the runway, then it has no method of gaining forward speed.. Link to comment Share on other sites More sharing options...
hihosland Posted February 9, 2008 Share Posted February 9, 2008 Reading some of the explainations one could conclude that if the aircraft did land on aforesaid traveling runway it would have a roll out of zero. !!! Envisage doing a touch and go on this runway,. If a touch and go is possible then the original aircraft would take off. dem's my thoughts Davidh Link to comment Share on other sites More sharing options...
eyecast Posted February 9, 2008 Share Posted February 9, 2008 Will the plane take off Movement through air gives lift. Ground movement has no effect unless air moves over a flying surface. It is possible to take off and fly backwards in relation to the ground. eyecast:pig::pig: Link to comment Share on other sites More sharing options...
Guest Dukes Posted February 10, 2008 Share Posted February 10, 2008 If you run the treadmill at 5mph and turn on the plane's engines just slightly, they will provide enough thrust, pushing against the air, to keep the plane still. If you then increase the treadmill speed to 500 mph, you won't need to adjust the throttle on the airplane - it will remain stationary. That's because it's seeing the same frictional force that it was at 5mph. Thus, it doesn't matter how fast the treadmill is moving - if the pilot does not want to remain stationary, then he won't. Brent I must say this selection I quoted is the key to the question. Took some convincing but I can see the light now. Everyone wrap your head around this aspect and you will say yes. The conveyor simply cant keep the aeroplane still relative to the surrounding. Chris Link to comment Share on other sites More sharing options...
Bigglesworth Posted February 12, 2008 Share Posted February 12, 2008 Obviously planes and Physics don't occur together This is an interesting question. It is also a very stupid question. It is interesting that no-one else realises this. Consider: 2 possible scenarios: 1 The conveyor reference ground (VcrefG) = -VprefG (plane refence ground) 2 Or,VcrefG = -VprefC (plane refence conveyor) In scenario 1, any vehicle will move to any speed providing that it can tun its wheels twice as fast as it is traveling. Disagree? Take VvrefG=x (vehicle reference ground) Then VcrefG=-x The difference between them is |VvrefG-VcrefG|=2x And that holds true for any value of x. Therefore scenario 1 would not hold a car in position. And I think that is the scenario that most people envisage. So, scenario 2: VprefC (plane ref. conveyor) = -VcrefG I.E the conveyor belt will do its best to hold the vehicle in position. In a car scenario: VvregC=x where x is any speed up to about 200km/h depending on vehicle. Vcrefg=-x which is feasible. Therefore VvrefG= x+-x=0 i.e. vehicle is stationary (I hate to think of the driver getting out to push:black_eye:) Now The Finale A plane is on the belt. Where x=0 the situation is the same as for a car. BUT, when the plane applies power: VprefG=>0 which means that VcrefG increases (or, to be exact, decreases {goes backward}) in order to put the situation back to equilibrium. But since the speed of the plane is not influenced by the belt, VprefC>-VcrefG. This holds true for any value of VcrefG and so the conveyor will continue to increase its speed until it reaches infinity. (or self-destructs first). Either that of the wheels fall off the plane which would mean it would not take off. So it comes to a structual contest between the plane and conveyor. And, as such, the question is stupid, because it cannot be solved theoretically. Link to comment Share on other sites More sharing options...
Admin Posted February 12, 2008 Share Posted February 12, 2008 ;) Link to comment Share on other sites More sharing options...
flie43 Posted February 12, 2008 Share Posted February 12, 2008 Bloody hell Biggles, my head is spinning now, can I get off the conveyor ;) Tezza 1 Link to comment Share on other sites More sharing options...
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