A plane on a runway, how smart is r3vlimited?

[chucko]

I agree 100% with you here. Obviously this is a hypothetical situation, because there no real conveyor belt runway. Therefore I completely agree with you that max wheel speed is infinite, max belt speed is infinite, and most importantly, wheel bearing friction is neglected. I'm glad we both agree with that.

Me too!

Of course the wheels on the missile are moving faster than the conveyor belt. That is how you prove that the missile is accually moving forward (relative to the air).

Ok, how can the wheels go faster than my belt which has a speed of infinity? What speed are the wheels going? infinity + 20mph? Let's just say (for my benefit) that the wheels can not go faster than the belt, does the object still move forward? The answer is No, the object can not move forward unless the wheels are no longer on the belt. That's my take on it.

According to the original question, the wheels NO NOT have to go the same speed as the conveyor belt. The plane has to go the same speed as the conveyor belt.

Why? I don't see how the original question separates the wheel speed from the plane speed. Say for the sake of argument, that I am standing on the conveyor belt with my feet strapped to the belt, and you start this scenario in motion. The plane will be going very fast relative to my position on the belt. I could very easily reference speed of the plane based on my observations standing on the belt. Or in a more realistic situation, say you had a conventional auto speedometer attached to the wheels of the plane and that was how you were measuring speed (speed is a function of wheel diameter and revolutions per unit time). The problem does not say how you are measuring the planes speed, people are jumping to their own conclusions.

If someone were to stand next to the conveyor belt, with a radar gun measuring the speed of the plane, that is how the speed is measured. Not by the wheel speed. The wheels are just free spinning and can do whatever they want. They are not a correct form of measurement of the accuall plane speed.

Ok, lets say the problem says plane speed is referenced to a fixed point outside of the moving system. The belt will always be moving the same speed that the plane is moving relative to that fixed point. So if the plane speed is 100, the belt would be 100, right? How does the plane ever begin to move? If the belt reaction is instantaneous, and the belt can only go as fast as the plane is moving, the force reactions should cancel eachother out. :-) We are tedering on a paradox here.


I'm going to bed now, but thanks for the interesting discussion.

[Rocla]

Yes, the plane is moving relative to the landscape.

The reason it is different than a runner on a treamdill is because the wheels on the plane are free-spinning. The plane plane is being pushed though the air by the jet engines. The conveyor belt has no effect on the airplane besides making the wheels spin faster than the plane is moving.

-Erik

That basicly means that as long as the wheels are rollers(not driven), the belt will not cancel the movement of plane compared to landscape?

I think that I'm finaly getting this. thanks Erik.

[325iCabrio]

Okay, since I've only read three pages, I don't know if this has been discussed. But I did read the arguments for the "correct" answer, and I do understand the plane would be able to move forward due to the speed of the plane and free spinning wheels.

However, I stand by my vote that the plane wouldn't fly because of one thing. Since the conveyor is moving at the speed of plane in the opposite direction, I don't think the plane could get quite enough thrust to take off on a runway-sized conveyor. I think we can all agree that the plane would move backwards if the conveyor belt started running and the jets were off and the plane was stationary. The conveyor would really have to jerk on and then slow down in order to move the qheels while the plane was sitting in one place. So, since that's the case, there's obviously some initial inertia (I think) that the plane would have to overcome to start moving in a forward direction, and thus limit the amount of thrust the jet engines would regularly have if on a stationary runway. So, if the conveyor runway was longer than a conventional runway, I'd say it could take off. But since it's runway-sized, I'll stick by my vote of no.

[gstuning]

Amazing,

Alright picture this ,
The plane is on a GIANT GIANT lake of frozen even water,
that supports it and what not,

Now push the plane backwards 100mph (lets say this is possible)
turn on the engines max thrust, what happens?????

The jets will slow the plane down to a stop and then accelarate and fly away,
The jets thrust is pushing on the wings and thus the body of the plane,
the tires are just there to support the plane while it´s on the ground,

Another example,

Push a skateboard down a hill with a little rocket attached to it,
wich is remote controlled, lets imagine the board goes backwards down the sloap without problems, turn on the rocket what happens??

The board will slow down from the thrust and then if the thrust remains will go back up,

Don´t you guys remember the Mythbusters show where they strapped a rocket to a impala and made it accelarate real fast,
Why did it accelarate ??? from AIR THRUST., totally independed from the drivertrain,

A engine doesn´t have a drivertrain!!!!!!!!!
Only thrust, it´s like this
Put you car on a conveyor belt, into neutral, rope that extends beyond the conveyor belt, enough people to pull the car (1 should be enough really)
the start the belt, what happens????

The car will be stationary,, the wheels will be spinning, if you pull on the rope the car will move forward (if you don´t belive that I´ll need a answer to that)
as the only variable changing is the rope lenght and nothing else,

Another one,

If you got your skateboard on a treadmill going 100,000,000mph and tied to the front with a rope. You smack it in the back with a giant baseball bat , what happens??
it goes flying off it,

Once you get it you´ll see the error of your ways

[RobertK]

That basicly means that as long as the wheels are rollers(not driven), the belt will not cancel the movement of plane compared to landscape?

I think that I'm finaly getting this. thanks Erik.

That's wrong. Go back and look at my roller skate guy on a treadmill example. By saying that the rolling belt on the conveyor is not able to cancel out movement then theoretically you could set a matchbox car on a treadmill and it would stay completely still without any opposite force required to keep it there.

No wheel is frictionless nor free rolling. Energy is lost in the transfer of friction between the wheels and the conveyor surface.

To put another spin on it what about his scenerio?

You are trying to take off in an amphibious airplane by going upstream on a river. A system is created to match the speed of the water flowing downstream with the speed of the plane. Will the plane take off?

a=b

[gstuning]

This is not the same, the resistance in the water is way way more then in the wheels,

[RobertK]

This is not the same, the resistance in the water is way way more then in the wheels,

How do you know what the exact resistance of the wheels is on the conveyor is if you do now know the weight of the plane and the weight of the wheels?

Some of you guys are arguing like there is absolutely no resistance generated by the wheels. Hopefully the plane in the river scenerio will make you think twice about that.

[gstuning]

I´m getting what your saying,
But does the resistance in the wheels change with speed? and in what fashion,
linear or unlinear,

I think the factor really is how much thrust the engines are allowed to overcome the resistance

[Jonathan M3]

I can't believe this is still being argued over! I haven't bothered to read 90% of this thread so what I say may have already been said.

The plane will take off.

Airplane engines exert their thrust on the plane by "pushing" on the air around it. The plane might as well be being pushed or pulled by something outside the realm of the conveyor system. This is why when in a vacuum, jet/propellor engines do not work and some sort of chemical reaction creating rocket engine would then be needed to propel the plane through the vacuum(read: space).

The conveyor, and the wheels as well, will continue to accelerate in vain while the plane is being "pushed" through the air by the engines until the plane reaches its take-off speed and lifts off the conveyor. The frictional resistance of the wheels against the ground is trivial and nowhere near enough of an opposing force compared to the thrust generated by the engines to keep the plane from accelerating.

[uofom3]

^^
so then I am at a loss - bassically you are saying airspeed has nothing to do with it...

[RobertK]

I can't believe this is still being argued over! I haven't bothered to read 90% of this thread so what I say may have already been said.

The plane will take off.

Airplane engines exert their thrust on the plane by "pushing" on the air around it. The plane might as well be being pushed or pulled by something outside the realm of the conveyor system. This is why when in a vacuum, jet/propellor engines do not work and some sort of chemical reaction creating rocket engine would then be needed to propel the plane through the vacuum(read: space).

The conveyor, and the wheels as well, will continue to accelerate in vain while the plane is being "pushed" through the air by the engines until the plane reaches its take-off speed and lifts off the conveyor. The frictional resistance of the wheels against the ground is trivial and nowhere near enough of an opposing force compared to the thrust generated by the engines to keep the plane from accelerating.

WRONG. Again.. Someone else who seems to think that the wheels are trivial. You are not considering the weight of the plane being exterted downward on the wheels which increases friction at the surface of the conveyor. The wheels would have to be extremely efficient to allow a trivial amount of energy to be lost there.

Lets remember something. The plane must be push or pulled off the ground before airspeed can even become a factor in whether or not it can take off.

[uofom3]

to add to my previous post,
If the plane is on a treadmill, and is stationary, and no matter how much you crank up the engine it is going to remain stationary, it is not going to go anywhere. There is no air flowing over the wings to generate lift.

I have been under the impression that the engines do not "push" air around it. They generate thrust to propel the plane forward and create air flow over the wings surface. At a given speed (depending on the plane) enough air will be flowing over (under) the surface of the wings to generate lift and get the plane off the ground.

With that said, if the plane is stationary, regardless of how much thrust you throw at it, it is not going anywhere. There is no air flowing over the wings.

I understand that the treadmill is like a virtual runway - and bassically the plane is rolling down a quasi runway. The part that is missing is the fact that the plane is not actually going ANYWHERE. With a run way, the plane IS going somewhere...

[Jonathan M3]

Of course airspeed has to do with it. Once the engines propel the plane through the air up to the airspeed required for lift, the plane will take off.

[uofom3]

here is even a better example - why do you think that you need to turn on a fan when you run on a treadmill to cool yourself? Think about it, you run on the treadmill and no air is hitting your face other than the stagnant air in the room because YOU ARE NOT MOVING.... just moving relative to the treadmill.

[uofom3]

Of course airspeed has to do with it. Once the engines propel the plane through the air up to the airspeed required for lift, the plane will take off.

right, but airspeed will never be achieved relative to the ground, only relative to the treadmill. Because you are not actually moving through the air this is irrelevant. It's like a car on a dyno - you can ramp it up to 160+ but you're strapped to the dyno. Regardless of how fast you stand on it you're not going 150+ relative to the ground - just relative to the dyno. Compared to the ground you're doing 0 on the dyno, and the plane is doing 0 relative to the ground while on the treadmill and generating no lift.