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12/06/2006 04:01:40 PM · #101 |
Originally posted by Gordon:
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. |
So then how does the plane physically move relative to its surroundings, such that there is airflow over the wings to generate lift? |
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12/06/2006 04:02:06 PM · #102 |
I give up, everyone is so bogged down with their own theories that the obvious just isn't anymore. My neighbour Brain Johnson, World Champion Top Fuel Bike, didn't do 6.1 second run cos of friction. On a treadmill matching his acceleration, he was staionary.
I await the truth later in this post. |
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12/06/2006 04:02:14 PM · #103 |
Originally posted by Zal:
I think the original question is poorly worded. :-) |
You've hit the nail on the head. The confusion and arguments generated by this riddle are a direct result of the ambiguity and misinterpretation of the wording of the riddle itself. |
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12/06/2006 04:03:05 PM · #104 |
Originally posted by sodoff:
I give up, everyone is so bogged down with their own theories that the obvious just isn't anymore. My neighbour Brain Johnson, World Champion Top Fuel Bike, didn't do 6.1 second run cos of friction. On a treadmill matching his acceleration, he was staionary.
I await the truth later in this post. |
Bikes and planes don't move forward in the same way. Drag racers exactly do move forward because of friction and more specifically traction. That's why they have that huge engine, turning even bigger wheels.
Planes don't drive the wheels to create thrust. Bikes do. It is a totally different principle of motion.
If you try to apply the principles of one to the other, you end up confused like many people in this thread.
Message edited by author 2006-12-06 16:05:10.
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12/06/2006 04:06:24 PM · #105 |
Originally posted by sodoff:
On a treadmill matching his acceleration, he was staionary.
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I think that's the whole crux of the confusion. Speed and acceleration are two different things. (speed being the first derivitave of acceleration, and acceleration being the square of speed). The original question said that the treadmill will always match the speed of the plane, and did not mention acceleration. The "pro-takeoff" people are assuming that the plane will out-accelerate the treadmill, eventually achiive forward motion, and hence lift and takeoff.
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12/06/2006 04:08:41 PM · #106 |
Originally posted by sodoff:
I give up, everyone is so bogged down with their own theories that the obvious just isn't anymore. My neighbour Brain Johnson, World Champion Top Fuel Bike, didn't do 6.1 second run cos of friction. On a treadmill matching his acceleration, he was staionary.
I await the truth later in this post. |
A bike driveline generates thrust by turning a wheel that pushes horizontally on the ground.
An airplane engine generates thrust by pushing a stream air out the back at high velocity, pushing on the air behind it.
The ground is an important element in the first case and irrelevant in the second. |
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12/06/2006 04:09:31 PM · #107 |
Originally posted by Zal:
The "pro-takeoff" people are assuming that the plane will out-accelerate the treadmill, eventually achiive forward motion, and hence lift and takeoff. |
No we don't really. We just assume that the wheels don't drive the plane. So it doesn't matter how fast they spin. A car would go nowhere. The forward motion comes from the action of the wheels against the ground.
A plane does not move forward that way, so the treadmill doesn't have much if any affect.
Message edited by author 2006-12-06 16:10:15.
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12/06/2006 04:09:47 PM · #108 |
Originally posted by Zal:
The "pro-takeoff" people are assuming that the plane will out-accelerate the treadmill, eventually achiive forward motion, and hence lift and takeoff. |
Not true. We are saying that regardless of how fast the treadmill goes, it will never impart any significant force on the airplane because all of the treadmill's motion is imparted to the wheels of the plane which spin and do not themselves impart any significant force to the plane. Therefore, there is no force from the treadmill countering the force from the plane's engines. |
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12/06/2006 04:09:55 PM · #109 |
Originally posted by Gordon:
Originally posted by sodoff:
I give up, everyone is so bogged down with their own theories that the obvious just isn't anymore. My neighbour Brain Johnson, World Champion Top Fuel Bike, didn't do 6.1 second run cos of friction. On a treadmill matching his acceleration, he was staionary.
I await the truth later in this post. |
Bikes and planes don't move forward in the same way.
If you try to apply the principles of one to the other, you end up confused like many people in this thread. |
No, I;m not confused. If I can remember back to my school days, acceleration is, 'To accelerate an object is to change its velocity, which is accomplished by altering either its speed or direction (like in case of uniform circular motion) in relation to time.' This was Einstein's theory. Now if you apply the same velocity in the opposite direction, you stay stationary.
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12/06/2006 04:11:37 PM · #110 |
Originally posted by sodoff:
Originally posted by Gordon:
Originally posted by sodoff:
I give up, everyone is so bogged down with their own theories that the obvious just isn't anymore. My neighbour Brain Johnson, World Champion Top Fuel Bike, didn't do 6.1 second run cos of friction. On a treadmill matching his acceleration, he was staionary.
I await the truth later in this post. |
Bikes and planes don't move forward in the same way.
If you try to apply the principles of one to the other, you end up confused like many people in this thread. |
No, I;m not confused. If I can remember back to my school days, acceleration is, 'To accelerate an object is to change its velocity, which is accomplished by altering either its speed or direction (like in case of uniform circular motion) in relation to time.' This was Einstein's theory. Now if you apply the same velocity in the opposite direction, you stay stationary. |
Yes, if you applied the same speed in the opposite direction (negative velocity - as velocity is speed with a directional component), the plane wouldn't move.
But you don't apply the same speed in the opposite direction. You rotate the wheels.
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12/06/2006 04:13:14 PM · #111 |
Originally posted by Zal:
The "pro-takeoff" people are assuming that the plane will out-accelerate the treadmill, eventually achiive forward motion, and hence lift and takeoff. |
No, the treadmill will simply make the wheels spin faster, but it does nothing to retard the forward motion of the plane itself. |
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12/06/2006 04:16:40 PM · #112 |
You are matching whatever the forward motion, with rear motion. Whether wheels, thrust, velocity. The treadmill in this question matches it. I believe this was the original part of the question.
If my supposition is right re: the original question. Then the aircraft will not move, even it moved slightly, this would not generate enough lift for the aircarft to take off. |
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12/06/2006 04:17:33 PM · #113 |
IMO I still say no. The reason for this is the information given to us by the OP. If the treadmill aways goes at the same speed as the plane then the plane will be at a standstill. The engines on a jet/plane serve three functions. 1) while on the ground it propels the jet forward on it's wheels. 2)once the speed reaches minimum forward velocity then the engines propels the jet forward through the air. And 3) when landing the air forced through the engine is forced forward by an air-brake, slowing the jet to save braking speed.
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12/06/2006 04:17:54 PM · #114 |
Originally posted by eqsite:
all of the treadmill's motion is imparted to the wheels of the plane which spin and do not themselves impart any significant force to the plane. Therefore, there is no force from the treadmill countering the force from the plane's engines. |
Originally posted by Gordon:
But you don't apply the same speed in the opposite direction. You rotate the wheels. |
From this link that someone posted earlier;
"the treadmill is accelerating the wheels and in the process imparting some angular (rotary) but some linear (backward) momentum to them." |
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12/06/2006 04:18:01 PM · #115 |
| The whole focus on the treadmill is a red herring. No matter how fast a car moves it will not lift off a level surface. If the plane can generate enough air speed it can take off. In a wind tunnel a plane can lift off the ground without it's wheels ever turning, because it has air moving past it.How that air speed is generated is beside the point, the question is can the plane pull enough air towards it to get lift off. My answer, some can (jets) some can't (props). |
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12/06/2006 04:18:13 PM · #116 |
Originally posted by Gordon:
Originally posted by Zal:
The "pro-takeoff" people are assuming that the plane will out-accelerate the treadmill, eventually achiive forward motion, and hence lift and takeoff. |
No we don't really. We just assume that the wheels don't drive the plane. So it doesn't matter how fast they spin. A car would go nowhere. The forward motion comes from the action of the wheels against the ground.
A plane does not move forward that way, so the treadmill doesn't have much if any affect. |
But when a plane goes from rest, on a runway, to takeoff speed, that's exactly how the plane moves forward. It doesn't matter if the engine is connected to a drivetrain or if the engine is providing thrust against the air. The plane gets to takeoff speed by moving along the runway.
If I put wings on a Formula 1 car, guess what? It would take off. That's why they have rear spoilers--they are essentially upside down wings that provide downforce ("upside down lift") to keep the car on the pavement.
Obviously the Formula 1 car wouldn't stay airborne because there is no thrust, but we aren't talking about flying (thrust), we are talking about taking off (lift). |
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12/06/2006 04:20:02 PM · #117 |
Originally posted by Southern Gentleman:
1) while on the ground it propels the jet forward on it's wheels. |
Still making the same wrong assumption. While on the ground it propels the jet forward. The wheels turn.
You are assuming the first and second are connected. The wheels turning is a side effect of the plane moving forward. It is not an essential part of the forward motion.
So they could turn in circles in any direction. It doesn't matter, other than providing vertical support.
Suspend the plane from a crane and crank up the engines. Does the plane go forward and then turn in circles around the crane ? Yes it does. The wheels are nothing to do with the forward motion, at any speed.
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12/06/2006 04:21:29 PM · #118 |
Originally posted by Zal:
If I put wings on a Formula 1 car, guess what? It would take off. That's why they have rear spoilers--they are essentially upside down wings that provide downforce ("upside down lift") to keep the car on the pavement.
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repeat after me ;) Cars are not planes. They do not move forward in the same way. If I think about one in terms of the other I will be confused.
Got it ? :) F1 cars provide downforce to increase the traction of the wheels with the ground. So that the engine, turning the wheels, can drive the car faster. The foils also provide some stability, but mainly they increase traction and acceleration to transfer more speed to the car.
They use the same principle (Bernoulli's) to provide the down force, opposite from the lift.
But the forward motion is provided by an entirely different mechanism than in a plane. This is the key part of the riddle that those who think the plane will not take off, are not grasping.
Message edited by author 2006-12-06 16:23:11.
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12/06/2006 04:24:50 PM · #119 |
Originally posted by BrennanOB:
the question is can the plane pull enough air towards it to get lift off. My answer, some can (jets) some can't (props). |
Lift isn't caused by 'pulling air towards' the wings. It's caused by pushing the wings through the air at a high speed.
The props or the jets both serve the same purpose, they push the plane forwards at a high enough speed to cause air to move over the wings and generate lift. |
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12/06/2006 04:24:57 PM · #120 |
Originally posted by Gordon:
Originally posted by Southern Gentleman:
1) while on the ground it propels the jet forward on it's wheels. |
Still making the same wrong assumption. While on the ground it propels the jet forward. The wheels turn.
You are assuming the first and second are connected. The wheels turning is a side effect of the plane moving forward. It is not an essential part of the forward motion.
So they could turn in circles in any direction. It doesn't matter, other than providing vertical support.
Suspend the plane from a crane and crank up the engines. Does the plane go forward and then turn in circles around the crane ? Yes it does. The wheels are nothing to do with the forward motion, at any speed. |
But this is the assumption. The aircraft is on the treadmill, the wheels turn. The treadmill turns, the thrust is increased, the wheels turn faster, the treadmill runs faster. Ad infinitum.
Explain where thrust actually makes the plane go faster than the wheels can go/where the treadmill cannot keep up. |
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12/06/2006 04:26:03 PM · #121 |
Originally posted by Gordon:
Originally posted by Zal:
If I put wings on a Formula 1 car, guess what? It would take off. That's why they have rear spoilers--they are essentially upside down wings that provide downforce ("upside down lift") to keep the car on the pavement.
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repeat after me ;) Cars are not planes. They do not move forward in the same way. If I think about one in terms of the other I will be confused.
Got it ? :) F1 cars provide downforce to increase the traction of the wheels with the ground. So that the engine, turning the wheels, can drive the car faster. The foils also provide some stability, but mainly they increase traction and acceleration to transfer more speed to the car.
They use the same principle (Bernoulli's) to provide the down force, opposite from the lift.
But the forward motion is provided by an entirely different mechanism than in a plane. This is the key part of the riddle that those who think the plane will not take off, are not grasping. |
I totally understand everything you are saying. I think we are talking past each other because the original question is purposely worded ambiguously in order to provoke long debates on internet forums. :-)
I am going to go home and have a beer. |
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12/06/2006 04:26:11 PM · #122 |
Originally posted by sodoff:
Explain where thrust actually makes the plane go faster than the wheels can go/where the treadmill cannot keep up. |
We have. Ad infinitum. Repeating it isn't apparently going to help you get it. You assume the wheel speed and plane speed are somehow connected.
That is true for a car.
Not for a plane.
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12/06/2006 04:27:27 PM · #123 |
Originally posted by legalbeagle:
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? |
Think I see the light. Let's define what is meant by "Plane's speed" and "Treadmill's speed". I propose:
"Treadmill's speed" is the linear speed of the part of the treadmill belt in contact with the plane.
The theory of relativity will help us define "Plane's speed". Is the speed of the plane relative to:
1. the speed of the top of the treadmill belt?
2. the center of mass of the ground the treadmill sits on?
3. the air passing by the airplane?
The correct answer depends on which way you measure the plane's speed. IMO, 2 and 3 are more correct than 1, so if the plane can attain a high enough velocity, it will take off. OTOH, if you measure the speed of the plane relative to the speed of the belt, the plane will not take off unless a strong wind comes by. |
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12/06/2006 04:27:58 PM · #124 |
Originally posted by Gordon:
You assume the wheel speed and plane speed are somehow connected.
That is true for a car.
Not for a plane. |
But they are connected! - Okay, lets say I glued the wheels to the treadmill using an incredibly powerful glue. What then? Are they connected? |
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12/06/2006 04:28:03 PM · #125 |
Originally posted by Zal:
That's why they have rear spoilers--they are essentially upside down wings that provide downforce ("upside down lift") to keep the car on the pavement. |
Put that F1 on our treadmill and floor the engine. How much downward thrust is being exerted? None. The spoiler works because it cuts through the air. No relative air movment, no push on the spoiler. The planes are differnt in that the could push the air past themselves while stationary relative to the ground. |
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