JuZ Posted March 3, 2006 Share Posted March 3, 2006 Lifted from another forum.. A plane is standing on a runway that can move (some sort of conveyer belt). The plane moves in one direction, while the conveyer moves in the opposite direction. 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). Can the plane take off? Opinions please Quote Link to comment Share on other sites More sharing options...
Chesterfield Posted March 3, 2006 Share Posted March 3, 2006 My vote is No. A plane takes off due to how air flows over its body and wings, in a nutshel the air takes a longer route over the plane than it does under it, which creates lower pressure above the plane, and higher pressure below it. This causes the plane to lift. Its excatly what happened to the lemans car some years ago that went of the track, it acted like a plane wing, and took off. No air going over the plane and its wings, no lift, no take off. Quote Link to comment Share on other sites More sharing options...
M13KYF Posted March 3, 2006 Share Posted March 3, 2006 Exactly what i thought. The plane needs air flow over the wing to cause lift which is why I also voted no. Quote Link to comment Share on other sites More sharing options...
Chesterfield Posted March 3, 2006 Share Posted March 3, 2006 If you could get a big enough fan, and tie a plane to the ground with a long chain.. if the fan blows hard enough, you could get the plane to lift off the ground. What a surreal conversation... LOL Quote Link to comment Share on other sites More sharing options...
Andy Posted March 3, 2006 Share Posted March 3, 2006 Agreed!! Quote Link to comment Share on other sites More sharing options...
JuZ Posted March 3, 2006 Author Share Posted March 3, 2006 Well I'm saying yes, as a plane uses thrust generated by the engines to move and that isn't affected by how fast the wheels are turning so it would take off as normal although the wheel speed would be much higher! Quote Link to comment Share on other sites More sharing options...
M13KYF Posted March 3, 2006 Share Posted March 3, 2006 But the plane isn't moving forward so their is no flow of air over the wings generating lift. Quote Link to comment Share on other sites More sharing options...
JuZ Posted March 3, 2006 Author Share Posted March 3, 2006 What's stopping the plane from moving forward? Quote Link to comment Share on other sites More sharing options...
Traktion Posted March 3, 2006 Share Posted March 3, 2006 Voted no, but then thought about it - Of course it's YES. The wheels are just holding the stationary mass of the plane off the floor. The thrust comes from the jet engine, which pushes against the air and NOT through the wheels. As a result, the only thing the wheels can do to "slow" the plane down, is cause some resistence. It doesn't mention that the wheels have brake, so I'll conclude that it'd only be the resistence in the bearings that would slow the plane at all. Essentially, the wheels have a negligible affect on the acceleration of the plane from the jet thrust. If the power was coming through the wheels, then the belt could keep the plane still, thus not providing the wings with air flow to create lift. TBH, the question is clever though as it fools you into thinking the plane would be stationary, when in fact, it'd take off with the same easy as if it was on a normal runway Quote Link to comment Share on other sites More sharing options...
Guest prescience Posted March 3, 2006 Share Posted March 3, 2006 But the plane isn't moving forward so their is no flow of air over the wings generating lift. Quite right, the plane only moves forward on a runway in normal circumstances to generate lift by the angle of incidence of the wing to the air flow. Lift can be generated in other ways; jump jet, helicopter etc but you need it to take off Quote Link to comment Share on other sites More sharing options...
Kuro Posted March 3, 2006 Share Posted March 3, 2006 have I missed something here? So you are saying that no matter how fast the plane travels (i.e thrust generated) the conveyor matches the speed? If so, then the plane is theoretically stationary despite running at full power? (as if it's on a rolling road is easier to relate) Every reaction has an equal and opposite and all that, so the thrust generated is cancelled by the opposing motion of the conveyor belt. So, the plane will in fact never get off the ground, due to, as already stated, lack of lift. I hope. Could be quite embarrasing if I get this one wrong Quote Link to comment Share on other sites More sharing options...
Guest prescience Posted March 3, 2006 Share Posted March 3, 2006 It would be interesting to consider what would happen if the conveyor belt suddenly stopped when the plane was very close to take-off speed. It would then have a huge instantaneous acceleration relative to the ground (theoretically) although the tyres wouldn't hold the traction and might disintegrate - don't know Quote Link to comment Share on other sites More sharing options...
AK47 Posted March 3, 2006 Share Posted March 3, 2006 Chesterfield is on the ball! Sounds like a quote from a textbook...exactly right! Quote Link to comment Share on other sites More sharing options...
Traktion Posted March 3, 2006 Share Posted March 3, 2006 But what is keeping the plane stationary? Are you saying that by spinning the planes wheels on a conveyer belt, the plane won't be able to move forward? IF the plane increased it's velocity by powering the *wheels*, this would be the case. BUT it's isn't - it uses a jet engine to provide thrust (or props instead), which is pushing against the *air* at the back and not against the conveyer belt. The wheels have little/no bearing on the speed of the plane, so it makes little/no difference what speed the conveyer belt is running at. If it was a car with wings, powering itself up a runway from powing the wheels, then the conveyer belt *could* keep the car stationary (and stop any lift), but we're talking about engines that are pushing against the air - the wheels just stop it from sitting on it's belly and help when landing EDIT: P.S. I want my vote switched to YES! Quote Link to comment Share on other sites More sharing options...
Guest prescience Posted March 3, 2006 Share Posted March 3, 2006 The wheels are the only thing in touch with the ground (conveyor belt) and the only thing that can move the plane forward (imagine you are an observer off to the side of the plane) so as they increase their rotation rate in this hypothetical example the conveyor belt increases its speed and the plane remains stationary. Like a person running on a treadmill Quite a good Friday topic though Quote Link to comment Share on other sites More sharing options...
Chesterfield Posted March 3, 2006 Share Posted March 3, 2006 Im starting to think that it could take off... But then again maybe not.... As the plane starts to move forward, the speed of the conveyor increases and moves it back at the same rate. It makes no difference where the thrust comes from - the conveyor can still counteract the thrust by way of moving the plane back at the same rate it attempts to move forward. head hurts now, and I have to get some work done! Quote Link to comment Share on other sites More sharing options...
Nick U Posted March 3, 2006 Share Posted March 3, 2006 An intresting question. Well as im a fly boy for her Majesty (helicopters) I can quiet honestly say the plane is going nowhere fast. Due to the conveuir belt matching the aircraft speed, the airspeed of the plane would be 0, even though the wheels speed would be huge. therefore no airflow over the wings=no lift at all. Of course there is wind to take into acount, asuming that the plane is facing into wind, which is the norm for any takeoff and landing (although not essentual). This would provide a minute amount of lift (irrelevent if you are talking about anything bigger than a microlite though!! God, and here is me trying to get away from work. I'll dig out some pics if anyone is interested. Quote Link to comment Share on other sites More sharing options...
Traktion Posted March 3, 2006 Share Posted March 3, 2006 The wheels are the only thing in touch with the ground (conveyor belt) and the only thing that can move the plane forward The wheels are the only thing that can move the plane forward? If that was the case, how would the plane keep moving forward after it had left the ground? In fact, I can't remember ever sitting in a plane, while those tiny wheels frantically spun and slipped trying to accelerate a large passenger plane up a run way either! The jets/propellers power a plane forward and they do so by working against the air (not the ground). It's a trick question which is trying to make you think that the ground has something to do with it; it doesn't. Whether the conveyer belt is moving at 500mph or 1mph, the air above the conveyer belt (which is the important bit) with still be relatively stationary (maybe some turbulence from the belt, but that's it). The wheels are just spinning freely and it wouldn't matter how fast or slow they spun, they would still have practically zero affect on the acceleration of the plane - the only difference would be that the wheels would be spinning faster as the plane moved forward. I guess the only time you could say the ground would have something to do with it, would be if the wheel brake were applied. This would give some reverse force against the jet/props, but even then, given the tires would have finite grip, the huge power from the engines would eventually overcome this too. Quote Link to comment Share on other sites More sharing options...
Traktion Posted March 3, 2006 Share Posted March 3, 2006 It makes no difference where the thrust comes from - the conveyor can still counteract the thrust by way of moving the plane back at the same rate it attempts to move forward. But the wheels are spinning freely, yes? So the only affect this would have on the speed of the plane would be the friction in the bearings. IMO, the conveyor belt is just a red herring as the wheels have little to do with the equation. Quote Link to comment Share on other sites More sharing options...
Kuro Posted March 3, 2006 Share Posted March 3, 2006 I'll dig out some pics if anyone is interested. Sadly, yup Quote Link to comment Share on other sites More sharing options...
Nick U Posted March 3, 2006 Share Posted March 3, 2006 Trust me when I say 'the plane will not take off'. 0 airspeed = 0 lift. When you run on a treadmill you remain stationary with reference to the air around you. Same as the plane. It remains effectivly stationary to the air, hence no lift, no matter how fast the engine makes it go , the moving ground counteracts it. No airflow, no lift. Unless of course it was a harrier, and had some kind of vertical thurst!! Quote Link to comment Share on other sites More sharing options...
Nick U Posted March 3, 2006 Share Posted March 3, 2006 I'll dig out some pics if anyone is interested. Sadly, yup I'll stick some pics in a seperate topic. Quote Link to comment Share on other sites More sharing options...
Traktion Posted March 3, 2006 Share Posted March 3, 2006 I have no argument with their being no lift if the plane isn't moving. That's totally true. I do however think that the conveyor belt wouldn't provide enough resistence to freely spinning wheels to stop the plane going forward when it powered its jets/props. Quote Link to comment Share on other sites More sharing options...
Sarnie Posted November 23, 2006 Share Posted November 23, 2006 Bump No way it could take off Quote Link to comment Share on other sites More sharing options...
stevie_350z Posted November 23, 2006 Share Posted November 23, 2006 Yes the plane will take off, but its wheels will be moving twice as fast! Quote Link to comment Share on other sites More sharing options...
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