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12/06/2006 03:18:18 PM · #76 |
Originally posted by Zal:
which is generated by the thrust of the engines against air
NOT correct. The engines do generate thrust, but this thrust is converted to kinetic energy by the wheels on the runway to generate forward motion. It is this forward motion that moves the air over the wings to create lift, and hence, take off.
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So then immediately the wheels are retracted, the plane crashes ? After all, you claim planes move forward by the engines turning the wheels, right ?
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12/06/2006 03:19:09 PM · #77 |
| I have great respect for aircraft engineers, I had no idea how hard their job was. Up to now I thought their biggest worries were providing enough thrust and designing wings to give enough lift, but now I have been enlightened. The engines are just for show, the plane actually accelerates using its wheels and gets up enough speed to last for the entire flight. |
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12/06/2006 03:23:42 PM · #78 |
Originally posted by ShorterThanJesus:
I have great respect for aircraft engineers, I had no idea how hard their job was. Up to now I thought their biggest worries were providing enough thrust and designing wings to give enough lift, but now I have been enlightened. The engines are just for show, the plane actually accelerates using its wheels and gets up enough speed to last for the entire flight. |
So your theory is that the plane will fly without wheels/skids/floats?? Would make a nice display of sparks down the runway. |
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12/06/2006 03:27:54 PM · #79 |
Originally posted by Gordon:
Originally posted by Zal:
which is generated by the thrust of the engines against air
NOT correct. The engines do generate thrust, but this thrust is converted to kinetic energy by the wheels on the runway to generate forward motion. It is this forward motion that moves the air over the wings to create lift, and hence, take off.
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So then immediately the wheels are retracted, the plane crashes ? After all, you claim planes move forward by the engines turning the wheels, right ? |
No, that's not what I said. The plane's engines don't turn the wheels. The plane's engine provide thrust. At takeoff, thrust moves the plane forward down the runway until the airspeed over the wings is sufficient to create enough lift to overcome the weight of the plane, and then the plane takes off.
Thrust is not the same force as lift. I agree that in this scenerio, there is thrust. However, unless the wing physically moves through the air, there is no lift and no take off.
I still haven't heard a "pro-takeoff" person explain where the lift comes from to allow the plane to take off. |
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12/06/2006 03:31:03 PM · #80 |
Originally posted by Zal:
I still haven't heard a "pro-takeoff" person explain where the lift comes from to allow the plane to take off. |
It took me some time to come around to this argument, but the lift comes from the fact that the plane will in fact move forward regardless of the treadmill. The treadmill is imparting no force (or negligle anyway) on the airplane as a whole. It does impart force on the wheels which rotate in response to it, but that force is not translated to the airplane. Therefore, the thrust from the engines which is imparted to the airplane is not countered by the treadmill, and therefore the plane moves forward, thus generating lift. |
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12/06/2006 03:32:52 PM · #81 |
Originally posted by Zal:
I still haven't heard a "pro-takeoff" person explain where the lift comes from to allow the plane to take off. |
The lift comes because the plane is in actuality moving down the runway. The point is that the treadmill CANNOT prevent this because it's trying to slow the plane down through the wheels. The wheels are not connected to anything important and so the treadmill can't do its job of preventing the plane from moving.
The only way it can work is if the treadmill makes the wheels spin so fast they fall off before the plane gets up to the critical speed to produce lift.
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12/06/2006 03:37:03 PM · #82 |
Originally posted by sodoff:
So your theory is that the plane will fly without wheels/skids/floats?? Would make a nice display of sparks down the runway. |
The wheels on planes provide about the same amount of power as the rear wheels on a front-wheel drive car (ie. none). They spin, but only when the vehicle is moved by another force. My sarcasm was not clear enough evidently.
Originally posted by DrAchoo:
The lift comes because the plane is in actuality moving down the runway. The point is that the treadmill CANNOT prevent this because it's trying to slow the plane down through the wheels. |
Exactly! I am glad that someone could put that clearly, I obviously missed the mark earlier. |
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12/06/2006 03:37:55 PM · #83 |
Originally posted by Zal:
I still haven't heard a "pro-takeoff" person explain where the lift comes from to allow the plane to take off. |
The source of thrust doesn't change at any point in a plane's flight though. It doesn't drive down the runway under direct power from the wheels. The same thrust that allows it to accelerate in flight is the same power that provides that initial acceleration.
The lift is provided by airflow over the wings, but the forward motion that generates that airflow occurs, wheels on the ground or not.
The wheels are there to stop it dragging along the ground during the initial take-off. Not to provide forward motion. They reduce friction, they don't provide acceleration. The treadmill scenario just makes the wheels spin a bit faster. To see that, consider a sea plane. There is no thrust/acceleration provided by the floats against the water. There are no hidden propellers etc. Boats can take off going up or down stream equally well.
Consider a rocket sled on ice. No traction or friction there either. No lift, no take off either, but forward motion. The plane would move forward. The wings would then generate lift.
Message edited by author 2006-12-06 15:39:37.
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12/06/2006 03:41:00 PM · #84 |
I fail to see this argument. As thrust is increased on an aircraft, this causes it to move forward at an increasing speed until it reaches sufficient lift caused by airflow to tak off.
As thrust is increased, and the aircraft would normally move, the treadmill counters this, so the aircraft does not move. Open up the throttle, the aircraft tries to move forward, but the treadmill just accelerates. |
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12/06/2006 03:41:43 PM · #85 |
Originally posted by eqsite:
Therefore, the thrust from the engines which is imparted to the airplane is not countered by the treadmill, and therefore the plane moves forward, thus generating lift. |
But wasn't the premise of the original statement that the treadmill will always speed up and prevent the plane from moving forward? |
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12/06/2006 03:43:47 PM · #86 |
Originally posted by Zal:
But wasn't the premise of the original statement that the treadmill will always speed up and prevent the plane from moving forward? |
Ahh, that was my initial assumption as well, but it is an assumption, not part of the initial problem. The key is "prevent the plane from moving forward". The treadmill cannot prevent the plane from moving forward as it is only apply force to the wheels and that force does not get translated in any useful amount to the rest of the airplane. |
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12/06/2006 03:43:52 PM · #87 |
OK, let's try this again.
We have a plane. it's sitting still, on a treadmill. If the plane tries to move forward, the treadmill reacts by running rearward, thus no forward motion. Right?
OK, now let's hook up a rope to the plane, and attach it at some far point off the treadmill. Now start the treadmill moving rearward. The plane will be carried rearward, until the rope stretched taut. The plane will then be motionless, with the treadmill still running rearward. The force on the rope is pulling the plane toward the front, while the friction of the wheels is causing a rearward force that keeps the rope stretched tight. Still right?
Now, imagine that the engines are started. They provide thrust, which tries to move the aircraft forward. The thrust is created solely between the engines and the air, the ground has nothing to do with it. As thrust is increased, at some point it equals the frictional force of the wheels (mind, the plane is still stationary), and the rope begins to go slack. If thrust is increased, the plane *will* move forward. The only way to keep it from doing so is to increase the friction of the wheels to the point where it equals thrust.
Now, commercial jet engines each put out between 50,000 and 100,000 pounds of thrust (multiply by the number of engines per aircraft to get total thrust) while wheel friction is at most a couple thousand pounds. Thrust trumps wheel friction by a factor of over 100:1. Doubling the wheel friction (by doubling the relative speed of the aircraft and ground (treadmill) will have a negligible effect on the aircraft, and it *will* move forward. As it moves forward, it generates lift, and takes off normally. |
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12/06/2006 03:44:13 PM · #88 |
Originally posted by Zal:
Originally posted by eqsite:
Therefore, the thrust from the engines which is imparted to the airplane is not countered by the treadmill, and therefore the plane moves forward, thus generating lift. |
But wasn't the premise of the original statement that the treadmill will always speed up and prevent the plane from moving forward? |
No. The premise of the original statement is that the treadmill moves faster to match the speed of the plane.
You then provide the misunderstanding that this prevents the plane from moving forward. This is because you are assuming the wheels are in some way driving the plane against the ground. That's the basic misunderstanding.
The engine, acting on the air drives the plane forward, creates airflow, causes lift.
The wheels are just along for the ride. If they spin faster, they spin faster. It doesn't change where the plane is.
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12/06/2006 03:45:56 PM · #89 |
Originally posted by sodoff:
As thrust is increased, and the aircraft would normally move, the treadmill counters this, so the aircraft does not move. |
Not true, the motion of the treadmill only increases the speed at which the wheels rotate. |
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12/06/2006 03:46:26 PM · #90 |
Originally posted by Zal:
But wasn't the premise of the original statement that the treadmill will always speed up and prevent the plane from moving forward? |
If the plane was somehow stopped from moving forward, then it would never be able to take off. You are right about that. The argument which has been made is that there is no way a treadmill could stop the plane from moving forward (in relation to the surroundings) and therefore the plane would take off normally. But yes, if this treadmill was somehow able to actually stop the plane from moving forward, the plane would be unable to take off. As you said, a plane cannot generate lift while not moving forward.
Message edited by author 2006-12-06 15:48:53. |
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12/06/2006 03:52:05 PM · #91 |
Originally posted by Gordon:
Originally posted by Zal:
Originally posted by eqsite:
Therefore, the thrust from the engines which is imparted to the airplane is not countered by the treadmill, and therefore the plane moves forward, thus generating lift. |
But wasn't the premise of the original statement that the treadmill will always speed up and prevent the plane from moving forward? |
No. The premise of the original statement is that the treadmill moves faster to match the speed of the plane.
You then provide the misunderstanding that this prevents the plane from moving forward. This is because you are assuming the wheels are in some way driving the plane against the ground. That's the basic misunderstanding.
The engine, acting on the air drives the plane forward, creates airflow, causes lift.
The wheels are just along for the ride. If they spin faster, they spin faster. It doesn't change where the plane is. |
What utter tripe! Everyone is so bogged down now with friction, drive to wheels etc.
The amount of friction caused by the wheels on the ground doesn't even count. The wheels are holdy up things, no more. Thrust provides the forward motion, increased thrust increases ground speed enough for lift off.
Forget sped of wheels etc, they are under the aircraft and as the aircraft accelerates, the wheels move faster...logical. However, the aircraft is moving. If the treadmill matches that forward motion, the plane ain't gonna move! Stick a Saturn rocket on it and allow the treadmill to match the forward speed and the aircraft will not move. For every action there is an equal and opposite reaction. Match forward propulsion with rear propulsion(treadmill), plane don't go nowhere fast. |
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12/06/2006 03:55:34 PM · #92 |
Originally posted by sodoff:
Forget sped of wheels etc, they are under the aircraft and as the aircraft accelerates, the wheels move faster...logical. However, the aircraft is moving. If the treadmill matches that forward motion, the plane ain't gonna move! Stick a Saturn rocket on it and allow the treadmill to match the forward speed and the aircraft will not move. For every action there is an equal and opposite reaction. Match forward propulsion with rear propulsion(treadmill), plane don't go nowhere fast. |
You are confusing speed with acceleration. There is a way to accelerate the treadmill fast enough to provide the equivalent opposite force, but that is not mentioned at all in the original question. Read the two links above, they clearly explain where you went wrong.
Put a rollerskate on your foot. Stand on a treadmill with that one foot. Run the treadmill as fast as you like. Can you move your foot forward ? Of course you can. Because the wheels are not connected directly to your foot through any drive mechanism. They spin. Big deal. It doesn't force your leg backwards much, beyond some basic component of the angular momentum. It certainly doesn't stop you moving it forward or holding it still.
Message edited by author 2006-12-06 15:57:10.
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12/06/2006 03:55:39 PM · #93 |
Originally posted by ShorterThanJesus:
Originally posted by Zal:
But wasn't the premise of the original statement that the treadmill will always speed up and prevent the plane from moving forward? |
If the plane was somehow stopped from moving forward, then it would never be able to take off. You are right about that. The argument which has been made is that there is no way a treadmill could stop the plane from moving forward (in relation to the surroundings) and therefore the plane would take off normally. But yes, if this treadmill was somehow able to actually stop the plane from moving forward, the plane would be unable to take off. As you said, a plane cannot generate lift while not moving forward. |
OK, now I think we are on the same page. I thought that the original question implied that the treadmill will always speed up to prevent the plane from moving forward relative to the surroundings.
So now we are saying that once the treadmill "maxes out" at a certain speed, the thrust from the engine will push the plane down the treadmill (even though the treadmill is moving in the opposite direction), move it through the air, and hence, take off? If so, then I agree.
I think the original question is poorly worded. :-) |
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12/06/2006 03:55:59 PM · #94 |
Originally posted by sodoff:
Originally posted by Gordon:
Originally posted by Zal:
Originally posted by eqsite:
Therefore, the thrust from the engines which is imparted to the airplane is not countered by the treadmill, and therefore the plane moves forward, thus generating lift. |
But wasn't the premise of the original statement that the treadmill will always speed up and prevent the plane from moving forward? |
No. The premise of the original statement is that the treadmill moves faster to match the speed of the plane.
You then provide the misunderstanding that this prevents the plane from moving forward. This is because you are assuming the wheels are in some way driving the plane against the ground. That's the basic misunderstanding.
The engine, acting on the air drives the plane forward, creates airflow, causes lift.
The wheels are just along for the ride. If they spin faster, they spin faster. It doesn't change where the plane is. |
What utter tripe! Everyone is so bogged down now with friction, drive to wheels etc.
The amount of friction caused by the wheels on the ground doesn't even count. The wheels are holdy up things, no more. Thrust provides the forward motion, increased thrust increases ground speed enough for lift off.
Forget sped of wheels etc, they are under the aircraft and as the aircraft accelerates, the wheels move faster...logical. However, the aircraft is moving. If the treadmill matches that forward motion, the plane ain't gonna move! Stick a Saturn rocket on it and allow the treadmill to match the forward speed and the aircraft will not move. For every action there is an equal and opposite reaction. Match forward propulsion with rear propulsion(treadmill), plane don't go nowhere fast. |
All of the motion of the treadmill is translated into rotation of the wheels. The wheels simply spin faster. The plane flies. |
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12/06/2006 03:57:41 PM · #95 |
Put a jet on a runway without it wheels down in a locked position. Now imagine a jet sitting on it belly (underside) now throttle up. It will just sit there. Maybe move from a little but will not get airborne because it will not be able to reach the minimum take off speed.
Why do you think on an aircraft carrier that have to have a catapult to get the jet up to speed (on the deck) [substituting deck for ground] to achieve flight by the time the jet reaches the release point at the end of the runway. Keep in mind that a jet on the aircraft carrier has it's engines at full throttle while sitting still until it is released by the catapult arm.
Message edited by author 2006-12-06 15:59:46. |
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12/06/2006 03:58:05 PM · #96 |
Originally posted by legalbeagle:
An interesting problem:
A plane equipped with fixed horizontal engines and wheel landing gear is placed on a huge treadmill runway. The treadmill has a clever design and always matches the speed of the plane, but runs in the opposite direction. Will the plane take off and fly or not?
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12/06/2006 03:58:37 PM · #97 |
Originally posted by Zal:
OK, now I think we are on the same page. I thought that the original question implied that the treadmill will always speed up to prevent the plane from moving forward relative to the surroundings.
So now we are saying that once the treadmill "maxes out" at a certain speed, the thrust from the engine will push the plane down the treadmill (even though the treadmill is moving in the opposite direction), move it through the air, and hence, take off? If so, then I agree.
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No. The treadmill always matches the speed of the plane. Which makes the wheels spin twice as fast as they normally would. The force is not converted to the plane, but transferred into the rotation of the wheels.
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12/06/2006 04:00:40 PM · #98 |
Originally posted by Southern Gentleman:
Put a jet on a runway without it wheels down in a locked position. Now imagine a jet sitting on it belly (underside) not throttle up. It will just sit there. Maybe move from a little but will not get airborne because it will not be able to reach the minimum take off speed.
Why do you think on an aircraft carrier that have to have a catapult to get the jet up to speed (on the deck) [substituting deck for ground] to achieve flight by the time the jet reaches the release point at the end of the runway. Keep in mind that a jet on the aircraft carrier has it's engines at full throttle while sitting still until it is released by the catapult arm. |
Nobody said the wheels don't stop the plane from dragging along the ground (or pitching into the sea in the case of an aircraft carrier)
But they don't provide lift, or acceleration.
The catapult on a deck is to provide more acceleration than is possible with a sane size of jet engine and conserve fuel, because of the short runway.
It isn't because the deck is moving or the wheels are spinning fast.
The kitty hawk for example didn't use wheels. It had a sled.
Message edited by author 2006-12-06 16:01:31.
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12/06/2006 04:01:08 PM · #99 |
Both answers are right;
Using 'real world' physics the plane will evenutally take off. No matter how efficient you make the treadmill eventually it will accelerate to its maximum speed, and the thrust of the jet engines will win.
Using 'imaginary' physics, you could build a treadmill that allows infinite acceleration, and a jet engine that supplies infinite thrust. The plane will forever stand still in some strange quantum state once the treadmill starts moving at the speed of light and the jet engine consumes all the matter in the universe... ;-) |
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12/06/2006 04:01:24 PM · #100 |
It seems to me that the answer you get depends on the assumption you make as to the type of aircraft you are thinking of.
If I assume the plane is a Cessna, it can't take off because the design depends on generating lift at the wings to get altitude. It's motor and prop will never pull enough air past the wings to create the needed imbalance of air pressure under and over the wing to create liftoff, even if the wheels are frictionless.
If I imagine a military powered jet on that same treadmill, a plane witht the same glide potential as a brick, it ought to take off no problem. With it's huge thrust it needs very little lift from the wings and you could have hundreds of pounds of friction on the wheels, given enough thrust it could launch.
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