A jet plane on a large treadmill

["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?

[ill-advised strategy]

ummm. seems like thrust would push the plane off a treadmill.

am I missing something?

[acostiga]

similarly, i always wondered how kitt was able to drive onto the back of that truck at highway speeds and not crash through the truck cab? physics is phun.

[Steezus Christ]

similarly, i always wondered how kitt was able to drive onto the back of that truck at highway speeds and not crash through the truck cab? physics is phun.

I wondered about that too, but then I saw a commercial for Fear Factor and it showed the contestants driving vehicles onto a trailer while it was moving fast. I think it might be possible.

But I hope The Big One is joking about that question because the jet wings would need lift to take off into the air.

[Steezus Christ]

ummm. seems like thrust would push the plane off a treadmill.

am I missing something?

Yes... The treadmill would negate the thrust.

[ill-advised strategy]

so, to have a spinning treadmill, and a jet plane stationary on that treadmill, wouldn't the thrust just have to overcome the rolling resistance of the wheels? that rolling resistance doesn't seem like much compared to a jet engine.

I give up, retards should not attempt to answer things such as these.
that is all.

[Vinman]

forget about thrust, it has to do with lift. There would be no air moving over the wings. The plane would be essentiall stationary. No air moving over the wings = no lift = plane stays on the ground.

[Sutter]

similarly, i always wondered how kitt was able to drive onto the back of that truck at highway speeds and not crash through the truck cab? physics is phun.

I have ALWAYS wondered about that move, when a car on the highway drives up the back of another moving vehicle at high speeds. explain, please.

[Tippster]

The car hits the brakes as soon as the tires hit the ramp, or simply get off the gas. As soon as the wheels stop the car does too. Kinda like stepping on and off a moving sidewalk in the Airport.

[DJSapp]

Draw a free body diagram, the answer is obvious. The plane cannot remain stationary on a tredmill.

[smmokan]

Its good to know I'm not the only one wondering about KITT. I actually asked my girlfriend that exact question last week... she didn't know either.

[Shredhead]

forget about thrust, it has to do with lift. There would be no air moving over the wings. The plane would be essentiall stationary. No air moving over the wings = no lift = plane stays on the ground.

There would still be lift unless you were in a vacuum. I've flown a 152 backwards in 60Knt wind.

[cj001f]

There would still be lift unless you were in a vacuum.

Air not moving relative to wing = no lift. Unless the treadmill has airmoving at the same speed as the belt (like a windtunnel) you'd get no lift.

[Tippster]

There's no wind mentioned. It's on a treadmill and the forward speed relative to the ground is assumed to be zero.

[Sensisnow]

There would still be lift unless you were in a vacuum. I've flown a 152 backwards in 60Knt wind.

No there wouldn't be any lift. The plane would remain stationary on the treadmill so there would be no wind to serve as lift for the plane. Now if there happened to be a significant headwind at the same time things would be different although it would take a hell of a lot more than a 60Knt wind to give a 747 enough lift to take off.

[Beaver]

Are you "no takeoff" guys fucking stupid or what? Twice for those who don't get it.

The jet plane will take off because it is irrelavant how fast it is going in relation to the ground as it is not driven by wheels but by jets (hence the name of the craft). It's wheels will be spinning like a motherfucker but it will have it's normal airspeed and thus take to the air like, well, a jet. It better have some good wheel berings and some well ballanced wheels.

If the plane does not use it's jets it will not move therefore the treadmill will never need to turn. If it uses it's jets it will begin to move forward, as it moves forward the treadmill will begin to move backwards twice as fast as the jet is moving forward. The jet will continue to accelerate gaining airspeed while the tradmill continues to accelerate twice as fast as the jet. Eventually (slightly longer than normal due to added rolling resistance) the jet will take off. It really does not matter how fast the ground appears to be passing by because like I stated, ground speed is irrelevant. The jets push air not ground so who fucking cares if it is on a treadmill.

[cj001f]

This thread should be a sticky in Tech Talk - let it serve as a lesson to those who ask maggot advice!

[Dirty Hoar]

Are you "no takeoff" guys fucking stupid or what? Twice for those who don't get it.

The jet plane will take off because it is irrelavant how fast it is going in relation to the ground as it is not driven by wheels but by jets (hence the name of the craft). It's wheels will be spinning like a motherfucker but it will have it's normal airspeed and thus take to the air like, well, a jet. It better have some good wheel berings and some well ballanced wheels.

If the plane does not use it's jets it will not move therefore the treadmill will never need to turn. If it uses it's jets it will begin to move forward, as it moves forward the treadmill will begin to move backwards twice as fast as the jet is moving forward. The jet will continue to accelerate gaining airspeed while the tradmill continues to accelerate twice as fast as the jet. Eventually (slightly longer than normal due to added rolling resistance) the jet will take off.

Let me ask you this... have you ever used a stationary bicycle? Did you feel wind blowing through your hair at any speed? Of course not, because your not moving forward. Thus there is no air resistance to enable lift. Oh ya... here's your sign.

[mcsquared]

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That is awesome. I want to live in your world. All I need is a treadmill and I can hanglide in my living room.

[mrw]

The jet would not go anywhere. The engines supply thrust to the airframe which the wings then use to create lift. Each aircraft has a speed at which you rotate to get airborne. That speed is the point where the wings have developed enough lift to support the aircraft.If the aircraft were in fact stationary due to a moving runway passing under the aircraft then the wings will have not developed enough lift to get airborne.Airspeed would essentially be zero.
To the guy who has flown backwards in a Cessna 150, go get yourself a Grumman

[Shredhead]

Opps: Remove head from vacuum, then post.

[Tippster]

Actually Beaver is right. Whether or not the ground the plane is on is moving or not is irrelevant, since the Jets push the air, not the ground.

Now, if the original poster wasn't as smart as Beaver and meant the 747 to maintain a relative ground speed of zero mph, then we're back to "...no, it wouldn't fly unless there was a headwind sufficient to lift the plane."

[DJSapp]

Ummm, Beaver is right. The only thing slowing the 747's velocity RELATIVE TO AIR is the rolling friction from the treadmill.

Simple example: A person is on a treadmill wearing roller skates and a rocket pack. The tredmill starts moving, and the person moves backward slowly due to rolling friction. The person flips on the rocket pack and blasts forward off the front. Why? The propulsion does not come from the wheels, it comes from the rocket pack.

Complex example: If the minimum takeoff speed for the 747 required 200 mph airspeed (speed of air moving over wing), the treadmill will be moving at 200 mph, the wheels will be spinning at 400 mph. The plane will be slowed by friction so the engines will need to overcome this. In theory the plane will take off.

Now to actually answer this in a more real situation I need to know what the strength and rolling resistance of the tires are, and the maximum thrust of the 747's engines. There are two ways the plane would not take off: 1. The tires blow out because they are rotating too fast to achieve the required airspeed, or 2. The rolling resistance is too great for the engines to overcome.

Got to go, Ogre is knocking on my door.

[mrw]

Opps: Remove head from vacuum, then post.

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.

[Beaver]

Let me ask you this... have you ever used a stationary bicycle? Did you feel wind blowing through your hair at any speed? Of course not, because your not moving forward. Thus there is no air resistance to enable lift. Oh ya... here's your sign.

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.