JamesN Posted March 11, 2008 Report Share Posted March 11, 2008 Got this off another forum, but it's an interested question: Heres the scenario... An airplane is on a conveyor. The conveyor is set to match the speed of the airplane in the backwords direction. However fast the plane moves, the conveyor moves just as fast. Can the plane take off? Quote Link to comment Share on other sites More sharing options...
Bill Posted March 11, 2008 Report Share Posted March 11, 2008 If it can we would'nt need mile long runways. Quote Link to comment Share on other sites More sharing options...
Louis Posted March 12, 2008 Report Share Posted March 12, 2008 It would take off Quote Link to comment Share on other sites More sharing options...
MikeO Posted March 12, 2008 Report Share Posted March 12, 2008 It would take off ...or not . Quote Link to comment Share on other sites More sharing options...
jacko 1990 Posted March 12, 2008 Report Share Posted March 12, 2008 (edited) it wouldnt, theirs no uplift under the wings, its just the wheels going round. Edited March 12, 2008 by jacko1990 Quote Link to comment Share on other sites More sharing options...
CraccerC Posted March 12, 2008 Report Share Posted March 12, 2008 (edited) it wouldn't take off. not really rocket science, no momentum no uplift (as jacko said) either Edited March 12, 2008 by CraccerC Quote Link to comment Share on other sites More sharing options...
jacko 1990 Posted March 12, 2008 Report Share Posted March 12, 2008 Looks like louis didnt take rocket science at school Quote Link to comment Share on other sites More sharing options...
CraccerC Posted March 12, 2008 Report Share Posted March 12, 2008 i don't see how anyone could think it would take off Quote Link to comment Share on other sites More sharing options...
Bill Posted March 12, 2008 Report Share Posted March 12, 2008 Unless its a vertical take-off plane (Harrier jump Jet), or a Helicopter. Quote Link to comment Share on other sites More sharing options...
MikeO Posted March 12, 2008 Report Share Posted March 12, 2008 (edited) i don't see how anyone could think it would take off Well, having given it a little thought....this is a pure mechanics problem, and we must examine it in terms of forces applied on the plane. While still on the conveyor/runway, the plane is subjected to the following forces: Gravity, oriented downwards, constant. Lift, oriented upwards, function of airspeed. Thrust, oriented forwards, assumed constant. Drag, oriented backwards, superposition of air resistance and friction in the wheels. Obviously the plane will not take off until the force due to lift overcomes the force due to gravity. For this to happen, the plane must achieve some minimum forward velocity relative to the air. Forward acceleration will only occur as long as the force of thrust overcomes the drag force. Because the conveyor belt speed is the inverse of the plane's airspeed, we can say that the conveyor belt does not move until the plane begins to accelerate forward. Thus, rolling resistance has already been overcome by thrust when this problem really begins. Initially, the engine thrust is considerably higher than the drag—this is what allows aircraft to take off on regular runways. So the question is, once we start moving and the conveyor belt starts up, does it impose some force on the aircraft that can overcome the force of thrust? Certainly not initially. Consider the case where the aircraft has just begun to move through the air at, say, 1 m/s. The conveyor belt is then moving backwards at 1 m/s and therefore the speed of the aircraft relative to the conveyor is 2 m/s. If a 2 m/s rolling speed created enough friction to overcome the engines, then no aircraft could ever accelerate beyond this speed on a regular runway. In fact, we know that at the moment of takeoff the wheels are still on the ground and rolling and the aircraft is still accelerating. So up to that rolling speed at least, the engines still win the thrust vs. friction competition. Would rolling at twice that speed cause sufficient friction? Consider the thrust of an aircraft engine. I won't give you figures here because it depends widely on the type of engine. But... it's a lot of force. Now contrast this with rolling friction. I'm guessing here, but I think that before a wheel could provide enough friction (without using the brakes) to completely counteract an aircraft engine at full throttle, the wheel would be spinning so fast that it would come apart. At any rate, it certainly would not reach this tremendous amount of friction at a rolling rate twice that of a standard takeoff speed. The airplane would take off normally, with the wheels spinning twice as fast as normal and a *slight* reduction in acceleration due to added friction. I think where people get thrown off is the idea that if the conveyor is moving backwards and the plane is on the conveyor, then this must translate to some backwards momentum that has to be overcome by the thrust. But of course the conveyor doesn't start up until the plane is already moving forward. It is the conveyor that must overcome the forward momentum imparted by the engines—and the forces just aren't comparable. That's my opinion anyway . I'm sure you may be able to find someone on the internet who agrees with me.....almost word for word I shouldn't wonder . Edited March 12, 2008 by MikeO Quote Link to comment Share on other sites More sharing options...
Bill Posted March 12, 2008 Report Share Posted March 12, 2008 :blink: so is that a :smile_anim: or a :sad01_anim: Mike. :sleeping_03anim: Quote Link to comment Share on other sites More sharing options...
JamesN Posted March 12, 2008 Author Report Share Posted March 12, 2008 Well, having given it a little thought....this is a pure mechanics problem, and we must examine it in terms of forces applied on the plane. While still on the conveyor/runway, the plane is subjected to the following forces: Gravity, oriented downwards, constant. Lift, oriented upwards, function of airspeed. Thrust, oriented forwards, assumed constant. Drag, oriented backwards, superposition of air resistance and friction in the wheels. Obviously the plane will not take off until the force due to lift overcomes the force due to gravity. For this to happen, the plane must achieve some minimum forward velocity relative to the air. Forward acceleration will only occur as long as the force of thrust overcomes the drag force. Because the conveyor belt speed is the inverse of the plane's airspeed, we can say that the conveyor belt does not move until the plane begins to accelerate forward. Thus, rolling resistance has already been overcome by thrust when this problem really begins. Initially, the engine thrust is considerably higher than the drag—this is what allows aircraft to take off on regular runways. So the question is, once we start moving and the conveyor belt starts up, does it impose some force on the aircraft that can overcome the force of thrust? Certainly not initially. Consider the case where the aircraft has just begun to move through the air at, say, 1 m/s. The conveyor belt is then moving backwards at 1 m/s and therefore the speed of the aircraft relative to the conveyor is 2 m/s. If a 2 m/s rolling speed created enough friction to overcome the engines, then no aircraft could ever accelerate beyond this speed on a regular runway. In fact, we know that at the moment of takeoff the wheels are still on the ground and rolling and the aircraft is still accelerating. So up to that rolling speed at least, the engines still win the thrust vs. friction competition. Would rolling at twice that speed cause sufficient friction? Consider the thrust of an aircraft engine. I won't give you figures here because it depends widely on the type of engine. But... it's a lot of force. Now contrast this with rolling friction. I'm guessing here, but I think that before a wheel could provide enough friction (without using the brakes) to completely counteract an aircraft engine at full throttle, the wheel would be spinning so fast that it would come apart. At any rate, it certainly would not reach this tremendous amount of friction at a rolling rate twice that of a standard takeoff speed. The airplane would take off normally, with the wheels spinning twice as fast as normal and a *slight* reduction in acceleration due to added friction. I think where people get thrown off is the idea that if the conveyor is moving backwards and the plane is on the conveyor, then this must translate to some backwards momentum that has to be overcome by the thrust. But of course the conveyor doesn't start up until the plane is already moving forward. It is the conveyor that must overcome the forward momentum imparted by the engines—and the forces just aren't comparable. That's my opinion anyway . I'm sure you may be able to find someone on the internet who agrees with me.....almost word for word I shouldn't wonder . lol good answer... well thought out and explained Quote Link to comment Share on other sites More sharing options...
JamesN Posted March 12, 2008 Author Report Share Posted March 12, 2008 :blink: so is that a :smile_anim: or a :sad01_anim: Mike. :sleeping_03anim: The airplane would take off normally, with the wheels spinning twice as fast as normal and a *slight* reduction in acceleration due to added friction. Quote Link to comment Share on other sites More sharing options...
MikeO Posted March 12, 2008 Report Share Posted March 12, 2008 :blink: so is that a :smile_anim: or a :sad01_anim: Mike. :sleeping_03anim: Not a clue mate....you don't suppose I actually read it do you ? Quote Link to comment Share on other sites More sharing options...
Bill Posted March 12, 2008 Report Share Posted March 12, 2008 Have tried to understand your theory on reletive thrust and drag forces but i cant. What i will say is ............ No matter how much power the thrust of the engines can muster to overcome the drag force of the Conveyor, it is still not possible for a stationary plane to leave the ground unless its a VTOL, Vertical Take off & Land. Its not designed that way, it needs uplift from under the wings (Air pressure) vertical engines, and other design factors that aid it in the lifting process. My answer is still the same as before, if it could take off we wouldnt need the mile long Runways. Quote Link to comment Share on other sites More sharing options...
CraccerC Posted March 12, 2008 Report Share Posted March 12, 2008 the plane needs to move it can't remain still, gravity and friction will remain and cause the plane to stand grounded no matter how powerful the engine is. there needs to be an upward force, upthrust, for the plane to take off (hence the wings) this is only produced by travelling at speed through the air, the air lifting the wings and ultimately the plane taking off. Without the force of the air beneath the wings, the plane can't take off, it would need to defy many physics laws. planes are designed the way they are for a reason. if a plane could take off from a conveyor belt, it wouldn't need wings would it, it's not like its a paper plane and you just throw it and it glides. Quote Link to comment Share on other sites More sharing options...
Bill Posted March 14, 2008 Report Share Posted March 14, 2008 Here you go, this is interesting. http://www.kottke.org/06/02/plane-conveyor-belt Quote Link to comment Share on other sites More sharing options...
CraccerC Posted March 15, 2008 Report Share Posted March 15, 2008 dammit it wont take off Quote Link to comment Share on other sites More sharing options...
zequist Posted March 16, 2008 Report Share Posted March 16, 2008 (edited) The TV show Mythbusters did an episode where they tested out this issue (the two parts below are about 10 minutes each): Part 1: http://www.youtube.com/watch?v=MuXK1nr9_jg Part 2: http://www.youtube.com/watch?v=U9STo3fjfGg Snoopy was pleasantly surprised at the results. Edited March 16, 2008 by JD in DC Quote Link to comment Share on other sites More sharing options...
MikeO Posted March 16, 2008 Report Share Posted March 16, 2008 Amazing the lengths some people will go to to prove Craccer wrong . There was never any concern for Snoopy though JD, the Red Baron can sleep no easier . Quote Link to comment Share on other sites More sharing options...
jacko 1990 Posted March 17, 2008 Report Share Posted March 17, 2008 The plane was still going forward, it wasnt stood still. Quote Link to comment Share on other sites More sharing options...
jacko 1990 Posted March 17, 2008 Report Share Posted March 17, 2008 actaully, the few words he said at the end 'what people dont understand is that the plane sends power to the engines not to the wheels' makes me think differently. Quote Link to comment Share on other sites More sharing options...
CraccerC Posted March 17, 2008 Report Share Posted March 17, 2008 you thought it would send the power to the wheels? im not having it, that mythbusters that was hardly a conveyor belt it was a piece of cloth dragged on concrete, and it pitted a car against a plane. Quote Link to comment Share on other sites More sharing options...
Bill Posted March 17, 2008 Report Share Posted March 17, 2008 No i'm not accepting that as a true test, the plane was travelling forwards even tho the car was pulling the canvas sheet in the opposite direction, on a true conveyor the conveyor would travel the same speed going in the opposite direction, therefore the plane would remain motionless and would not be able to take off. I stick with my original statement ..... if it could work we would'nt need half a dozen or so Mile long runways at the airports. Quote Link to comment Share on other sites More sharing options...
jacko 1990 Posted March 17, 2008 Report Share Posted March 17, 2008 The plane was still going forward, it wasnt stood still. No i'm not accepting that as a true test, the plane was travelling forwards even tho the car was pulling the canvas sheet in the opposite direction, on a true conveyor the conveyor would travel the same speed going in the opposite direction, therefore the plane would remain motionless and would not be able to take off. I stick with my original statement ..... if it could work we would'nt need half a dozen or so Mile long runways at the airports. agree, i just didnt see why they were jumping about when the plane was obviously accelerating forward, which isn't what the test why to try and prove. Quote Link to comment Share on other sites More sharing options...
MikeO Posted March 17, 2008 Report Share Posted March 17, 2008 (edited) agree, i just didnt see why they were jumping about when the plane was obviously accelerating forward, which isn't what the test why to try and prove. Because even if the experiment was extremely flawed and the plane was moving forward it certainly wasn't moving forward fast enough over the ground for take off had it not been for the "conveyor" moving beneath it. Forward motion alone doesn't disprove it.... Would a 747 take off at 50mph? No. Would a 747 take off at 50mph over the ground with the engines at full thrust and the earth slipping backwards at, say, 75mph so that its actual speed is 125mph? Apparently it would. Edited March 17, 2008 by MikeO Quote Link to comment Share on other sites More sharing options...
jacko 1990 Posted March 17, 2008 Report Share Posted March 17, 2008 The Ground and the plane would have to be traveling at the same speed for the plane not to move as the question says, so if both are travveling at 50mph they balance each other out and nothing happens, the plane just stays still and doesnt take off. Quote Link to comment Share on other sites More sharing options...
Bill Posted March 17, 2008 Report Share Posted March 17, 2008 Because even if the experiment was extremely flawed and the plane was moving forward it certainly wasn't moving forward fast enough over the ground for take off had it not been for the "conveyor" moving beneath it. Forward motion alone doesn't disprove it.... Would a 747 take off at 50mph? No. Would a 747 take off at 50mph over the ground with the engines at full thrust and the earth slipping backwards at, say, 75mph so that its actual speed is 125mph? Apparently it would. Dont think they'd be able to prove that one Mike, they'd need ten miles of canvas sheet to act as a conveyor. You only have to read the question again to prove that it should be stationary, the belt moves backwards to exact speed of plane therefore its not going 125 mph, it should be stationary. Even the pilot said it wouldnt take off, the fact that it did is in your own words.... because the experiment was extremely flawed. Quote Link to comment Share on other sites More sharing options...
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