A jet plane on a large treadmill

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Originally Posted by focus

Absofuckingloootely was overstating it. The point remains. Pressures, airspeeds, etc. are not what the treadmill is measuring. The treadmill is measuring the PLANE'S (or cars) velocity, not airspeed, not pressures, not wheel speed. Dumbass of the year? I'll wear it. Doesn't change the fact that you still don't seem to fucking get it.

;)

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Actually we agree damnit. Just not on the car thing. :)
BUT my head hurts so fucking much from this thread that I haven't given it much thought.

so im not going to get in any further of an argument, but I still dont see the plane moving.

the acceleration thats counteracting your air mass from the turbines is from inertia, ie gravity.


heres my thought process (so you can either correct me or figure out what im saying)


normal plane normal runway
engines spin up, thrust applied to plane, plane rolls, rolls fast enough to take off

normal plane on conveyor
engines spin up, thrust applied to plane, plane rolls but instantly so does the conveyor, and the plane never gains any airspeed.


if the plane isnt allowed to move forward, and gain airspeed and lift, by a moving ground, then it doesnt takeoff.

seriously, not trying to get in an argument, so theres no need to get so pissy and defensive. If you guys are right, which Im not 100% certian yet, Id like to understand why. Im not there yet.

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Originally Posted by MeatPuppet

The question, as it is asked, is unanswerable. There is simply not enough information provided. Everybody is (mostly unknowingly) inserting their own information into this question to make it answerable. Insert different information, get different answers.


If the bearings are of this earth and are affected by friction: then, if the thrust provided by the engines is sufficient to accelerate the plane to a speed that creates enough airflow across the wings to create enough lift for the plane to take off...the plane takes off. But, if the counteracting force generated by the friction in the bearings is not able to be overcome by the engines...it doesn't.

As I said, the question, as it is presented, is unanswerable. More information has to be introduced and people are unconsciously doing this based on perceived characteristics of devices(i.e. a treadmill makes objects remain stationary/jet engine makes airplane take off) without really considering the individual forces involved.

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Go reread my post on page 7.

woah
howd you quote me before i posted? ;)
I deleted that and clarified my post

hah

anyway, "not allowed to roll forward" because the conveyor matches the plane's speed at the same instant. Im thinking 100% in velocity here, because thats whats important for a plane to takeoff.

Pechelman, think of it this way, say the plane is landing on the treadmill, which is rotating at the same speed as the plane, it would be no different then landing on a stationary strip, his wheels will just spin faster. The conveyer is not applying a force that counteracts that create by the engines.

and a pitot tube will give you the absolute air speed measured from pressure, not the relative ground speed.

this is why racecars have pitot tubes in addition to wheel speed sensors.

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Originally Posted by pechelman

normal plane normal runway
engines spin up, thrust applied to plane, plane rolls, rolls fast enough to take off

normal plane on conveyor
engines spin up, thrust applied to plane, plane rolls but instantly so does the conveyor, and the plane never gains any airspeed.

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There's the problem. Say the airplane begins moving at 1 mph. The treadmill moves backward at 1 mph. The wheels are free spinning and do not transfer any force to the airplane. The airplane continues to roll. The treadmill cannot go faster than the airplane to increase rolling friction and stop it.

Once the plane begins rolling, the conveyor cannot stop the airplane. The frictional force between the conveyor and the wheels is almost insignificant.

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Originally Posted by DJSapp

Go reread my post on page 7.

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Dude, why didn't you tell me to go look at your post before I went through the trouble of writing all that down?!? You even used numbers and pictures...for all the good it did...:biggrin:

LMAO!

Page 20 here we come.

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Originally Posted by P_McPoser

Pechelman, think of it this way, say the plane is landing on the treadmill, which is rotating at the same speed as the plane, it would be no different then landing on a stationary strip, his wheels will just spin faster. The conveyer is not applying a force that counteracts that create by the engines.

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gonna have to disagree again

the plane when landing already has velocity

in this scenario i just dont see it moving.

i just dont see your guys' logic yet.
gah maybe i need to let this settle for a bit
it was always these problems i sucked at anyway

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Originally Posted by pechelman

anyway, "not allowed to roll forward" because the conveyor matches the plane's speed at the same instant. Im thinking 100% in velocity here, because thats whats important for a plane to takeoff.

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Dude, if it doesn't move forwards, the conveyor doesn't roll backwards. In your world, nothing will ever move. Ever. Not the plane, not the wheels, not the treadmill. There's so much tension in your world, man.

The treadmill matches the plane's forward speed. No more, no less. It isn't overcoming anything. The conveyor is not charged with the task of keeping the plane stationary. It just rolls backwards at the same speed that the plane is rolling forwards. The wheels do double-time.

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Originally Posted by pechelman

woah
howd you quote me before i posted? ;)
I deleted that and clarified my post

hah

anyway, "not allowed to roll forward" because the conveyor matches the plane's speed at the same instant. Im thinking 100% in velocity here, because thats whats important for a plane to takeoff.

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The conveyer matches the velocity of the plane, but the only connection between the plane and conveyer is a roller mang, thing back to dynamics. Even if you want to talk friction beteween the tires and the conveyer, and the wheel bearings, that horizontal force element is never going to be significant compared to that of a jet engine.

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Originally Posted by focus

Dude, if it doesn't move forwards, the conveyor doesn't roll backwards. In your world, nothing will ever move. Ever. Not the plane, not the wheels, not the treadmill. There's so much tension in your world, man.

The treadmill matches the plane's forward speed. No more, no less. It isn't overcoming anything. The conveyor is not charged with the task of keeping the plane stationary. It just rolls backwards at the same speed that the plane is rolling forwards. The wheels do double-time.

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my understanding of "instantaneously" means at the same time

if the plane would normally move forward at a certain velocity and acceleration on a normal runway, then the conveyor would match this velocity and acceleration profile exactly and at the same time indicies.
instantly. at the same time.

thats where i think we're assuming different things or reading it differently
maybe?

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Originally Posted by DJSapp

The airplane continues to roll. The treadmill cannot go faster than the airplane to increase rolling friction and stop it.

Once the plane begins rolling, the conveyor cannot stop the airplane. The frictional force between the conveyor and the wheels is almost insignificant.

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And this is your problem.

The treadmill cannot go faster than the forward velocity of the airplane. Let me say that again - it cannot accelerate faster and faster and faster to stop the airplane. It matches speed exactly with the forward velocity of the airplane.

The conveyor is not there to stop the airplane. That's not what it does. Less than insignificant, frictional force really has nothing to do with the problem.

again, not arguing
just explaining my thought process and reasoning so i can get to the bottom of this and sleep tonight :p

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Originally Posted by pechelman

gonna have to disagree again

the plane when landing already has velocity

in this scenario i just dont see it moving.

i just dont see your guys' logic yet.
gah maybe i need to let this settle for a bit
it was always these problems i sucked at anyway

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Net forces cause accelerations. The engines cause a force forward due to them throwing air molecules backward(like throwing a basketball in a wheeled office chair) The spinning treadmill creates a force backward due do friction in the whole wheel system. Most likely at all treadmill speeds the force of friction is never bigger or equal to thrust(though it may be at crazy speeds assuming the wheel system doesn't break apart, see my above post) thus there is a net force forward and thus an acceleration

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Originally Posted by P_McPoser

The conveyer matches the velocity of the plane, but the only connection between the plane and conveyer is a roller mang, thing back to dynamics. Even if you want to talk friction beteween the tires and the conveyer, and the wheel bearings, that horizontal force element is never going to be significant compared to that of a jet engine.

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no, youre right, its not

but theres the whole thing about the plane's inertia that is significant compared to the engines' thust.

the whole idea behind the thrust, is that its meant to make the plane move.
when its at rest, the plane must roll or slide or whatever.

in order for it to have any relative airspeed, it must roll or slide or whatever, with relationship to the ground, in this case, which is moving in the opposite dir at the same velocity\accel profile.

gah we seem to be talking in circles

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Originally Posted by "The Big One"

A plane (747 passenger jet) is sitting on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the planes speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The question is:

Will the plane (747 passenger jet) take off or not?

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my understanding of "instantaneously" means at the same time

if the plane would normally move forward at a certain velocity and acceleration on a normal runway, then the conveyor would match this velocity and acceleration profile exactly and at the same time indicies.
instantly. at the same time.

thats where i think we're assuming different things or reading it differently
maybe?

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Exactly. At the same time. Now, does that really matter? You're hung up on instantaneous and really, that isn't where you're going wrong.

Put a car on the same treadmill. The treadmill measures how fast the car is moving forwards. Step on the gas, and the car moves forwards, while the conveyor moves backwards. The wheels turn at twice the speed. Your acceleration is probably around half of what it could be on dj's dyno.

The treadmill doesn't care if the plane or car moves forwards or not. It just goes the same speed backwards. It isn't locked into battle with the plane or car.

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Originally Posted by pechelman

my understanding of "instantaneously" means at the same time

if the plane would normally move forward at a certain velocity and acceleration on a normal runway, then the conveyor would match this velocity and acceleration profile exactly and at the same time indicies.
instantly. at the same time.

thats where i think we're assuming different things or reading it differently
maybe?

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You are assuming that the wheels will impart enough force to keep the airplane still. This is not the case. The airplane pushes against the air to accelerate, the wheels do nothing. The treadmill causes the wheels (which do nothing) to spin faster.

No, you're understanding it correctly, other than that the speed of the conveyor does nothing but spin the wheels faster and faster, it never does anything to counteract the forward motion of the aircraft.

Lets look at it this way, say the a/c wheels are perfectly frictionless. Fire up the conveyer, the wheels start spinning, the plane goes nowhere. This is the equilibrium case. The conveyer imparts no force on the plane other than turning the wheels.

Okay, the other case, the conveyer is turned off, the jet takes off, the wheels of the jet impart no force on the conveyor, it doesn't move.

The interaction of the wheels and conveyor do nothing to impart a force on the aircraft.

As far as inertia, the only thing the aircraft has to overcome is the rolling resistance of the tires, which isn't going to be a problem.

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Originally Posted by P_McPoser

No, you're understanding it correctly, other than that the speed of the conveyor does nothing but spin the wheels faster and faster, it never does anything to counteract the forward motion of the aircraft.

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... I'm saying this one more time and then you retards ( ;) ) are on your own.

P_McP and Mr. Wong -- the treadmill cannot -- CANNOT -- spin faster and faster and faster. It matches the forward velocity of the plane. Again, that is ALL it does. It isn't trying to do ANYTHING else (anything as in, keep the plane stationary).

WHY would the treadmill spin faster and faster? How would it know?? Is it measuring the force generated by the engines? Does it care? Does the question have anything to do with the treadmill being charged with the duty of stopping the plane? The only thing the treadmill knows is how fast the plane is going forwards. The only thing the treadmill is ALLOWED to do is match the FORWARD VELOCITY of the plane.

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Originally Posted by focus

... I'm saying this one more time and then you retards ( ;) ) are on your own.

P_McP and Mr. Wong -- the treadmill cannot -- CANNOT -- spin faster and faster and faster. It matches the forward velocity of the plane.

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Wait, so this forward velocity of the plane, it's increasing right? And then the conveyer matches that right? And the wheels do not speed up how now? :smile:

Your brain is starting to get scrambled as well. ;)

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Originally Posted by pechelman

no, youre right, its not

but theres the whole thing about the plane's inertia that is significant compared to the engines' thust.

the whole idea behind the thrust, is that its meant to make the plane move.
when its at rest, the plane must roll or slide or whatever.

in order for it to have any relative airspeed, it must roll or slide or whatever, with relationship to the ground, in this case, which is moving in the opposite dir at the same velocity\accel profile.

gah we seem to be talking in circles

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Ok, here's the disconnect.

Inertia = Mass * Velocity

For the plane to gain inertia, it sucks in huge amounts of lightweight air and pushes it out at great speed. Inertia must be balanced. Therefore:

380,000 lbs (weight of air moved by four 747 engines at full throttle) * 1,000 mph (est. exit speed of air from engine) = 910,000 lbs (weight of 747) * 417.58 mph

Since the plane begins at zero mph, the inertia begins to move the plane. The ground has nothing to do with it.

Thanks DJ. I didn't feel like cracking out books to do that.

You people are funny and a notable proportion of you just don't get it.

I really hope that none of you who think that the plane won't take off are engineering/building anything with dynamic components.

But, this is a nice blast from the past.

um thats momentum.
isnt inertia is F=ma or that ability to change something's momentum?

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Originally Posted by Telenater

You people are funny and a notable proportion of you just don't get it.

I really hope that none of you who think that the plane won't take off are engineering/building anything with dynamic components.

But, this is a nice blast from the past.

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good thing im in the structures group and not in propulsion :p

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Originally Posted by Telenater

You people are funny and a notable proportion of you just don't get it.

I really hope that none of you who think that the plane won't take off are engineering/building anything with dynamic components.

But, this is a nice blast from the past.

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be scared, because (don't know about focus) but the last few pages has been an enginerd circle jerk.

and im still thinking this through
but ive got other things i need to do atm

Pechelman, this is easy, I have access to a FUCKIN HUGE conveyor, remember? All the way through the divide. Just get some rockets up here. ;)

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Originally Posted by pechelman

um thats momentum.
isnt inertia is F=ma or that ability to change something's momentum?

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You are correct, however inertia is a concept, regularly used by laymen to describe momentum and not force. Momentum is the deritive of force, so it really doesn't matter which you are talking about, the principle remains the same.

F = m*a

P = m*v

ok i think im half way there
just need to convince myself now it would really move relative to the ground\air

thanks to most of you for being patient

edit:

humility is important in situations like this, and im definitely not short of it

You guys are right, its moving relative to the ground\air, thanks for sticking with me here.

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Originally Posted by P_McPoser

Pechelman, this is easy, I have access to a FUCKIN HUGE conveyor, remember? All the way through the divide. Just get some rockets up here. ;)

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"News Flash! The missing 747 has been found with the nose wedged into an entrance of a mine in Northern Nevada. Authorities are baffled, saying the only clue they have is a notepad, left behind by the thief's when they fled, with the words "must get to conveyor" scrawled on it."


Film at Eleven.

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Originally Posted by pechelman

ok i think im half way there
just need to convince myself now it would really move relative to the ground\air

thanks to most of you for being patient

edit:

humility is important in situations like this, and im definitely not short of it

You guys are right, its moving relative to the ground\air, thanks for sticking with me here.

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DPPS :D

edit: predicting at least 20 pages before this dies

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Originally Posted by P_McPoser

be scared, because (don't know about focus) but the last few pages has been an enginerd circle jerk.

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Yes, I know. I just find it entertaining and scary that the geographer/Biologist gets this and some engineers don't. Now, I won't claim to be your typical geographer or biologist, but the ability to understand that the engines pushing against air can accelerate an airplane regardless of a stupendously large conveyor belt should be a prerequisite for an engineering degree.

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Originally Posted by P_McPoser

Pechelman, this is easy, I have access to a FUCKIN HUGE conveyor, remember? All the way through the divide. Just get some rockets up here. ;)

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I GUARANTEE that bitch is far from frictionless!!!

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Originally Posted by Powow

It matches the speed, but then what does that mean? The treadmill doesn't put any force on the plane.

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And there you have the answer. can we move on now?

Gotta say, T-Luv - I'm a bit surprised.

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Originally Posted by rideit

I GUARANTEE that bitch is far from frictionless!!!

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The conveyor belt doesn't need to be frictionless. It just needs to be able to match the speed of the idiot in a wheelchair holding onto a firecracker (but in the opposite direction).

No, it doesn't. The firecracker/motorcycle/Jet Engine isn't pushing against the treadmill. QED.

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Originally Posted by MeatPuppet

"News Flash! The missing 747 has been found with the nose wedged into an entrance of a mine in Northern Nevada. Authorities are baffled, saying the only clue they have is a notepad, left behind by the thief's when they fled, with the words "must get to conveyor" scrawled on it."
Film at Eleven.

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Or Colorado....