A jet plane on a large treadmill

[Vinman]

I agree with mrw. it the treadmill is tued to the EXACT speed of the jet no matter what is driving the wheels it is not going anywhere. The ground spped of the jet would be 0. And with 0 ground speed there would be no air moving over the wings.

The rollerskates and treadmill example doesn't work because in the given example the belt of the treadmill is tuned to exactly the same speed as the plane. In the jet pack example your belt would need to stay at the same speed for that to work.

[Dirty Hoar]

maybe I'm misunderstanding the premise but if the thrust of the jet is equaled by a runway moving equal and opposite the jet, the jet is going nowhere.

You mean nowhere really fast don't you?

[Viva]

What if both the plane and the treadmill were moving at the speed of light?

[DJSapp]

maybe I'm misunderstanding the premise but if the thrust of the jet is equaled by a runway moving equal and opposite the jet, the jet is going nowhere.

Speed of aircraft is generally measured by airspeed (speed of wing reltative to air).

[Vinman]

Speed of aircraft is generally measured by airspeed (speed of wing reltative to air).

true, but its not in the air.

[crashnburn'd]

I agree with mrw. it the treadmill is tued to the EXACT speed of the jet no matter what is driving the wheels it is not going anywhere. The ground spped of the jet would be 0. And with 0 ground speed there would be no air moving over the wings.

I don't think the plane would take off either. Assuming no wind, the air speed would be 0 too, so you have no lift. Sure, there would be lots of thrust, but in order to gain air or ground speed, the plane needs to be able to move and because of the treadmill, it can't. The only way I could see it going anywhere would be if the jets were actually providing lift...

[Cornholio]

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).

AAAAAAHHHHHH!

THERE IS NO FUCKING AIRFLOW.

NO AIRFLOW.

NO LIFT.

It doesn't matter if I'm pushing the fucking jet or it has a transmission or 32154 jet engines.

The treadmill "tracks the plane's speed and moves in the opposite direction."

The plane is moving quickly RELATIVE to the treadmill.

The plane is NOT MOVING relative to the ground the treadmill is standing on.

The AIR is not moving relative to the ground the treadmill is standing on.

There is no airflow.

There is no lift.

[DJSapp]

AAAAAAHHHHHH!

THERE IS NO FUCKING AIRFLOW.

NO AIRFLOW.

No airflow? No airspeed. No moving treadmill.

Chicken, egg. Egg, chicken.

[cj001f]

No airflow? No speed. No moving treadmill.

Chicken, egg. Egg, chicken.

Uh, um, no.

[mrw]

If the jet has a rotation speed of 110k, and turns onto a runway with a 120k headwind, will the jet perform a vertical takeoff?

[Cornholio]

The treadmill tracks the GROUNDSPEED of the plane. As it rolls forward, the treadmill rolls backward.

It says so right in the problem statement.

It doesn't matter HOW the plane starts moving forward.

[Beaver]

Unbefuckinglievable. Think about it. Jet engines push air not dirt. Therefore a tradmill can not & will not be able to halt or even slow the progress of a jet which is not getting velocity from pushing dirt but from the jet pushing air. If a jet used big fucking engines attached to the wheels to gain enough speed to fling itself into the air then it started it's jets it would never get off the ground. BUT IT DOESN'"T, it uses air to create it's forward velocity.

[Dirty Hoar]

WOW yer smrt!!

So if I simply stand on a conveyor belt with roller skates on I can hold onto a running jet engine without getting pushed forward into the wall.

No, you would probably take off into the air like a balloon with a hole in it. My stationary bike example didn't have anything to to with a treadmill. I was trying to explain how there is no air resistance to enable lift of the plane. The 747 is very heavy and the engines probably don't have enough force to make it take off like a rocket. You watch too much Wiley Coyote and you're trying to turn the 747 into a rocket which it's not. The jet engines are simply there to propel the airplane forward fast enough to enable lift. Rockets don't need wings to gain lift because their engines far exceed the thrust to mass ratio needed to overcome gravity.

I suggest you stick with the porn gig.

[Cornholio]

This is why I keep saying it doesn't matter what's pushing the plane forward.

the wheels are contacting the treadmill and that interaction is pushing the plane back.

[Vinman]

No airflow? No airspeed. No moving treadmill.

Chicken, egg. Egg, chicken.

OK so when you are running on a treadmill at 8 mph do you feel wind in your face?...no, because your relative ground speed is 0. you aren't actually going anywhere.

This is no different for the jet plane example, its just a faster treadmill

[DJSapp]

The question did not state what the conveyor's speed was linked to. The argument here (and the answer) is based on this. If the conveyor's speed is linked to wheel speed (speed of wheel moving reltative to ground) the 747 will never take off. If the conveyor's speed is linked to airspeed, see my previous example, the plane might take off. Aircraft generally measure speed by airspeed, even when they're on the ground.

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 Relative to WHAT??? and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

[dubu]

so what happens if a plane has its breakes locked up and engine at full power? according to the no takeoff people it would not move right?

anyway
the plane would take off since it would be moving forward at 1/2 the speed of the convator belt (eventually, it would take some time longer, ).

more geeky explonation

force of engines > force of rolling resistance of spinny wheels..


1 747 engine has an power output of about 35Kw

so 4 would have 140kwatt power output.. power = (force * distance) over time

and I really dont thing that the wheel bearings will produce that much power from friction..

therefore the plane will move in the direction of the thrust vector ie, engine thrust.

A car on the other hand is different and it will not move.. since the force is directly applied to the treadmill.

[Beaver]

Note to self - flabango is not too bright.

[EPSkis]

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?

If I might:

We have to assume that as the engines rev higher, it *will*, in fact - move forward. As a result of the plane moving forward, the conveyor senses this and advances its' speed to negate any further forward advance by the planes wheels. Result: Plane stands still, but only AFTER moving forward.

Repeat this process.

2 obvious variables that are NOT taken into account:

1) The *length* of the conveyor
2) The available power of the engines.

Assuming an unlimited amount of thrust and a short conveyor, the final result will be that the plane will FALL off the end of the conveyor, not "take off". At no point is there sufficient forward movement to produce enough lift.

[Beaver]

This is why I keep saying it doesn't matter what's pushing the plane forward.

the wheels are contacting the treadmill and that interaction is pushing the plane back.

So if you are running on a treadmill and I put a rope on you and pull really hard and you turn up the treadmil speed as I pull you will not come towards me? Think hard about this and remember I am not on the treadmill. Your feet will be going like mad but I WILL pull you off the front of the treadmill because I (like the Jet engine pushing air) have nothing to do with the treadmill. The jet engine will push air regardless of what the wheels/treadmill are doing. IT WILL PUSH AIR which is NOT affected by a treadmill. THINK HARD, I have confidence that you are not stupid.

[cj001f]

THINK HARD

work on it Beaver and you'll see why jets need runways.

[Dirty Hoar]

Note to self - flabango is not too bright.

hahahaha

beep beep!

[Beaver]

When the treadmill notices that the plane is moving forward and it speeds up the jet will continue forward because all the treadmill does is spin the wheels of the craft faster. It does not stop the thrust of the enjine against the air which is not affected by the wheel speed.

AIRCRAFT DO NOT USE THIER WHEELS TO ACCELERATE THERFORE THE TREADMILL SPEED IS IRRELEVANT TO THE AIRCRAFT'S MOTION.

[wicked_sick]

who puts a jet on a treadmill anyways?

[cj001f]

Oh yeah.