A jet plane on a large treadmill

[pechelman]

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.

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

[focus]

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?

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?

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.

[DJSapp]

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?

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.

[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.

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.

[P_McPoser]

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.

[focus]

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.

... 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.

[P_McPoser]

... 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.

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?

Your brain is starting to get scrambled as well.

[DJSapp]

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

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.

[P_McPoser]

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

[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.

[pechelman]

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

[pechelman]

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.

good thing im in the structures group and not in propulsion

[P_McPoser]

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.

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

[pechelman]

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

[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.

[DJSapp]

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

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

[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.

[MeatPuppet]

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.

"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.

[DJSapp]

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.

DPPS

edit: predicting at least 20 pages before this dies

[Telenater]

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

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.

[rideit]

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.

I GUARANTEE that bitch is far from frictionless!!!

[Tippster]

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

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

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

[Telenater]

I GUARANTEE that bitch is far from frictionless!!!

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).

[Tippster]

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

[P_McPoser]

"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.

Or Colorado....