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Plane Problem

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Moz

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Nov 21, 2005
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Would you all agree that you need the ground in some fashion to take off (water included)?

Ok, then if the ground is matching your forward speed in the opposite direction, how do you get moving forward to create enough air causing lift?

Plane is at a full stop. Applies thrust, starts to creep forward. At the exact same instant the treadmill moves with equal speed in the opposite direction. The plane is never allowed to move forward or gain momentum due to the treadmill matching the speed made by the plane's thrust - in the opposite direction. Where does the force of the air come from in an example that has the plane, in effect, standing still?
WARNING; I'M HAVING A BRAIN FART AND CAN'T REMEMBER BASIC MATH, SOMEONE PLEASE CLEAN THIS UP FOR ME

There is a curve.

For every x thrust , there is an equal and opposite y thrust of the treadmill.
Z is the motion.

(x)-(y)=z

So mr car goes 10mph, treadmill goes 10mph back

(10)-(10)=0

So the car goes nowhere.

The wheels on the car are set to neutral, and a ROCKET engine on top is putting out the equivalent of 100mph of thrust. So the treadmill puts 100mph back in to the wheels that the car are resting on.

(100)-... wait.

The wheels are free rolling, so when you push not all the force is transfered, so maybe 0.1 of the force is transfered to the plane.

So thats

(100)-(100x0.1)=z
(100)-(10)=90

So the plane moves 90kph forward. Instead of 100kph.

Of course the 0.1 would actually be a curve because more force the more resistance, blah blah blah but Its early morning and I should be working.
 

BuddyLee

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Apr 12, 2006
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Would you all agree that you need the ground in some fashion to take off (water included)?

Ok, then if the ground is matching your forward speed in the opposite direction, how do you get moving forward to create enough air causing lift?

Plane is at a full stop. Applies thrust, starts to creep forward. At the exact same instant the treadmill moves with equal speed in the opposite direction. The plane is never allowed to move forward or gain momentum due to the treadmill matching the speed made by the plane's thrust - in the opposite direction. Where does the force of the air come from in an example that has the plane, in effect, standing still?
The ground is not acting against the plane (since the ball bearings/wheels/axle eliminate friction) at the same time the engines are pushing off the air like normal.

The speed/force/motion of the treadmill is transferred to the ball bearings, (it is acting against the wheels, not the fuselage) so that force is cancelled out. Which leaves you with the thrust, thrust is working against the air, creating forward momentum. This forward momentum will continue until the plane has reached take off speed.

Someone E-Mail Bill Nye the Science Guy. "Bill!! Bill!!"
 

melipone

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Mar 22, 2006
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Exactly.. if the plane cant move forward, than no lift will occur. The question is poorly worded. The plane cannot move faster than the treadmill according to the question. So without forward movement through the air... lift cannot be generated.
You dont see though that some or most of the speed from the treadmill is absorbed by the wheels turning. Look at the rollerskates on a treadmill analogy - would you go straight back at the same speed as the treadmill if you didn't have an external force or would the wheels absorb most of it? You can add whatever force you like now - and make yourself go as fast as the treadmill if you like.,.you still go forwards because of the wheels
 

Cuddles

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Nov 2, 2006
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Even if the plane gets enough lift the dreadmill is to short and the plane will fall down


Ah enough of words Someone get a RC plane and put it on a dreadmill.
 

Moz

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Nov 21, 2005
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My Hypothesis isn't that the plane is going to take of the spot.

I didn't state this clearly in the video because I am an idiot. (sorry, I know :( )

The plane will move forward regardless and will not be effected very much. It is not going to take off on the spot.

http://www.youtube.com/watch?v=5pbaO4NbYEQ

Also I have to get to work now but getting paid **** money seems a lot better than arguing over the interbutt as my alternative
 

REZ

FNG / Fresh Meat
Nov 21, 2005
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The treadmill keeps up with the body of the plane, no? Am I wrong? Or is this a question trying to get you to figure the .00001 (or whatever) friction value. Cause if it is, then I'm looking at it in a different way.


Spra is right... the question is poorly worded, and can be interpreted in different ways.
 

MkH^

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Mar 11, 2006
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Of course it won't take off. No matter how much thrust can the engines provide, the dreadmill will just pick up more rotations per minute and the plane will remain at static position - the dreadmill nullifies the thrust generated by the planes engines.

Everyone who knows sh*t about planes, knows that the lift is generated by airflow. The airflow is generated by the thrust from the engines pushing the plane forward - when the plane is standing still, there is no flow of air. That's why it's not necessary to tie parked aircraft on the ground on an airfield.

Take a really fast and light car, add huge airplane wings onto it and speed up on an airport. When you gain enough speed, the car will very likely take off, or at least partially lift up with disastrous effects. Now, put the car on a giant dreadmill and put pedal to the metal - nothing will happen. No airflow whatsoever is generated. The car is practically standing still.

Of course, the purpose of the wheels on planes is to minimize the friction, while in cars it is to create it, but it does not really matter on this case.
 

BuddyLee

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Apr 12, 2006
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lol, nice response. But 2 questions: how is the plane getting pushed through the air if the treadmill is stationary? if the treadmill exactly matches the speed of the plane then there is no airspeed.

how can the plane accelerate forward without actually moving forward?

BTW, after watching the video again I think they guy shoots himself in the foot. He said lift is irrelevant for this example, but any pilot will tell you that lift is REQUIRED for an airplane to get airborne. Just because an engine creates thrust does not mean it will generate any actual speed or lift. He also contradicts himself by saying that somehow the wheels move faster than the treadmill. "if the jet is moving at 200 kph forward and the treadmill is moving at 200 kph in the other direction, then the wheels are moving at 400 kph." ????
If there were no treadmill then the wheels would be moving at 200kph?

I just don't see any airspeed with this example or explanation. Airspeed is the motion of moving through the air, and a treadmill negates that motion. The treadmill merely moves the ground under the plane, not the airspeed around the wings. So....where is the airspeed being created?

So thrust creates motion, we all know that, and the motion is what creates the lift through air resistance, it doesn't just suddenly appear because you have thrust. the air needs to move around things to create lift. Consider putting your hand out of a window of a moving car, boat, plane, whatever. as you move your hand around, you fill the resistance, i.e. lift, acting on your hand. Take that same car and put it on a dyno. The same amount of forward motion is being generated but if you put you hand out the window, there is no air resistance, i.e. lift. I don't think where the thrust is generated makes a difference, it's the forward motion that creates lift. Even race cars will get airborne without their downforce, but it will never happen on a dyno because there is no air resistance being created.


Sorry for the book, :D
The wheels will always spin at the same speed as the treadmill while they are still in contact, even when the plane moves forward from the center of the treadmill as it accelerates, the wheels are still moving the same speed as the treadmills surface.

Even if it seems that the plane is moving faster than the surface of the treadmill because of forward momentum as it takes off, both the wheels surface, and the treadmils surface are still moving at the same speed because as the plane moves forward it increases the speed the treadmills surface is moving in relation to the wheels proportionately.

For instance, as two cars converge going opposite directions on the highway, one is moving at 25mph, the other at 75 mph. even though one car is traveling faster than the other, they are closing the distance at the same rate/speed. 100mph.
 

Cuddles

FNG / Fresh Meat
Nov 2, 2006
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Ok now I get it :D if theres no friction or very little friction the wheels will spin freely on the treadmill but the plane is moving forward and if the tredmill is long enough it could actually fly (in my mind) And I was always thinking the treadmill was as long as the plane itself.:(
 

REZ

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See that friction in the wheels thing has been added to validate a theory I'm afraid. The question is much simpler, and in my eyes is trying to say that even though there is force and movement... those things alone dont equal lift.

You have to have actual forward movement through the air for that.

The question isnt designed to single out a vague friction value (at least I dont think it is). I'm glad I only invested a few minutes into this and didnt go make a movie or some sh!t like that...

The engines causing the thrust have a peak... the treadmill is MAGICAL. The treadmill as stated keeps up with the plane. Therefore, no foward movement.
 

stebbs

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May 15, 2006
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The wheels will always spin at the same speed as the treadmill while they are still in contact, even when the plane moves forward from the center of the treadmill as it accelerates, the wheels are still moving the same speed as the treadmills surface.

Even if it seems that the plane is moving faster than the surface of the treadmill because of forward momentum as it takes off, both the wheels surface, and the treadmils surface are still moving at the same speed because as the plane moves forward it increases the speed the treadmills surface is moving in relation to the wheels proportionately.

For instance, as two cars converge going opposite directions on the highway, one is moving at 25mph, the other at 75 mph. even though one car is traveling faster than the other, they are closing the distance at the same rate/speed. 100mph.

I completely agree with that, but the wheels will still move at the same speed as the ground is underneath it. I was merely commenting on his words, that the wheels will move faster than the treadmill. Their combined speed would be as you said, but the wheels on their own will not be moving faster than the ground below it.

On a side note, how can the plane move past the center of the treadmill if the treadmill exactly matches the speed of the plane? My understanding of the original argument was that the treadmill will EXACTLY match the speed of the plane, so no forward movement by the plane would be possible.
 

worluk

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Nov 21, 2005
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On a side note, how can the plane move past the center of the treadmill if the treadmill exactly matches the speed of the plane? My understanding of the original argument was that the treadmill will EXACTLY match the speed of the plane, so no forward movement by the plane would be possible.
again, the plane can go in one direction by a velocity v, the treadmill in the opposite direction -v, the wheels would turn with the value of 2v.
Quite possible if you ask me. The only thing that would be influence by the treadmill matching the speed of the plane is the planes wheels.

I cant see were the problem is worded poorly.
 

Nestor Makhno

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Feb 25, 2006
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I am glad that the forward motion = lift scenario is now pretty clear in most people's minds.

The 'forward motion is impossible' axiom has still to be established but, I believe, can be - that the wheels will be carried backwards at a set rate by friction with the ground and then be pushed forward at the same magnitude but opposite direction velocity

To Moz - stop screaming and descending to personalities and I will attempt to explain my viewpoint.

At no point did I say that the wheels provide thrust to the entire plane. Clearly the engine provides thrust to the entire rigid body (i.e. the plane). The reason to focus on how that thrust affects the wheels is that that is the interface - the only point of contact - with the treadmill, and we are attempting to establish whether a treadmill set-up can produce a situation of no forward motion relative to the ground.

As far as I see it the crux of the matter is do the wheels end up moving at twice the speed of the plane or do they end up going at just the actual speed of the plane meaning that the plane stands still relative to the ground. If the plane were on skis or on its belly or something there would be no debate - it would not and could not fly.

I am not feeling inclined to discuss the matter further until people have calmed down and stopped name-calling. In the interim I will give full consideration to all aspects of the problem and post something that definitively puts the matter straight.

If I establish that the plane is capable of net forward motion despite being on a treadmill, I will happily post in this thread saying so. If I establish that there is a very good reason for no net forward motion I will also post a detailed explanation.
 

BuddyLee

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Apr 12, 2006
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OK if the treadmill is infinite in length.

And Newton's Law, for every action, there is an equal but opposite reaction applies in this magical scenario.

I see 3 forces at work in this problem.

1) The force applied by the surface of the treadmill against the surface of the tires. This force is predominately countered by the existing landing gear system combined with a small percentage of the engines thrust.

2) The engine thrust, this is working against the wind(drag).The plane has enough thrust to propel the airplane to takeoff speeds, and beyond.

3) At this point, the airflow over the wing surface will generate lift, working against gravity, enabling the plane to fly.

And as I said before, the surface of the tires and the surface of the treadmill are always in sync, closing at exactly the same speed until the point of liftoff.

There would be no extra drag applied to the body of the aircraft by the motion of the treadmill. The treadmills motion only effects the landing gear.

That is as simple as I can explain it.
 

Nestor Makhno

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Feb 25, 2006
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I just had a look at http://forum.physorg.com/index.php?showtopic=2417 where this problem has been discussed for 432 pages - count them - 432!!

A quick scan, flitering out the flame and so on suggests that Moz is right - the wheels, surface and so on are largely irrelevant to the airflow.

Imagine the turbines in the jets as a way of grabbing the air and skooshing it out of the back - giving you your equal and opposite reaction of thrust.

Imagine that the wheels' primary purpose is to stop the plane falling on its belly. That's all; as the majority of the reaction is against the air being dragged thru the turbine not against the ground.

The motion of the wheels will be fast as they attempt to deal with the forward motion of the plane and the backward motion of the treadmill.

Never let it be said that I am not able to recognise when I am in the wrong.

BTW Moz - your being right does not prevent your "big letter" post from looking rather churlish.

Possibly one reason the truth is so hard to determine on the internet is that everything sounds like rubbish when it is expressed in the kind of language that gains attention on forums etc.
 
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