Gaz6002 Posted February 7, 2008 Share Posted February 7, 2008 But the ground speed is measured against stationary ground, not a moving belt. That's where you're confusing things. No, that's where I'm basing my facts. An object won't move if 2 equal forces acting in different directions act on said force at the same time. Do you agree with that? Quote Link to comment Share on other sites More sharing options...
caseys Posted February 7, 2008 Share Posted February 7, 2008 This is symbolic of your struggle against oppression... So's this... http://www.straightdope.com/columns/060203.html Where the first post is listed word for word , the explaination too: First the obvious-but-wrong answer. The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly. A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily. Maybe we should also debate the 3-doors 1-prize puzzle Quote Link to comment Share on other sites More sharing options...
Ewen Posted February 7, 2008 Share Posted February 7, 2008 What a cool thread. Maybe it would help if you imagined the belt was already moving backwards, fast, but the plane was held stationary by a rope tied to its nose. The plane is not moving, but its wheels are due to the belt. There is a slight force pulling the plane backwards, due to the movement of the belt being transmitted through the (free to rotate) wheels, but the rope can easily resist this force. Now turn the engines on...it MUST be easy to imagine that the thrust of the engines pushing the plane forwards is far more than the small inertial force pulling it back against the rope. As the thrust of the engines is greater than the pull on the rope, the plane will move forward from its fixed point, and accelerate until its fast enough to take off. Now back to the original set up, instead of the rope holding the plane stationary, just imagine its just enough engine thrust to overcome the inertial force of the belt through the wheels. Then increase the thrust...again, the additional thrust will tip the balance and the plane moves forward etc etc. In both cases, the point is that the engines thrust is always going to be greater than the inertial pull from the belt through the freely rotating wheels, no matter how fast the belt goes... Besides, if the plane is described as MOVING, it must be moving forward, away from a fixed point. The faster the plane MOVES (fwd, away from a fixed point) the faster the belt moves granted, but eventually the plane will be MOVING fast enough (fwd, away from a fixed point) to achieve take off speed. Quote Link to comment Share on other sites More sharing options...
Rob Posted February 7, 2008 Share Posted February 7, 2008 No, that's where I'm basing my facts. An object won't move if 2 equal forces acting in different directions act on said force at the same time. Do you agree with that? I agree with that, BUT in this scenario the force from the conveyor belt is not acting against the force from the engine, all it encounters is some minor friction from wheel bearings. Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted February 7, 2008 Share Posted February 7, 2008 Correct, except the plane is desribed as "a plane is STANDING on a runway... Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted February 7, 2008 Share Posted February 7, 2008 Correct, except the plane is desribed as "a plane is STANDING on a runway... EDIT: this is in response to Ewens post. Quote Link to comment Share on other sites More sharing options...
Ewen Posted February 7, 2008 Share Posted February 7, 2008 EDIT: this is in response to Ewens post. :dLove your new sig. Its a bad thread this one and no mistake. In the beginning...A plane is standing on a runway that can move (some sort of band conveyer). It came to pass that....The plane moves in one direction, while the conveyer moves in the opposit Its not that well written, but basically, the plane is sitting (still at the moment) on a runway that CAN move (but isnt yet). Then the plane starts to move...as does the belt. As the belt (or belt surface) is going in the opposite direction to the plane, we have to accept the plane is going in the opposite direction to the belt....so if the belt is moving backwards, the plane must be moving forwards. It was stationary on the belt, but it has now begun to move, forwards. If its moving forwards, it can only be due to its engines, as the only other reaction involved is the belt which will tend to pull the plane backwards through its contact with the (free to rotate) wheels. Once we accept that the engines are in fact making the plane move forwards, all we have to do then is rev em up and fark off skyward:) Quote Link to comment Share on other sites More sharing options...
Ewen Posted February 7, 2008 Share Posted February 7, 2008 Crikey, its nearly 2 am...how time flies. Sorry, wrong thread. Quote Link to comment Share on other sites More sharing options...
Supra Pilot Posted February 7, 2008 Share Posted February 7, 2008 Yawn.... This is simple aerodynamics and common sense. Have not read through all the dribble, but it seems people are focusing on the wheels.... And the Thrust of the Engines! Rule 1 An Aeroplane flying.... an Aeroplane does not need 'Engines' to fly! Think of a Glider... no engines! It can stay up in the air for hours, days, not lose height and can travel great distances. This rules your propulsion and thrust ideas out. In the example, the A/craft is basically stationary as it's foward movement is being cancelled out by the conveyer.. Regard less of the the amount of thrust the engines are kicking out. Now you say it does not need wheels, you are partly right... it does not need wheels to fly... it needs foward movement... if no foward movement, then it will require a massive headwind to react and create lift to fly! Forgetting the theory, I will give you a real world example that has not only been carried out by me(several hundred times) but also by many others in my field. Doing Engine runs(ground test), the A/craft Brakes are on(like a handbrake) and wheels may also be chocked(blocked to prevent rolling). Nothing else connected to the A/craft... We take the Engines up to full power(several minutes), the A/craft is screaming and bouncing up and down like a loon... But does it go flying off... NO! The A/craft is stationary, no foward movement to create lift... it ain't flying! I can go on and on and on... but in one simple sentence... 'If there is no foward motion or an equivelant head wind... no lift will be created and such No flight is possible..' If that statement above is untrue, I suggest you 'yes' people, give all the UK airports a phone call, tell them to sell their Runways to land developers to make some money, as Aeroplanes don't need them... they can take off without foward motion!... Rgds Quote Link to comment Share on other sites More sharing options...
Supra Pilot Posted February 7, 2008 Share Posted February 7, 2008 ..... Sorry double post...... Quote Link to comment Share on other sites More sharing options...
Jake Posted February 7, 2008 Share Posted February 7, 2008 In the example, the A/craft is basically stationary as it's foward movement is being cancelled out by the conveyer errrr, no. It would still gain forward movement (and therefore take off), the conveyor belt just makes the wheels turn faster. The plane is pushing/pulling itself forward against the air, not the runway/conveyor. The air is not affected by the moving surface the plane is standing on. I can see where people are having trouble understanding this; it's because they assume that the moving belt stops the plane moving forwards, but it really wouldn't stop it doing that - it would just make the wheels revolve more quickly. Once you've grasped that it's fairly easy to understand, isn't it? Quote Link to comment Share on other sites More sharing options...
Darren-K Posted February 7, 2008 Share Posted February 7, 2008 Correct... 'The plane cannot take off!' I will make a simple reply maybe later on this, but first I need to go and put my skill/knowledge to use and make an Aircraft safe to fly! (in real life, not in wiki or some tele program) Rgds At last, someone who actualy knows planes, as i stated way back., an airplane needs FORWARD velocity to take of,the wheels allow forward velocity., The plane engines would just burn out. the thrust from the engines are not related to the wheels for thrust,but the wheels are responsible for forward motion. simple physics., in regards to the myth busters clip,. thats 100% flawed as the weight of the aircraft still allows some energy to be passed thru the thin material they use in the example, a better clip is available on youtube showing a model plane and a free running converyor belt. the plane doesnt take off in the clip. However, i guess so many people are so strongly opinionated on this that they will never accept that on paper the plane would take off... however in the real world physics wont allow this to happen. its like saying,. a human can fall of a 100 feet high bridge and live no problem because if you drop an ant of a 10 feet high bridge the ant lives. on paper it looks good,.. in reality - it doesnt work. Quote Link to comment Share on other sites More sharing options...
AndyT Posted February 7, 2008 Share Posted February 7, 2008 errrr, no. It would still gain forward movement (and therefore take off), the conveyor belt just makes the wheels turn faster. The plane is pushing/pulling itself forward against the air, not the runway/conveyor. The air is not affected by the moving surface the plane is standing on. I can see where people are having trouble understanding this; it's because they assume that the moving belt stops the plane moving forwards, but it really wouldn't stop it doing that - it would just make the wheels revolve more quickly. Once you've grasped that it's fairly easy to understand, isn't it? Yep. Quote Link to comment Share on other sites More sharing options...
tbourner Posted February 7, 2008 Share Posted February 7, 2008 Seems almost like people have assumed the wheels are braked or something? All you NO answer people, do you think the wheels on the plane are stopped from rotating by the brakes? Also, you're all saying that the wheels have to be going the same speed as the plane? Well yes, because they're attached, but the wheels don't have to ROTATE at a speed which is similar to the rotation speed normally. See here: If the belt matches the planes ground speed, the wheels don't turn, yet the plane still accelerates down the runway until it takes off. Get your head round that one. During flight the wheels aren't turning at a speed which is related to the flight speed!! Because the wheels are free to rotate at whatever speed they like, so when off the ground they stop. Similarly; when on the ground but on a medium which isn't 'normal' for the take off, the wheels will rotate at a speed which isn't related to the air speed of the plane, but so what! It still moves. How about this: Imagine for a minute a hypothetical example that the plane in question is able to hover above the runway/conveyer when stationary, the same rules apply yet the wheels now are only just touching the runway - no weight on them at all. When the engines spin up the plane will move forwards, right? Just like it would in the air, right? The conveyer will move backwards, spinning the wheels, but not affecting the plane. Now in our situation it's exactly the same, except for the weight of the plane. The friction created on large wheels by the weight of a plane is NOWHERE NEAR enough to overpower the engines, otherwise they'd have trouble taking off in normal conditions!!!!! Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted February 7, 2008 Share Posted February 7, 2008 errrr, no. It would still gain forward movement (and therefore take off), the conveyor belt just makes the wheels turn faster. The plane is pushing/pulling itself forward against the air, not the runway/conveyor. The air is not affected by the moving surface the plane is standing on. I can see where people are having trouble understanding this; it's because they assume that the moving belt stops the plane moving forwards, but it really wouldn't stop it doing that - it would just make the wheels revolve more quickly. Once you've grasped that it's fairly easy to understand, isn't it? How would it move forward though? If the ground moves backwards for every mm that the plane moves forwards it's not going anywhere. It will remain in a stationary position relative to the earth. If it isn't moving forwards against it's position on the earth then it can't create any lift as it's not forcing air to act as a force on the wings. In this case, the lift must equal zero because the plane isn't moving relative to the air. Quote Link to comment Share on other sites More sharing options...
tbourner Posted February 7, 2008 Share Posted February 7, 2008 It IS moving relative to the air. It IS moving relative to the Earth. The conveyer is allowed to move whichever way it wants, all it'll do is spin the wheels of the plane - BTW, in case you hadn't got it yet; the wheels DO NOT have any bearing on the planes movement, they are ONLY there so the plane doesn't crash!!! Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted February 7, 2008 Share Posted February 7, 2008 It IS moving relative to the air. It IS moving relative to the Earth. The conveyer is allowed to move whichever way it wants, all it'll do is spin the wheels of the plane - BTW, in case you hadn't got it yet; the wheels DO NOT have any bearing on the planes movement, they are ONLY there so the plane doesn't crash!!! How can it move if everytime it tries the ground counteracts the movement by moving in the other direction? It's not moving relative to the earth. That goes against the laws of physics. If you break it down into component forces it's just one equal force pushing against another. Quote Link to comment Share on other sites More sharing options...
Supra Pilot Posted February 7, 2008 Share Posted February 7, 2008 FFS........ This is my last post on this topic, you can argue the toss all you like.... I am not getting into a disscussion on this with people that have read a few books on Aeroplanes(including Thomas Cook holiday brouchers), surfed the net for rubbish info and who's Aircraft experience is limited to Holidays to Spain! 1. Think of yourself on a 'threadmill'.... either running or on rollerskates. You are running/skating like hell on the the threadmill, but the threadmill keeps you in check with it's matching speed. Strap a rocket on and give it full guns.... If the threadmill speeds up to match your potential foward motion regardless of how powerfull the rocket is or how potentially fast the rocket wants to move you foward....... then you arn't progressing foward are you! Therefore you are stationary! Another example with an Aeroplane for the hardheaded, Here we will forget the wheels, OK, forget the wheels! A Seaplane, no wheels..... In the water ready to take off, giving it max engine power to move, but the tide and current is soooo strong that it prevents the A/craft from moving foward.... So will it fly...... ... Will it F**k.. 'Wot?', it will just jump out of the water and start flying.... . Good day.... Quote Link to comment Share on other sites More sharing options...
johnnyknox Posted February 7, 2008 Share Posted February 7, 2008 FFS........ This is my last post on this topic, you can argue the toss all you like.... I am not getting into a disscussion on this with people that have read a few books on Aeroplanes(including Thomas Cook holiday brouchers), surfed the net for rubbish info and who's Aircraft experience is limited to Holidays to Spain! 1. Think of yourself on a 'threadmill'.... either running or on rollerskates. You are running/skating like hell on the the threadmill, but the threadmill keeps you in check with it's matching speed. Strap a rocket on and give it full guns.... If the threadmill speeds up to match your potential foward motion regardless of how powerfull the rocket is or how potentially fast the rocket wants to move you foward....... then you arn't progressing foward are you! You're wrong and your pompous attitude has just made you look like a fool good day to you too Quote Link to comment Share on other sites More sharing options...
Alex Posted February 7, 2008 Share Posted February 7, 2008 In the water ready to take off, giving it max engine power to move, but the tide and current is soooo strong that it prevents the A/craft from moving foward.... So will it fly...... ... Will it F**k.. 'Wot?', it will just jump out of the water and start flying.... . Good day.... You're explanation is only applicable in that situation....because of excess friction. The wheels do not create the same level of friction. The engines with push the aircraft fwd's relative to the air/earth irrespective of what the treadmill is doing. It's been proven via a telly program. It's been perfectly explained a thousand times. It will take off!! Quote Link to comment Share on other sites More sharing options...
tbourner Posted February 7, 2008 Share Posted February 7, 2008 1. Think of yourself on a 'threadmill'.... either running or on rollerskates. You are running/skating like hell on the the threadmill, but the threadmill keeps you in check with it's matching speed. Strap a rocket on and give it full guns.... If the threadmill speeds up to match your potential foward motion regardless of how powerfull the rocket is or how potentially fast the rocket wants to move you foward....... then you arn't progressing foward are you! Hang on, the questions said: This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same The conveyer speed is exactly the same as the forward speed of the plane, if it was tuned to go in reverse at a fast enough speed to prevent forward motion then yes you'd be correct (it'd be going BLOODY fast though!!!), but it's not, it's going the same speed as the plane. On your example, the treadmill is going at your runnng speed, if it doesn't increase it's speed and you strap a rocket to your back then you'd go flying off the end of the treadmill!!! Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted February 7, 2008 Share Posted February 7, 2008 You're explanation is only applicable in that situation....because of excess friction. The wheels do not create the same level of friction. The engines with push the aircraft fwd's relative to the air/earth irrespective of what the treadmill is doing. Except it won't - because everytime the plane moves forwards the treadmill counteracts it and moves backwards... it can't move off from a standstill. Quote Link to comment Share on other sites More sharing options...
Alex Posted February 7, 2008 Share Posted February 7, 2008 Except it won't - because everytime the plane moves forwards the treadmill counteracts it and moves backwards... it can't move off from a standstill. Who's the mod? Therefore who is always right!? Quote Link to comment Share on other sites More sharing options...
Gaz6002 Posted February 7, 2008 Share Posted February 7, 2008 Who's the mod? Therefore who is always right!? :d Quote Link to comment Share on other sites More sharing options...
tbourner Posted February 7, 2008 Share Posted February 7, 2008 Except it won't - because everytime the plane moves forwards the treadmill counteracts it and moves backwards... it can't move off from a standstill. What did I just say above, the conveyer is NOT set to go at a high speed; one that would be fast enough to overpower the engines. It's ONLY going as fast as the plane is moving forwards. As said this just spins the wheels it isn't enough to stop the engines from pushing the plane forwards!! Quote Link to comment Share on other sites More sharing options...
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