Will it take off?

[Nico Flax]

I'm wondering if you guys have had this question posted before here, I copied this from another forum. If it has then just delete it I guess but this thing has been spreading like wild fire! Also many debates and flames.

I suggest you really think about it before you answer, heh it can be a toughie if you over think it.


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

The question is:

Will the plane take off or not?


so what do you think?

I say no

Should we make this into a possible poll?

[Nico Flax]

Actually after reflecting on the idea I realize that the plane WILL take off. Yes believe it or not because its jets are not powering the wheels, the wheels are just spinning but the jet is moving forward.

[327]

if the plane is moving at one speed in one direction and the conveyor is moving the same speed in the opposite direction. the plane wont be moving,

the engines will be powered up all the way, But it needs Lift( air moving under the wings) to raise.

[Proetus]

If the wings never physically move forward then they generate 0 lift.

http://www.princeton.edu/~asmits/Bic...Bernoulli.html

[TuffG929]

Originally Posted by Proetus

If the wings never physically move forward then they generate 0 lift.

http://www.princeton.edu/~asmits/Bic...Bernoulli.html

Exactly. The answer has got to be no. Though the plane's instruments may tell you otherwise, the plane is acctually moving at 0 Miles Per Hour.

If case you where not aware 0 MPH = 0 Lift

[camaro chris]

no becuase it needs to have the the air going past to lift the wings.

[suckstobeyou]

no dude, unless a hurricane comes and blows some wind under the wings that plane ain't going nowhere

[Nico Flax]

No the plane will take off because the wheels aren't the problem. This is because the air from the jets will exert force on the plane. Think of it like this, the plane is going 0mph because of the belt, where does all that thrust go? It has to move forward, the force isn't on the runway its on the plane. It doesn't matter wheather the wheels are moving because the belt isn't stopping the forward motion of the plane itself, just the motion of the wheels. The jet engines just have to overcome the friction of the wheels, easily done, and then it starts to move.

[TomND]

As far as flight is concerned, it doesn't matter how fast the plane is moving in relation to the runway. The important thing is how fast the plane is moving in relation to the surrounding air. Since the plane in standing still in relation to the air, there will not be any lift. It doesn't matter how fast the wheels are turning, or how hard the engines are working. The jets will exert force on the plane, but there still won't be any forward motion in relation to the surrounding air. The jets don't generate lift, they just push on the plane, which is offset by the moving runway.


EDIT: ok, I'm not so sure about that now. I saw this discussion on another forum and now I'm totally confused.

Another edit: The plane will take off. The question is confusing because you think the conveyor belt keeps the plane stationary, but, since the wheels move freely, it does not. Even on a moving runway, the plane will still move forward and generate lift.

[SS4EVER]

Originally Posted by TomND

As far as flight is concerned, it doesn't matter how fast the plane is moving in relation to the runway. The important thing is how fast the plane is moving in relation to the surrounding air. Since the plane in standing still in relation to the air, there will not be any lift. It doesn't matter how fast the wheels are turning, or how hard the engines are working. The jets will exert force on the plane, but there still won't be any forward motion in relation to the surrounding air. The jets don't generate lift, they just push on the plane, which is offset by the moving runway.


EDIT: ok, I'm not so sure about that now. I saw this discussion on another forum and now I'm totally confused.

Another edit: The plane will take off. The question is confusing because you think the conveyor belt keeps the plane stationary, but, since the wheels move freely, it does not. Even on a moving runway, the plane will still move forward and generate lift.

You were right the first time, it will not take off. Just like driving a car onto a dynometer. The rollers match the speed of the wheels...eventually. They use the tiedowns and straps for safety reasons but basically the car cannot move forward. Now think of the planes wheels rotating faster and faster (the linear acceleration is transfered from the thrust of the engine to the wheels) while the conveyor matches the acceleration. Of course with no forward motion there will be no lift. Like serveral posters replied, 0 mph = no lift.

[Nico Flax]

Originally Posted by SS4EVER

You were right the first time, it will not take off. Just like driving a car onto a dynometer. The rollers match the speed of the wheels...eventually. They use the tiedowns and straps for safety reasons but basically the car cannot move forward. Now think of the planes wheels rotating faster and faster (the linear acceleration is transfered from the thrust of the engine to the wheels) while the conveyor matches the acceleration. Of course with no forward motion there will be no lift. Like serveral posters replied, 0 mph = no lift.

No because on a car the wheels are what is providing power but on a plane the thrust is not giving the wheels power, the wheels just move in response. Think of it like this, your on a treadmil where rollarblades, and the treadmile moves as fast as you in the oppisite direction. Now say you have a rocket tied to your back, do you really think the thrust from the rocket isn't going to power you foward?

[Nico Flax]

Originally Posted by TomND

As far as flight is concerned, it doesn't matter how fast the plane is moving in relation to the runway. The important thing is how fast the plane is moving in relation to the surrounding air. Since the plane in standing still in relation to the air, there will not be any lift. It doesn't matter how fast the wheels are turning, or how hard the engines are working. The jets will exert force on the plane, but there still won't be any forward motion in relation to the surrounding air. The jets don't generate lift, they just push on the plane, which is offset by the moving runway.


EDIT: ok, I'm not so sure about that now. I saw this discussion on another forum and now I'm totally confused.

Another edit: The plane will take off. The question is confusing because you think the conveyor belt keeps the plane stationary, but, since the wheels move freely, it does not. Even on a moving runway, the plane will still move forward and generate lift.

Yay you see! I'm not the only one! I'm right the plane will fly!

[327]

Originally Posted by Nico Flax

No because on a car the wheels are what is providing power but on a plane the thrust is not giving the wheels power, the wheels just move in response. Think of it like this, your on a treadmil where rollarblades, and the treadmile moves as fast as you in the oppisite direction. Now say you have a rocket tied to your back, do you really think the thrust from the rocket isn't going to power you foward?

yes but if you have the rocket tied to your back, powered up and you are moving the same speed as the conveyor is in reverse you wont be moving

the same with the plane. just think about it. the conveyor moving 150MPH, the plane moving 150 mph the other direction with the engine powered up. The plane would be sitting still on the conveyor (wheels rolling engine powered up) going 0 mph

Planes need to be moving fast enough to get air under their wings to create lift. If the wind was moving as fast as the conveyor in the same direction then it would lift

[miken123]

I have the answer. It's a little more complex than a yes / no but here goes.. try to follow me on this:

Thrust is coming from the jets so that TRIES to push the plane forward,
HOWEVER the plane doesn't move forward*YET* because the as the jets move the plane forward and so the wheels start rolling and the conveyer belt matches the wheels speed so the plane goes no where. Right now the wheels contacting the conveyer are NOT slipping (think not losing traction like a car peeling out) meaning the frictional force (i.e. the opposing force the conveyer belt is providing) will be EQUAL to the forward force (engine jet thrust). This is known as non-slip condition (where the coefficient of friction is u-s (static friction)).

Now is where complexities begin: Let's say the plane keeps on getting more and more thrust.
So the plane is thrusted forwards more and more at the jet's level (usually wings)-this causes the wheels to spin faster and faster (and so does the conveyer belt in the opposing direction in order to match the increasing wheel speed). HOWEVER At some point the thrust *MAY* be so great (and this depends on the particular jet engines' power output..) that the frictional force between the plane's wheels (tires) and the conveyer belt will not be able to match the thrust of the jets!!!

At this point (again this is assuming the jet engines can produce this thrust), the plane's tires start to SLIP at the contact point with the conveyer! At this point the friction between conveyer and tires is FIXED (its the normal force multiplied by the friction coefficient mu-k k for kinetic) Since The thrust of the jets is enough to cause this SLIPPING then the conveyer belt, can't hold the plane back anymore!!! WHY? Well to reiterate this paragraph in layman's terms: The tires are slipping so they are in effect being partially dragged over the conveyer belt (again this is what slip means).

DO NOT CONFUSE THIS WITH WHAT HAPPENS WHEN TIRES ON A CAR "SLIP"!!

THE DIFFERENCE: It's that on a car the tires (that are slipping) are what provide the thrust on the car, so if they slip the car in effect no longer gets all the tires thrust
ON A PLANE WITH TIRES SLIPPING: since its tires ARE NOT what provides the plane's thrust, the plane's thrust will still be based off of the engines at the wings or wherever they may be placed.
FINALLY: Since the tires on the Plane ARE what provide the "Opposing thrust" if you will, and since these "providers of opposite thrust" are now SLIPPING, the plane will get a net forward thrust!

Conclusion:
Once the jet engines provide enough forward thrust (force) to overcome the friction between its tires and the conveyer belt, THE PLANE WILL BEGIN TO MOVE FORWARD.

NEW QUESTION!!!!--> Now the plane is moving forward, but will it lift off?
Answer, assuming the jet engines can provide enough thrust --it takes more than it would if there wasn't that opposing force from the slipping wheels -- and that for the sake of this argument, the conveyer belt is long enough so that the plane's forward motion doesn't get to the point where it falls off the conveyer belt's end, YES it will eventually gain enough forward speed to lift off.

NOTE the assumptions of the engines having enough power to:
#1) Generate enough forward thrust to cause the wheels to go into 'slip'
#2) once in 'slip' generate enough added thrust on top of that to reach the lift of speed required.
IF ASSUMPTION 2 IS NOT MET:
The plane will move forward but never fast enough to take off
IF ASSUMPTION 1 IS NOT MET:
The plane won't even move forward


Thank you and have a nice day. If you don't understand the explanation contact a dynamicist or read one of the numerous Dynamics text books, i recommend Craige's Engineering Science and Mechanics: Dynamics.
You may need to read Craige's Statics text first, and possibly read a book in physics before that for a quick background.

[atomicshark]

I think it would end up with one mangled jet off the side of the runway.