A jet plane on a large treadmill

[AsheanMT]

Bullshit.

Dumbshit!

[Dantheman]

Dumbshit!

fkna, dumb as a rock.

[P_McPoser]

Shall we?


Best I've seen for explaining this..

[pechelman]

wow wtf, how did I miss this.

that motorcycle analogy is incorrect because the force is not transmitted through the skater but directly to the ground. I havent read everything in this massive thread but;

Heres how I read the problem
Plane is standing still on the treadmill to start, with 0 air and ground speed.
Plane fires its turbines and instantly, the treadmill matches to maintain this 0 velocity condition. The treadmill will always match the speed and the plane will never move.

Sure there is air moving through the turbines, and if you were to drop the landing gear, the plane would jut forward a little bit, but since the plane has mass, and is not moving, it will fall to the ground. If the engines had infinite thrust, the plane would take off. The impulse is just not enough to take the plane from 0 velocity to takeoff airspeeds of around 200mph.

Seriously folks, this is exactly the same as saying that a rope is tied to the back of the plane when the turbines are at full thrust. Cut the rope and what happens? There is no lift to keep the plane up in the air. The thrust of a passenger jet is NOT greater than the plane itself, so it will not just be held stationary in the air. The plane has mass, and has inertia, and will not just magically take off. It would either take a JETO-Pack or something with enough impulse to raise the thrust-weight ratio of the plane above 1 so it jets off.

thats it folks
it doesnt go anywhere.
seriously.

edit
as an fyi, a similar scenario actually occurs on carrier take off.
planes wheels are locked into a track, plane fires engines to 100%, track slingshots the plane from 0-150mph in 1.5 seconds, and because of the delta V imparted to the plane by the track, it is able to take off. Some fighter jets actually do have T-W ratios greater than 1, but need this extra velocity for aero stability as well. Otherwise id have the feeling the planes engine might just whip around on it, similar to a RWD car in the snow.

[horizon]

oh fuck it, I'll play.

Of course the rollerblader will get pulled off the treadmill. Of course the plane will fly.

All you need to assume is that the friction in the wheels is much much smaller than the force pulling the said rollerblader or pushing the said plane.

The initial problem is wrongly worded - in that friction cannot in the real world counteract the jet engine thrust - and hence the treadmill will be unable to move as fast as the plane but in the opposite direction. At some point the threadmill won't keep up using real-world assumptions about friction. This assuming of course that we can't build a treadmill moving at lightspeed.

But IF it could, which is what the thought experiment is asking us to contemplate, then the rollerblader would NOT move forward, and the plane would NOT fly.

Those of you answering that the plane would fly are talking about the real world. In the real world, the force of a jet engine or that of a motorbike pulling a rollerblader is hundreds of times higher than friction. Hence the initial thought experiment CANNOT be reproduced in reality. (You're simplifying this by talking about zero friction, which could be considered a reasonable assumption in the real world).

Slam on the brakes and suddenly things become more interesting. Rollerblader gets all mangled up but might well not move forward (rope might break though), and the plane won't fly. Yet this is just a special case of the first question. You've brought two previously imbalanced forces closer and now you can see how they affect the plane.

[AsheanMT]

for fucks sake!!! WRONG WRONG WRONG! It would be NO different than a plane with skis on a snow field. The landing gear is simply there to keep the plane off the ground it does not matter if its on a treadmill or on water or on snow or flying off a moving fuckin aircraft carrier.

[huck4bucks]

The only plane that's leaving this treadmill is this one:


As for everyone that mentions the word 'friction' in this thread, you're not paying attention to the correct principle: lift.

On an airplane wing (and even a helicopter wing) lift is created when the pressure above the wing is lower than the pressure below the wing dues to it's shape and wind speed. When a plane relative to the ground is motionless because the treadmill counteracts any forward motion from the plane with it's own counterbalancing reverse motion, there's never any wind over the wings. With no wind, the wings cannot create lift. Without lift, the plane will never leave the ground. Unless it's a Harrier jump jet or some other VTOL (Vertical Take-off or Landing) vehicle, or a helicopter with creates it's own lift by rotating it's wings in a circle.

[BeanDip4All]

As for everyone that mentions the word 'friction' in this thread, you're not paying attention to the correct principle: lift.

On an airplane wing (and even a helicopter wing) lift is created when the pressure above the wing is lower than the pressure below the wing dues to it's shape and wind speed. When a plane relative to the ground is motionless because the treadmill counteracts any forward motion from the plane with it's own counterbalancing reverse motion, there's never any wind over the wings. With no wind, the wings cannot create lift. Without lift, the plane will never leave the ground. Unless it's a Harrier jump jet or some other VTOL (Vertical Take-off or Landing) vehicle, or a helicopter with creates it's own lift by rotating it's wings in a circle.

Correct.

I can hook my bike up on my trainer and pedal as fast as I want, but still will never create any wind in my face. No wind, no lift, the plane does not take off.

[AsheanMT]

Lift is obviously needed. What you no brainers fail to see is that the plane WILL move forward as usual and therefore create lift as usual. In this scenario a prop plane could take off, you don't even need it to be a jet. The motion of the treadmill is not going to prevent the plane from moving forward. If the plane were a car THEN you would be correct to think that the car would not be able to move forward but the plane can because its not dependent on power through the wheels!

[pechelman]

The only plane that's leaving this treadmill is this one:


As for everyone that mentions the word 'friction' in this thread, you're not paying attention to the correct principle: lift.

On an airplane wing (and even a helicopter wing) lift is created when the pressure above the wing is lower than the pressure below the wing dues to it's shape and wind speed. When a plane relative to the ground is motionless because the treadmill counteracts any forward motion from the plane with it's own counterbalancing reverse motion, there's never any wind over the wings. With no wind, the wings cannot create lift. Without lift, the plane will never leave the ground. Unless it's a Harrier jump jet or some other VTOL (Vertical Take-off or Landing) vehicle, or a helicopter with creates it's own lift by rotating it's wings in a circle.

100% agree, except that you can have something with an obscene amount of thrust that will just horizontally launch something to takeoff speed in the same time that the plane would fall to the ground.

however, it would probably be so much thrust, it would shear the wings straight off the plane or crumple the fuselage as in a JETO-Pack

[Powow]

wow wtf, how did I miss this.

that motorcycle analogy is incorrect because the force is not transmitted through the skater but directly to the ground. I havent read everything in this massive thread but;

Heres how I read the problem
Plane is standing still on the treadmill to start, with 0 air and ground speed.
Plane fires its turbines and instantly, the treadmill matches to maintain this 0 velocity condition. The treadmill will always match the speed and the plane will never move.

Sure there is air moving through the turbines, and if you were to drop the landing gear, the plane would jut forward a little bit, but since the plane has mass, and is not moving, it will fall to the ground. If the engines had infinite thrust, the plane would take off. The impulse is just not enough to take the plane from 0 velocity to takeoff airspeeds of around 200mph.

Seriously folks, this is exactly the same as saying that a rope is tied to the back of the plane when the turbines are at full thrust. Cut the rope and what happens? There is no lift to keep the plane up in the air. The thrust of a passenger jet is NOT greater than the plane itself, so it will not just be held stationary in the air. The plane has mass, and has inertia, and will not just magically take off. It would either take a JETO-Pack or something with enough impulse to raise the thrust-weight ratio of the plane above 1 so it jets off.

thats it folks
it doesnt go anywhere.
seriously.

edit
as an fyi, a similar scenario actually occurs on carrier take off.
planes wheels are locked into a track, plane fires engines to 100%, track slingshots the plane from 0-150mph in 1.5 seconds, and because of the delta V imparted to the plane by the track, it is able to take off. Some fighter jets actually do have T-W ratios greater than 1, but need this extra velocity for aero stability as well. Otherwise id have the feeling the planes engine might just whip around on it, similar to a RWD car in the snow.

What you said is true if the treadmill matches the speed of the wheels.

It doesn't though, it matches the plane. Which means the wheels spin twice as fast as normal, but otherwise everything is the same.

edit: Imagine if this plane is on skis in antarctica, and the conveyer belt is made of snow. Then what happens?

[MeatPuppet]

Correct.

I can hook my bike up on my trainer and pedal as fast as I want, but still will never create any wind in my face. No wind, no lift, the plane does not take off.

If half the people in this thread were standing in front of the treadmill arguing about it, there would be no shortage of airflow across the wings.

[Gripen]

The only plane that's leaving this treadmill is this one:


As for everyone that mentions the word 'friction' in this thread, you're not paying attention to the correct principle: lift.

On an airplane wing (and even a helicopter wing) lift is created when the pressure above the wing is lower than the pressure below the wing dues to it's shape and wind speed. When a plane relative to the ground is motionless because the treadmill counteracts any forward motion from the plane with it's own counterbalancing reverse motion, there's never any wind over the wings. With no wind, the wings cannot create lift. Without lift, the plane will never leave the ground. Unless it's a Harrier jump jet or some other VTOL (Vertical Take-off or Landing) vehicle, or a helicopter with creates it's own lift by rotating it's wings in a circle.

Exactly! You need lots of air moving over the wing... the air moving thru the engine and the thrust only provides forward proplusion to GET the lift. If the treadmill negates this, then there is no lift!! no lift= no fly

[pechelman]

Lift is obviously needed. What you no brainers fail to see is that the plane WILL move forward as usual and therefore create lift as usual. In this scenario a prop plane could take off, you don't even need it to be a jet. The motion of the treadmill is not going to prevent the plane from moving forward. If the plane were a car THEN you would be correct to think that the car would not be able to move forward but the plane can because its not dependent on power through the wheels!

DUDE

the plane generates thrust
sure
100% agree

BUT, its not moving on a treadmill. ever. the wings relative to the ground and relative to the air are not moving. wings are stationary. 0. no velocity.

either im really really dense today, or theres something others are assuming thats not in the problem statement

inertia cant just magically be overcome

if you did have enough thrust to propell the plane to takeoff AIRSPEED velocity before it hit the ground, then yes, the plane would take off.
However like I said, its very unlikely this would ever work

[AsheanMT]

It would take the normal amount of thrust it always takes. the only difference is that in this scenario when the plane reaches its 220 mph the wheels and the treadmill will then be rotating exraordinarily fast.

[MeatPuppet]

DUDE

the plane generates thrust
sure
100% agree

BUT, its not moving on a treadmill. ever. the wings relative to the ground and relative to the air are not moving. wings are stationary. 0. no velocity.

either im really really dense today, or theres something others are assuming thats not in the problem statement

inertia cant just magically be overcome

if you did have enough thrust to propell the plane to takeoff AIRSPEED velocity before it hit the ground, then yes, the plane would take off.
However like I said, its very unlikely this would ever work

Treadmill is to car as wind tunnel is to airplane.

Maybe that will help.

[Shepherd Wong]

Plane would just jump off the front of the treadmill. That is unless if the treadmill were spinning fast enough to provide enough drag in the landing gear's bearings to counter the thrust of the engines.

[huck4bucks]

If half the people in this thread were standing in front of the treadmill arguing about it, there would be no shortage of airflow across the wings.

Bingo. Take a look at this. There's nothing in there that mentions wheels speed, jet packs, prop planes, snow-covered treadmills in Antarctica, fruit booters tied to motorcycles, or any other jackassery.

Planes fly because of lift. Not thrust, not Magnum V12 engines, not magical friction-less wheels. The Wright Brothers had none of that. What they had was a plane with a propeller that could push the plane fast enough to create lift under the wings. A 747 and any other plane today works from that same model.

[focus]

Jesus Christ -- this is back, and even most of you who say the plane will take off don't really get it.

If the plane generated all its take-off speed by transmitting power through the wheels, it would STILL be able to take off, assuming the motor(s) driving the wheels can push them at at least twice the required take-off speed.

How do we define the plane's speed? Is anybody really going to make a case that the plane's speed is truly measured by how fast the wheels are spinning? If not, shut up about friction, frictionless platforms, and planes not taking off.

now THINK about this: the treadmill is stationary unless the plane is moving forwards. If the plane is moving forwards at 1 mph, the treadmill is moving backwards at 1mph. If the plane is moving forwards at 100mph, the treadmill is moving backwards at 100mph. If the plane is not moving forwards, the treadmill IS NOT MOVING BACKWARDS. The wheels will always spin at twice the forward speed of the plane.

[pechelman]

gah
thrust has nothing to do with it, other than it gets the plane to a specific airspeed.

that analogy doesnt help MeatPuppet.

If ConveryVelocity = Plane Velocity then shits not moving anywhere except the air through the turbines...which as I said, is not a sufficient force to accelerate the plane to takeoff airspeed velocity in the time the plane would hit the ground if it dropped its landing gear.

[pechelman]

im also going to be super nit picky here, and say that at super high treadmill velocities, skin friction of the treadmill will generate a current thats might be significant enough to actually suck the plane to the ground.

high velocity = low pressure = suction

[P_McPoser]

You's guy's is being dumbasses.

Ok, the pilot firewalls the fucker, directing tens of thousands of pounds of thrust AGAINST THE AIR. The conveyor senses this and begins rotating the opposite direction at a speed\acceleration rate equal to that created against the air by the engines. BUT this opposite force IS NOT BEING APPLIED TO THE AIRFRAME. This force is being applied to the fucking wheels, which DO NOT transmit any fucking lateral force to the airframe itself, the force applied by the conveyer can by definition ONLY TURN THE FUCKING WHEELS, not counteract the force against the air emplaced by the engines. These pilots are fucking drunk anyways if they've managed to get themselves in this situation.

[Powow]

im also going to be super nit picky here, and say that at super high treadmill velocities, skin friction of the treadmill will generate a current thats might be significant enough to actually suck the plane to the ground.

high velocity = low pressure = suction

yeah but the treadmill will never be going faster than the plane's takeoff speed which is much lower than the velocity needed to do that.

[AsheanMT]

when the plane lifts off from the runway at 220 mph the wheels will be spinning at 440 mph in one direction and the treadmill will be spinning at 440 mph in the other direction. theoretically

[Powow]

when the plane lifts off from the runway at 220 mph the wheels will be spinning at 440 mph in one direction and the treadmill will be spinning at 440 mph in the other direction. theoretically

NO, the treadmill will be going 220 mph. the wheels go 440 mph (this statement doesn't even really make sense, but assume you are talking about the tread of the tire) because the plane is going 220 in the opposite direction.