Guest RogerRammedJet Posted February 5, 2008 Share Posted February 5, 2008 "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)." The senario does not say anything about frictionless bearings etc! So, let me try it this way. 1) the aeroplane is sitting stationary on the conveyor belt, which is also stationary 2) the aeroplane applies a small amount of thrust and moves forward at 0.00000001 kts. Instantaneously the conveyor belt moves in the opposite direction at 0.00000001 kts - the end result being that the aeroplane remains stationary relative to its surroundings and therefore the parcel of air that surrounds it ie there is no airflow over the wings and therefore no lift 3) the thrust is increased in increments that result in 0.00000001 kt increases speed relative to a stationary conveyor belt, but the conveyor belt also responds with matching speed increases - the end result being that the aeroplane remains stationary relative to its surroundings and therefore the parcel of air that surrounds it ie there is no airflow over the wings and therefore no lift 4) when the aeroplane is at full thrust - the conveyor belt has matched each of the thrust increases and the aeroplane has remained in a stationary position relative to its surroundings - there is no airflow over the wings - therefore no lift - therefore the aeroplane cannot fly 5) the aeroplane is just sitting there - engines screaming - wheels spinning like crazy - conveyor belt whizzing along. Rog Link to comment Share on other sites More sharing options...
tvaner Posted February 5, 2008 Share Posted February 5, 2008 I can see it now. ;) The large motorised conveyer arriving at Echuca behind a truck for everyone to try out their theories. What a weekend that would be. ;) Brilliant stuff. Link to comment Share on other sites More sharing options...
Guest brentc Posted February 5, 2008 Share Posted February 5, 2008 Rog, think about what I am saying. It doesn't have to be an aircraft. We are technically talking about an engine effectively on a stand, on wheels, which is on the treadmill. Think of it in your lounge room on the treadmill (that's where my treadmill is). Will the speed of the treadmill have any effect whatsoever on the engine? Because there is so little friction on the wheels, it will just zoom off and take off. eg. It will take 200rpm on the prop to counter say 10kmh of treadmill action. After that, any more RPM will cause the engine to move forward, because any increase in treadmill speed has neglibible effect on the wheels because the friction on a bearing based wheel doesn't increase over a certain speed! You are probably confused with a car. If a car with the wheels driven by the engine increases speed on the conveyor and the conveyor increases, it will never go anywhere, however we are being driven through the air just like a rope attached to the front of the treadmill and then we pull on that rope as someone said earlier and we move forward. Same goes with the prop. What we need is someone with one of those backpack powered aerochute things with the engine on their back and a large treadmill. Does anyone have an aerochute we can use for this? Link to comment Share on other sites More sharing options...
BigPete Posted February 5, 2008 Share Posted February 5, 2008 Look at it this way - remove the wings - will it fly? - NO Now put the wings back on and take away the lift - will it fly? NO regards Link to comment Share on other sites More sharing options...
Flyer Posted February 5, 2008 Share Posted February 5, 2008 What we need is a government grant of say 5 million. Then build an airpark with a small conveyor belt, buy a few planes and test the theory. Should be able to drag it out for a couple of years....;) Regards Phil 1 Link to comment Share on other sites More sharing options...
Guest Decca Posted February 5, 2008 Share Posted February 5, 2008 You guys are gunna drive yourselves NUTS!!! Decca. Link to comment Share on other sites More sharing options...
Guest Redair Posted February 5, 2008 Share Posted February 5, 2008 What we need is a government grant of say 5 million. Then build an airpark with a small conveyor belt, buy a few planes and test the theory. Should be able to drag it out for a couple of years....;)Regards Phil Will this conveyor be able to reach light speed? Just wondered as I would then be able to turn up long after the plane had gone, in time to see that it was indeed unable to take-off. Redair. Link to comment Share on other sites More sharing options...
Flyer Posted February 5, 2008 Share Posted February 5, 2008 No but I've just shortened an aircraft carrier. Spin a conveyor belt up to warp nine and then stop it. The conveyor should absolutely catapult the aircraft off the deck... Regards Phil Link to comment Share on other sites More sharing options...
Admin Posted February 5, 2008 Share Posted February 5, 2008 You guys are gunna drive yourselves NUTS!!!Decca. Link to comment Share on other sites More sharing options...
Guest RogerRammedJet Posted February 5, 2008 Share Posted February 5, 2008 All you need to test this is a treadmill and an electric R/C aeroplane. Been there done that! Trust me, I am right - which is generally the case! Rog Link to comment Share on other sites More sharing options...
Guest mudjeep Posted February 5, 2008 Share Posted February 5, 2008 Most of you are making it a lot more complicated than it actually is, but some of you have nailed it. Roger, please expain how the conveyor is exerting a force on the aircraft if the wheels are free-wheeling, other than negligible bearing friction. The wheels and conveyor have no influence on whether the aircraft takes off. The propeller will still bite at the air and pull itself forward with no resistance capable of being supplied by the wheels unless the brakes are on. The plane will fly no matter what the ground is doing. Link to comment Share on other sites More sharing options...
Guest brentc Posted February 5, 2008 Share Posted February 5, 2008 I took the time to watch Airsick's Youtube link just now. Quite convicing. He holds it at a certain throttle setting which is equivalent to 2 miles an hour and the model aircraft sits constant, then proceeds to speed up the conveyor to 10+ miles an hour and the aircraft stays exactly where it is on the treadmill because the friction on the wheels is the same. He then applies full power and the aircraft accelerates forward. I'm at the stage of betting money if that's what it's going to take! Link to comment Share on other sites More sharing options...
Guest JRMobile Posted February 5, 2008 Share Posted February 5, 2008 I am a convert, I voted "NO" but after reading Brentc #43 the lights came on. Link to comment Share on other sites More sharing options...
Guest Dukes Posted February 5, 2008 Share Posted February 5, 2008 Here is the answer: Ok guys just thought i'de throw in my 2 bobs worth here. I just saw the outcome of it tested in real life and the answer is YES! Visit here, read and there is a video that shows it. http://mythbustersresults.com/episode97 Although the ultralight aircraft is not fully stationary relative to the ground it causes some questions. Link to comment Share on other sites More sharing options...
Flyer Posted February 5, 2008 Share Posted February 5, 2008 Yeehar :) Beers and Bex for everyone.... regards Phil Link to comment Share on other sites More sharing options...
Guest brentc Posted February 5, 2008 Share Posted February 5, 2008 More evidence here from an expert. http://www.straightdope.com/columns/060203.html Link to comment Share on other sites More sharing options...
w3stie Posted February 5, 2008 Share Posted February 5, 2008 The original hypothetical doesn't give enough information, that's why everyone is disagreeing. I initially accepted that the treadmill was able to hold the aircraft still wrt the air. But how could it do this? Can the rolling friction generated by the treadmill overcome the thrust generated by the prop? How fast can the treadmill turn? In a perfect world where there is no rolling friction or centrifugal force on the wheels or heat build up from wheel bearings turning at 10000000 RPM, then there is nothing to prevent the thrust from pushing the a/c along wrt the air. In the real world it would still be possible to build a treadmill that could run at well over the design limits of the wheels, resulting in catastrophic failure of the wheels and a messy crash. So in that set of assumptions, the plane would not fly. So everyone is correct, depending on your assumptions :) Link to comment Share on other sites More sharing options...
Ross Posted February 5, 2008 Share Posted February 5, 2008 How long would a Jab engine go for sitting on a conveyor not moving with the engine going flat out? Link to comment Share on other sites More sharing options...
youngmic Posted February 5, 2008 Share Posted February 5, 2008 A bit of exam technique required here, RTBQ. The answer lies in three words in the question, everything else is a distraction. Like the statement from Deep Thought, it helps if you know the answer before the question. "A plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. 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). Can the plane take off?" So it is apparent the plane is moving, and this is the key to it, we can safely assume it is a conventional type which moves forward, as it moves forward toward lift off speed the conveyor speed is constantly matching the aircrafts forward velocity and moving in a reverse direction. But the plane is moving, so at say 100 kts GS and at lift off speed the conveyor is running in reverse at 100kts, the wheels rotational speed is now equivalent to a GS of 200 kts. Just for fun, if you had a manually operated limitless speed control on your conveyor do you think you could change the scenario by increasing the reverse speed on the conveyor so as to totally arrest any forward movement of the aircraft? Happy Hunting Link to comment Share on other sites More sharing options...
pudestcon Posted February 5, 2008 Share Posted February 5, 2008 I voted YES Why? Well the motion detectors on the conveyor will indeed make the conveyor move at the aircraft speed (nothing to do with wheel speed) and in the opposite direction but the conveyor cannot influence the movement of the aircraft because of the FREEWHEELING wheels. What would happen if the aircraft was suspended above the conveyor (ala hovercraft uplift) and then ran up the engine to provide forward thrust? I say the aircraft would accelerate to lift off speed and leave the conveyor behind furiously rotating backwards. Pud Link to comment Share on other sites More sharing options...
Guest Baphomet Posted February 5, 2008 Share Posted February 5, 2008 Roger is right ....again. BrentC and others, this has nothing to do with REAL WORLD it's a hypothetical! Read the question! 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)." So the inference here is that (despite the fact it's practically impossible) the conveyor has the ability to add an increasing amount of friction to the wheels to match the increasing levels of thrust provided by the engine. The fact that this wouldn't happen, is not the point. Link to comment Share on other sites More sharing options...
Guest airsick Posted February 5, 2008 Share Posted February 5, 2008 the conveyor has the ability to add an increasing amount of friction to the wheels to match the increasing levels of thrust provided by the engine. The fact that this wouldn't happen, is not the point. Firstly, it is impossible to do this. Friction is not related to speed in any way (aside from the fact that it may cause some wear in the bearings, a bit of heat, some expansion, etc. all of which is minimal and insignificant). Secondly, it is the point. This is a thought experiment. It isn't a trick question, there is a perfectly valid answer. We allow ourselves to simplify some of the difficulties when we consider it but we don't replace these difficulties with things like "although it is impossible we will assume away some fundamental laws of physics and just say that friction takes over and prevents the plane from moving." Brent posted a link to a good site that sums these things up. It is worth a read. PS. I thought Brent was really really smart when I read his post earlier on. Then I read the page he linked to and realised he had plaigarised it and wasn't that smart after all. Link to comment Share on other sites More sharing options...
slartibartfast Posted February 5, 2008 Share Posted February 5, 2008 Also, check the video linked earlier. They do the experiment and do it well (not the Myth Busters one - those guys suck). No matter how fast the conveyor moves, the aircraft has no problem remaining stationary. Then when it's time to take off, increase the throttle and away you go. The question says that the conveyor is able to increase its speed to match the speed of the plane. Not the plane's wheels, the plane. If the plane moves forward at 5 knots, the conveyor does 5 knots backwards. What affect does this have on the plane? About the same as the damage suffered by allowing the bulldozer to run over Aurthur Dent. Almost none at all. Remember the maximum speed the plane needs to go is the minimum take-off speed - say 50 knots. So the maximum speed the conveyor will go is 50 knots. Not light speed, not exponentially increasing - just 50 knots. There's no way the conveyor running at 50 knots and acting only on the freely spinning wheels can stop the plane from accelerating via the force the propellor exerts on the air. I'm amazed this discussion has taken off. I thought we killed it last time. And with the experiment actually done on video and discussed by experts, that people still say it won't take off. I argued strongly the first time that it wouldn't take off too. I realized eventually where I had made my error. I'm glad I didn't say something like "Physics: 101" when I was wrong. That would have been embarrassing. Link to comment Share on other sites More sharing options...
Deskpilot Posted February 5, 2008 Share Posted February 5, 2008 I told you so What's "Physics: 101?" School was soooo long ago! Link to comment Share on other sites More sharing options...
Guest palexxxx Posted February 5, 2008 Share Posted February 5, 2008 Well, everyone has had their say and proposed their theories on whether the plane will or won't fly. But how do we find out who is right? Looks like we need a demonstration. Link to comment Share on other sites More sharing options...
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