No, i get it.. Aerodynamics was my thing in school, remember? But I mean, I am assuming the original poster of the question MEANT that the plane remains motionless, know what i mean?
He definitely stated the plane moved-remember?
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10:13 PM
Firefighter Member
Posts: 1407 From: Southold, New York, USA Registered: Nov 2004
Wolfhound is ABSOLUTELY CORRECT. What's with you guys???? Read the original question again. The plane is NOT MOVING at all, therefore NO LIFT. You guys are thinking about how a helicopter might take off. Vertical flight was not the issue. The propeller can be spinning at 100,000 rpms, but on the ground it only makes the wheels turn faster . The conveyor belt is timed to keep up with movement in the opposite direction. Nothing is going anywhere. Ed
[This message has been edited by Firefighter (edited 12-03-2005).]
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10:17 PM
Wolfhound Member
Posts: 5317 From: Opelika , Alabama, USA Registered: Oct 1999
The propeller can be spinning at 100,000 rpms, but on the ground it only makes the wheels turn faster . Ed
And, where is all that energy from the propellor being expended to? Not the conveyor for sure. It's being expended in the air, which is independant of anything the conveyor does. That plane is flying, and all the conveyors in the world aren't going to stop it.
Yeah, yeesh! Nick, you have a point in that the weels help the plane take off, but lets look at why. If you were to draw an FBD of the plane's wheels, they, because they are bearings, provide no resistive moment, and no horizontal resistance. They only provide a force normal to the surface they are on, the conveyer in this case. The only force the wheels can apply to the plane is in the verticle direction (of course this is all assuming that there is no friction, which you stated that you were assuming). So, why do we have wheels? They provide an upward force on the airplane to keep it in static equilibrium the 2 feet or so it sits off the ground when parked. Why not take away the wheels? now we have a plane sitting on the ground. ok, the only force here is the equal and opposite forces of the plane on the ground and the ground on the plane. Can the plane still take off? not really because in this case, the friction would be way to large to neglect. while probably isn't enough to stop the plane from taking off, it would be enough to rip whatever material is on the bottom of the plane off therefore wrecking the structural integrity. So back on track. the wheels on the plane really only provide an upward force on the plane, no horizontal forces. this means that the plane is free to move forward or backward from any outside force. The treadmill will not provide this force as it is acting on the wheels which will rotate and not transfer any of the horizontal motion to the plane. same would happen in a car if you disengaged the engine from the trans and neglected all friction in the wheel assembly, which you are. turn on the treadmill and the car, plane, whatever on wheels will not turn. Next time you are at a gerocery store try this. it is fun. put a grape on their conveyer belts and it will remain stationary and rotate freely, provided you give it a little umph to overcome initial static friction, and that the grape is fairly round. So the treadmill cannot transfer any horizontal force to the plane. it only causes rotational force to the wheels, and this rotational motion is not transfered to the plane as the wheels are just on bearings, not hooked up to an engine of any sort. So the plane could be off and the treadmill speed wont affect it. Turn the engine on in the plane and now you have greated a force to give it horizontal motion. the propeller pushes against air around it pushing the air backward and the plane forward. the treadmill is helpless against this motion as it can transfer no horizontal movement to the plane. ok so..once the plane gets going to a significant speed due to the interaction between the props or what ever method of propulsion and the air (or exhaust if a rocket), the force of the moving air over the wings will give lift. The moving air occures because the plane is moving forward from the prop and the treadmill is helpless to impeed it. And then the plane can take off. Ok. Still don't believe me? get one of those gas powered model airplanes. hold it by your hand in the air. turn on the engine and rev it up. Feel that strong force? That is from the prop. now the plane is stationary because your hand provides the force to equalize gravity and the force to counteract the prop. if you let go the plane will go forward and start to drop in altitude untill it has reached enough forward momentum to gain lift (all from the props, not from wheels) and then it willl fly, unless it hits the ground first before it gained enough forward momentum. So there!
arrrrr!!! read it again guys!! the ground has force interaction on the plane except that of a normal reaction force with respect to the ground. it doesnt matter what the ground is doing horizontally!!!!!!!!!!!!!!!
No, he worded it in a way that contradicts what he said. He meant the plane stands still. The question is a paradox, which is not what he intended. He said the plane moves forward while the runway moved backwards. If what he said is exactly what he meant, then it is no different then "The plane moved faster and the runway stood still", which is a non issue, so I assume thats not what he was thinking. The way he worded it is like saying "If a car moves forward than will it drive" when what he originally meant to ask is "If a road is moving backwards and the car is rolling forward, is the car going anywhere".. Obviously a different example and two totally different questions, but an example of how someone can want to explain something but word it wrong.
OK!! Revelation and inspiration has come, with the early morning light!! The plane will fly!! How did I come to that conclusion? And why? 1. The engine would have to produce extra power, compared to a normal take-off.That percentage power increment would be governed on a decreasing sliding scale, in relation to two forces...one, the amount of extra friction caused by the fact that the wheels will be turning at a greater speed than normal.Therefore there will be a relative increase of friction.Very little, but sufficient to slow the plane down in comparison to moving over a fixed runway. The drag caused by the conveyor belt passing under the undercarriage would be very small, and would lessen as the plane gathered forward momentum, thereby starting to provide lift from the wings.The part that is confusing is not confusing st all, when you think logically, as many of you did.The wings are providing an increased amount of lift, from the moment the plane starts to move forward.So the more speed that is gathered, the more lift, and the less drag experienced by the tyres/wheelbearings.That is the key to it all. The wings don't suddenly provide lift just prior to take-off, but on a upward sliding scale, until the point of lift-off.And so, in reality, it would take a longer taxi path than normal to gather the lift, relative to the amount of drag coefficient through the undercarriage.Basically, if the length of runway needed to take off normally was 3000 yards, it would take 3000x(difference between drag of wheels turning on a fixed runway at 120 mph, and the drag coefficient at double that speed, 240mph, which would be the speed the wheels would be turning at, because of the moving conveyorbelt) Convert that to a fraction, and this is only an example to illustrate the mathematical process:drag on undercarriage normal take-off is 3%. double that, for argument's sake, in the moving conveyor belt scenario, would give 6%. Therefore the only difference between the two take-offs would be the extra distance required to overcome extra friction. would be 3%, since the normal take-off would be experiencing that 3% anyway. Giving a required taxi path of 3000+3% of 3000..3000+90 yards, or probably less, due to the reduction in friction caused by steadily increasing lift from the wings. I think I am right, although the percentages quoted are not scientific at all, just guesses!! So, to sum up, my new understanding of the theory is: The plane would take off, if the conveyorbelt was at least 3% longer than a normal taxi path. The only difference is the drag factor between wheels turning at 120 mph, and wheels turning at 240 mph!! I think i should go back to school!! I enjoyed this riddle immensely! Something I would have hated when I was at school. My learning and interest levels are far higher now than then!!!. The only real problem with life? It runs back to front!! Ability and desire to learn increases with age!! And when you are young, and have enthusiasm to do things, you are not earning enough money to enhance the scope of possibility! Nick
[This message has been edited by fierofetish (edited 12-04-2005).]
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04:35 AM
Patrick Member
Posts: 37857 From: Vancouver, British Columbia, Canada Registered: Apr 99
No, he worded it in a way that contradicts what he said.
His words are what he said. See wolfhound's post regarding assumptions. (Ass-U-Me)
quote
He meant the plane stands still. The question is a paradox, which is not what he intended.
Aren't you the guy who cautions people not to read anything into your posts other than exactly what you typed? It's a simple question with a simple answer once the irrelevant parts are disregarded.
quote
He said the plane moves forward while the runway moved backwards. If what he said is exactly what he meant, then it is no different then "The plane moved faster and the runway stood still",
Now, you got it. As you pointed out early on, the initial post is full of red herrings. Purposely put there IF I guess right, to cloud the issue. The mind plays illusion master with us, as we try to imagine the plane and conveyor. Our 1st thoughts are that it won't go anywhere, because we actually cannot see the invisible force being excerted on the air by the propellors/jet engines. We can visuallize the wheels turning, & the conveyor moving ever faster in the opposite direction to counteract the wheel rotation.
I was talking to Tom last night about this and I asked him if I was correct. I told him the plane would not fly and he told me I was correct in my thinking (well, that's a first!) He came up with all of these physics formulas about why it would not fly. But it all came down to not having any airflow over/under the wings. If it doesn't have that lift (he spouted some equations) then it will not fly.
He told me if you suspended a 747 in the air by some means and the engines were going full force, if you unleased the plane it would drop like a lead balloon. No lift, there might be a void in the air in front of the engines (which thrust is, from what he told me) but without the lift (air flowing over and under the wings) it would fall to the ground and go boom! Big Boom!! Now if you were to somehow, turn the plane into the drop and get air flowing over the wings then you could pull it out of the drop and fly it (well, under all the right circumstances)
I told him he should post his side, but he hates to type with a passion!! I couldn't believe he typed so much with the Flight 800 thread!!
I think i should go back to school!! I enjoyed this riddle immensely! Something I would have hated when I was at school. My learning and interest levels are far higher now than then!!!. The only real problem with life? It runs back to front!! Ability and desire to learn increases with age!! And when you are young, and have enthusiasm to do things, you are not earning enough money to enhance the scope of possibility! Nick
That's the way it is with most of us older people. I think it has a lot to do with the fact that we now have real life experiences to go along and equate to the dull theories that we saw presented to us in school so many years ago. Many retirees do go back to school to earn a degree just for the fun of learning. Been wanting to do it myself for about a decade now.
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07:08 AM
PFF
System Bot
Wolfhound Member
Posts: 5317 From: Opelika , Alabama, USA Registered: Oct 1999
I had said the plane would not fly. I have long since rethought my opinion. Now I "think" it might. As said, the scenario is unclear. Now let me throw another hitch in this get-a-long. I think we all can agree that if the conveyor is moving backwards (relative to the way the plane is facing) and the propellers are not moving, the plane would be moving backwards. Get them propellers turnin enough to keep it stationary (relative to ground position) then in fact it would have air speed. Would this be a way to shorten the runway on aircraft carriers ?
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08:05 AM
cliffw Member
Posts: 36870 From: Bandera, Texas, USA Registered: Jun 2003
PAGE 5?! How could this have gone on so long? I am an Airline transport pilot and will weigh in.
There are only two things that can make a plane fly, lift and thrust. With enough of either a plane will get off of the ground. Notice that I did not say ground speed. It is irrelevant. I have flown small planes at 70 mph in an 80 mph headwind. It had the effect of negative 10 mph. I was going backwards over the ground. I have even backed up planes as large as a Cessna 206 when the winds aloft were strong during my skydiving days.
Rockets fly on pure thrust. Aircraft fly because there is wind going over the wings. If a prop could make enough wind to create lift on a wing a plane could fly from a standing start. I have never seen one. I have seen one come close though.
Would this be a way to shorten the runway on aircraft carriers ?
No, it would not. Even ifthe conveyor were moving the same directionbas the take off direction, The ship's speed, natural wind direction, and cat already do a great job with a much less ponderous mechanicl device.
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08:36 AM
Wolfhound Member
Posts: 5317 From: Opelika , Alabama, USA Registered: Oct 1999
If a prop could make enough wind to create lift on a wing a plane could fly from a standing start. I have never seen one. I have seen one come close though.
It comes close on an unloaded MV33 with the nacells rotated forward (engines horizontal to the ground). Those are huge blades--and variable pitch to boot.
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08:41 AM
cliffw Member
Posts: 36870 From: Bandera, Texas, USA Registered: Jun 2003
I asked believing it would not. Not trying to confuse, just trying to get people thinking. I actually think a plane would have to work twice as hard to take off with a conveyor.
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08:56 AM
Cliff Pennock Administrator
Posts: 11808 From: Zandvoort, The Netherlands Registered: Jan 99
Glossary Plane: That thing with the two wings Belt: The conveyor belt Ground: Planet Earth, that thing the conveyor belt is placed on (<- This is important. Do not confuse "ground" with "belt")
"The plane moves in one direction, while the conveyer moves in the opposite direction."
This indicates that movement/speed is measured relative to the ground, not the conveyor belt or the plane (an object can't move in it's own 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)."
This is actually the culprit. Does the conveyor belt tracks speed relative to the ground or itself? And does it tune it's speed relative to the ground or the plane?
If all speeds are relative to the ground, then yes, the plane will fly because we can then safely assume two things: either both the plane and the conveyor belt are at rest, or they are both moving with the same speed in opposite directions relative to the ground in which case the plane has a forward ground speed no matter what is causing the plane to move: the wheels or the engines! So eventually, if the plane's ground speed is sufficient enough, it will take off.
If the conveyor belt measures the plane's speed relative to itself and tunes it's own speed relative to the ground, then no, the plane will not take off since the plane's ground speed is always zero.
If the conveyor belt measures the plane's speed relative to itself and tunes it's own speed relative to the ground, then it depends wether the plane gets its forward momentum from its wheels or it's engine. If the plane gets its forward momentum from the wheels, then the plane will not take off because its ground/air speed will remain unchanged. If the plane will get it's forward momentum from the engines, then the plane will take off because the speed of the conveyor belt has no influence whatsoever on the forward speed of the plane. In such a case the belt will actually reach an infinite speed in no time.
The third and final possibilty is that the belt measures the plane's speed realtive to itself and then tunes it's own speed relative to the plane (but in the opposite direction). In this case the plane will take off too since the belt's speed relative to the ground will remain zero - in other words, the belt will remain at rest. Why? Because the belt's speed realtive to the plane is the same as the opposite speed of the plane relative to the belt. So the belt stays at rest and the plane gains ground speed and takes off.
No, i get it.. Aerodynamics was my thing in school, remember? But I mean, I am assuming the original poster of the question MEANT that the plane remains motionless, know what i mean?
Actually, I didn't remember aero was your thing. But, the assumption that the plane remains motionless is the whole misdirection. The question leads you, the reader, to make that assumption, when it is not stated in the question. All that is stated is that as the conveyer tracks the plane's speed and turns the conveyer at exactly the same speed in the opposite direction. The reader then makes the assumption that the two velocities cancel, the plane is motionless, and therefore doesn't take off.
It's that incorrect assumption that leads everyone astray, and I think the whole point of the question was to illustrate that.
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09:26 AM
Formula88 Member
Posts: 53788 From: Raleigh NC Registered: Jan 2001
If the conveyor belt measures the plane's speed relative to itself and tunes it's own speed relative to the ground, then no, the plane will not take off since the plane's ground speed is always zero.
Question, then. How does the speed of the conveyer affect the speed of the plane? If you assume the wheels have negligible rolling resistance and there are no brakes in use, the speed of the conveyor is irrelevant. If you ignore static friction and inertia for a moment, you could have the plane sit still, and start the conveyer and it would begin moving under the plane, without the plane moving.
Regardless of your frame of reference, there is no mechanism for the movement of the conveyer to exert a force on the plane. Remember, ground speed is irrelevant. Only air speed will decide if/when the plane takes off.
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09:32 AM
Wolfhound Member
Posts: 5317 From: Opelika , Alabama, USA Registered: Oct 1999
Question, then. How does the speed of the conveyer affect the speed of the plane? If you assume the wheels have negligible rolling resistance and there are no brakes in use, the speed of the conveyor is irrelevant. If you ignore static friction and inertia for a moment, you could have the plane sit still, and start the conveyer and it would begin moving under the plane, without the plane moving.
It doesn't matter if the speed of the belt (relative to the ground) affects the speed of the plane (relative to the belt). Again, in this case the belt measures the speed of the plane relative to itself and moves the belt in opposite direction relative to the ground. So no matter what the effect of the belt's speed is on the plane's speed (relative to the belt and due to friction or inertia or whatever), the plane's ground/air speed will remain unchanged and the plane will not lift off.
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09:42 AM
Formula88 Member
Posts: 53788 From: Raleigh NC Registered: Jan 2001
My point is the belt has NO effect on the plane. The only way the belt can affect the plane is through the contact patch on the tires. If the wheels roll freely, the belt cannot speed up or slow down the plane. It doesn't matter what you measure the speed of the belt relative to.
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09:50 AM
Cliff Pennock Administrator
Posts: 11808 From: Zandvoort, The Netherlands Registered: Jan 99
My point is the belt has NO effect on the plane. The only way the belt can affect the plane is through the contact patch on the tires. If the wheels roll freely, the belt cannot speed up or slow down the plane. It doesn't matter what you measure the speed of the belt relative to.
Oh, I see your point. I made the mistake that in such a case the wheels were what is causing the plane to move. Let me correct that in my original story so I can get out of this thread.
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09:58 AM
blackrams Member
Posts: 32295 From: Covington, TN, USA Registered: Feb 2003
Originally posted by jstricker: And exactly HOW are you going to make it's groundspeed zero if the wheels supply no motive force? John Stricker
Quite frankly, I'm amazed at some of the responses here. Ground Speed has absolutely nothing with whether or not the aircraft will fly. It's all about airspeed. Heck even us rotor heads know that!
------------------ Ron Freedom isn't Free, it's always earned. My imagination is the only limiting factor to my Fiero. Well, there is that money issue.
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10:24 AM
Cliff Pennock Administrator
Posts: 11808 From: Zandvoort, The Netherlands Registered: Jan 99
Quite frankly, I'm amazed at some of the responses here. Ground Speed has absolutely nothing with whether or not the aircraft will fly. It's all about airspeed. Heck even us rotor heads know that!
At take off, Ground Speed =~ Air Speed. Unless you are taking off in the middle of a hurricane. (so under normal circumstances and without wind)
Actually, I didn't remember aero was your thing. But, the assumption that the plane remains motionless is the whole misdirection. The question leads you, the reader, to make that assumption, when it is not stated in the question. All that is stated is that as the conveyer tracks the plane's speed and turns the conveyer at exactly the same speed in the opposite direction. The reader then makes the assumption that the two velocities cancel, the plane is motionless, and therefore doesn't take off.
It's that incorrect assumption that leads everyone astray, and I think the whole point of the question was to illustrate that.
THen yes, if you want to do it exactly as the question is worded, then it's just like saying "The plane moved faster and the runway stood still" as I said a few posts ago, kind of, atleast regarding wheels and what not.. This question has been answered 2000 times by both sides of the fence..
I have a couple RC airplanes and a treadmill, should I put an end to this shenanigans?
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11:53 AM
jstricker Member
Posts: 12956 From: Russell, KS USA Registered: Apr 2002
If you think an airplane takes off at the same groundspeed in Denver, CO as it does at Houston, TX, you're in for a very rude awakening. And that's given a no wind situation. Ground speed rarely equals airspeed.
John Stricker
quote
Originally posted by Cliff Pennock:
At take off, Ground Speed =~ Air Speed. Unless you are taking off in the middle of a hurricane. (so under normal circumstances and without wind)
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12:01 PM
Wolfhound Member
Posts: 5317 From: Opelika , Alabama, USA Registered: Oct 1999
There are, in my opinion,(now!!!!) only two relevant factors to be considered.Thrust and air resistance to it.The only difference between a plane taking off from a fixed runway, and taking off from a a runway moving in the opposite direction to the trajectory of the plane, is the resistance to forward motion experienced in the undercarriage.The resistance to the motive power thrust by the surrounding air, is the same in both cases.The weight of the plane is the same.The only difference is the tyre/wheelbearing friction. The resistance provided by the undercarriage in the moving runway, as opposed to the fixed runway, is slightly higher because the wheels are turning faster.So, if it normally requires for example, 5000 revs to take off from the fixed runway, and the resistance involved in the moving runway is 15% higher, it will require opening the engines up to 5750 rpm to provide the extra 15% more thrust to overcome the higher resistance. Purely speculative figures, ignoring torque, power curves etc etc. Once the plane is rolling, the undercarriage resistance would be even less than 15%, because of lift from the wings.
She flies!!!!!! Nick
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01:45 PM
Formula88 Member
Posts: 53788 From: Raleigh NC Registered: Jan 2001
Originally posted by JohnnyK: I have a couple RC airplanes and a treadmill, should I put an end to this shenanigans?
Yes, and video tape it. A real world test is the best way to settle it. Just for grins you could set the treadmill faster than take off speed. So the plane should not only not take off, but should be going backwards. (according to the "won't fly" arguments)
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01:58 PM
blackrams Member
Posts: 32295 From: Covington, TN, USA Registered: Feb 2003
eQUOTE]Originally posted by Cliff Pennock: At take off, Ground Speed =~ Air Speed. Unless you are taking off in the middle of a hurricane. (so under normal circumstances and without wind)[/QUOTE]
Sorry Cliff, way off on this one, if that was true, how would a helicopter ever develop lift. Air passing over/under the air foil is what develops the lift, therefore, ground speed has no effect. Think about an aircraft carrier, they turn into the wind to get more air passing over the aircraft for take off and landing.
------------------ Ron Freedom isn't Free, it's always earned. My imagination is the only limiting factor to my Fiero. Well, there is that money issue.
[This message has been edited by blackrams (edited 12-04-2005).]
The plane would be able to take off. Groungspeed has little to do with airspeed. Beacause an aircraft moves against air rather than ground a conveyer belt moving backwards would not keep it from taking off. A plane flying into a 100mph headwind with a 100mph airspeed though it would have 0 ground speed it would be flying. At takeoff, a tailwind would cause a plane to need more groundspeed and more runway to get airborne but the airspeed requirement for takeoff would be the same as flying into the wind. A plane with a take off airspeed requirement of 80mph flying into a 35mph headwind would take off at 45mph groundspeed. The same plane taking off with a 35mph tailwind would have a 115mph groundspeed at takeoff. A highspeed conveyer belt moving against the aircraft would merely delay the takeoff due to rolling resistance.
But I mean, I am assuming the original poster of the question MEANT that the plane remains motionless, know what i mean?
But, the assumption that the plane remains motionless is the whole misdirection. The question leads you, the reader, to make that assumption, when it is not stated in the question. All that is stated is that as the conveyer tracks the plane's speed and turns the conveyer at exactly the same speed in the opposite direction. The reader then makes the assumption that the two velocities cancel, the plane is motionless, and therefore doesn't take off. 
