Airplane on a Conveyor Belt...

[Fishu]

To keep the plane from moving there would have to be an opposing force to the thrust, which would be headwind. However that wouldn't stop it from moving in vertical axis, but the effect would then be the same as with car on a conveyor belt - it wouldn't move forward.

[AKIron]

The problem here Sluggish is that since the problem was ill defined, it can be resolved in many ways with different outcomes. Some of us, perhaps foolishly, enjoy exploring the possibilities.

[WWhiskey]

Originally posted by Fishu
To keep the plane from moving there would have to be an opposing force to the thrust, which would be headwind. However that wouldn't stop it from moving in vertical axis, but the effect would then be the same as with car on a conveyor belt - it wouldn't move forward.

headwind? think that will cause lift, then flight,hence movement in the vertical= flying!!!

[sluggish]

If we have a "magic" conveyor belt that can instanly and infinately compensate and react to the movement of a "normal" wheel that must remain within the parameters of mass and enertia, the wheel masses acceleration enertia will prevent the plane from moving.


the plane doesn't fly.

[BlkKnit]

I still say that this is impossible.  The tires do not turn unless the plane moves, but the belt will counteract the tire rotation, in which case, the aircraft does not move.  

I understand the argument of:

Originally posted by eskimo2
Imagine that “exactly matches” really means that YOU have a big acceleration control dial for the conveyor and your job is to keep it in place.  The plane fires up and begins to inch forward; you see this and crank the acceleration dial.  You go too far, however, so the plane drifts back behind its starting point.  Seeing this you back off on the acceleration dial and adjust it to keep the plane pretty much where it started.

Look at the concept; don’t get caught up in the semantics of the word “exactly”.

But very soon after tire rotational speed is established, it is only maintained by forces from the belt, NOT by forces from the aircraft.  This skews the results.   I do realize that there are forces from the aircraft involved, but those same forces would move the aircraft, only spinning the tires as a result of the movement.  If the aircraft does not move the tires do not spin unless there is an outside force applied to the belt (which is a given as stated in earlier arguments).  Thats what I want to see:  Make the belt match the tires rotation.  Is it mathematically possible?  I'm no math guru, but I am pretty sure that you can prove anything mathematically if you go about it right.

[AKIron]

Originally posted by sluggish
If we have a "magic" conveyor belt that can instanly and infinately compensate and react to the movement of a "normal" wheel that must remain within the parameters of mass and enertia, the wheel masses acceleration enertia will prevent the plane from moving.


the plane doesn't fly.

Don't blame those of us that say in scenario #2 the plane may not fly for a magic conveyor belt. We did not establish the criteria. The problem as interpreted one way requires a "magic" conveyor belt. I'm simply assuming that the poser of the problem intended this.

In reality, something far less than infinite speed would be required to keep the plane stationary for several minutes or hours at full power.

[Chairboy]

This is kinda embarrassing to watch.  Since you're fixating on wheels, consider for a moment the vectors involved.  Bottom of the wheel goes one way, top goes the other.  They cancel each other out, there is _no effect_ on the plane.



And since the friction coefficient remains the same at any speed, the end result is that the plane takes off.

[2bighorn]

Originally posted by AKIron
Okay but think about this. Assume the tire riding in the back seat has the same mass as the tire rolling under the plane. Which has more energy at takeoff? They are both moving forward with the same amount of energy relative to the fixed starting point but the one rolling has more energy because it is both moving forward and spinning. That energy could have been used to accelerate the plane but wasn't.

OK, the only thing I wanted to know is why Eskimo thinks planes wouldn't have same take off distance, and I wanted to see what kind of logic he applied.

Obviously, we live in the same universe, unless by some internet magic, I actually communicate with the parallel universe Eskimo, the same laws of physics apply.

And whilst we mention laws and physics, I'll remind you and Eskimo about one of the most important, most basic and most known law applicable in this story.

The second law of motion. F=ma

Until the time you guys actually apply that, there's no point going any further.

[sluggish]

Eskimo’s wheel story is very relevant.  Inertia = an object at rest stays at rest, an object in motion stays in motion.  The energy required to make a still object move is directly proportional to the amount it moves.

Rotational inertia.  The amount of time a planes wheels remain spinning after take-of is directly proportional to the energy used to get them spinning.  Said energy is subtracted from the energy used to move the plane forward.

If enough continuous (acceleration) rotational inertia is applied to the wheels of the plane, (enough so that the total power of the thrust is consumed) the plane will not move forward.

The plane doesn't fly.

This is all based on the theory of a "magic" conveyor belt which doesn't have to follow the same rules as the wheel.  If the wheel lives in the same fairy tale land as the belt, and is not bound by the rules of physics, there will be no rotational inertia.

The plane flies.

[john9001]

Originally posted by sluggish

Rotational inertia.  The amount of time a planes wheels remain spinning after take-of is directly proportional to the energy used to get them spinning.  Said energy is subtracted from the energy used to move the plane forward.

 

the energy to spin the wheels comes from the belt, not the planes thrust.

according to the "it won't fly" people it would be impossible to drive a car because all the power from the engine would be used up putting "Rotational inertia" into the cars wheels.

[WWhiskey]

magic belts exist in what world?
might be the same one where magic wheels exist,and it doesnt matter!
if you want to test a relavent question you must use real objects not mythical devices!
there are no belts that will run at unlimited speed, they to will fail, just like the wheels of the plane if you turn them to fast, tires explode,drive boxes in belts fail, the belt itself will separate at some point!
if you go too this magical world where belts have unlimited speed then the plane must also have wheels that can go at unlimited speed as well!
its not fair too say the belt is indestructible but not the plane's wheels.

the plane will fly  as long as you dont destroy it trying to run the speed of the belt to an equal amount of the wheel speed of the plane! just try landing at over 200mph in AH and what happens? you die! if you run the landing gear up to a speed like that it will fail then the plane crashes into the belt and you die!

to have an expirement like this you must have control factors.
the relavant factor in this test is that the belt is going an equal amount of speed to the takeoff speed of the plane, to find this speed you must takeoff in the plane without the belt and find out how fast that speed is,
that speed is the speed of the belt no more no less!

[sluggish]

Think of that big wheel on The Price is Right.  It takes quite a bit of force to get that thing moving.  Once it’s moving it spins freely, but that’s not what we’re concerned with.  We’re talking about rotational acceleration.  Now imagine the wheel instead of being mounted in a fixed position, mounted on the end of a pendulum.  Now spin the wheel, but instead of pushing down on the wheel to turn it, push it along the arc of the pendulum.  What happens?  The wheel begins to turn but is also pushed along the arc of the pendulum.  The amount of the spin (speed and time) is directly proportionate to the amount of thrust (your push) that was used to overcome the rotational resistance (inertia) of the wheel and not used to propel the wheel forward (hence wasted).  Looking at it from this point, if enough rotational inertia (the conveyor belt going faster and faster, continuously exerting more and more rotational resistance) is exerted, all of the planes thrust will be used just to overcome it and the plane will not fly.  This theory requires a super god-like conveyor belt which completely ignores the very rules it is using to keep the plane in place, but under these circumstances the plane will not fly.  However, if the plane’s wheels have zero mass (thereby bringing the wheels into the same la-la land as the conveyor belt), and therefore zero rotational resistance (inertia), the plane will fly.

As far as automobiles go, rotational inertia is very important.  Lighter flywheels, drive shafts and wheels are all used to make engines rev faster (as in getting from one rpm to another more quickly) and to make cars accelerate faster.

Another example of rotational inertia is a car engines torque.  Back in the old days when you put a car in neutral and revved the engine up it would rock from side to side.  Today when you do that the car rocks back and forth (most front-wheel-drive cars have the engine in sideways…).  This is a direct result of the engine reacting to the rotational inertia of the crank shaft and the fly wheel and is also an example of energy loss due to the nasty laws of physics.

[SaburoS]

The only way the wheel starts moving is through the forward movement of the plane. The wheel rotates/increases because the forward speed of the plane is increasing.
The wheel speed is in reaction to the plane's speed.

The plane will take off.

What some are confusing (IMHO) is if the speed of the plane was the result of the wheels being powered (like in a car).
The car (or any vehicle relying on it's movement from its wheels) will remain stationary in this conveyor bely sample.

[2bighorn]

Originally posted by sluggish
Looking at it from this point, if enough rotational inertia (the conveyor belt going faster and faster, continuously exerting more and more rotational resistance) is exerted, all of the planes thrust will be used just to overcome it and the plane will not fly.

Do you understand that conveyor is rotating the wheel in the same direction plane would do on the normal runway? By what logic thrust has to overcome that angular momentum?

Normal runway: Planes thrust alone has to overcome wheels inertia

Plane on the conveyor: Plane gets little helper (conveyor)

[trax1]

Put a plane on ski's it will still take off, the wheels moving have nothing to do with a plane taking off.