Debate!

[Hagar]

If however all friction in the system, that being conveyor and wheel bearings is eliminated (theory only) and friction only exists between wheel and conveyor then if the speed of the conveyor equals the thrust of the engines then you have a tread mill and the aircraft will not move forward.

If the aircraft was bolted to the ground, the we would all agree that it would go nowhere. So when we put a system under the aircraft that says forward thrust is neutralised by the reverse motion of the conveyor on the wheels do people then think that the aircraft will fly.

You're the only one here that actually did a practical test so I find it difficult to disagree with your findings. Surely the point here & what most people seem to be missing is that the conveyor should have no effect on the forward thrust of the aircraft at all.

Going back to the original question, it's not clear what the speed of the conveyor is relative to, groundspeed or airspeed of the aircraft. This would make a big difference to the maximum speed of the belt but in either case it assumes that the aircraft can in fact move or it would all remain stationary.

[EGNX]

Of course, when you build in the factor that there will be lag in both the conveyor belt control system, and in the motor system itself it becomes all the more complicated. Depending on that I believe, will be the take off distance required, which will be quite a long way.

Now this is definitely something we need to take into account.....  ;)

(Remember, dont worry about aircraft limitations just yet!)

[TSC.]

I hate this question, but it always makes me laugh - if the conveyor belt was going to be running at such a speed as to keep the plane from moving, IE: no airflow - why does the conveyor belt have to be soooooooooooooooo long

Couldn't they make do with a conveyer belt that is only marginally longer than the wheelbase?

Just remember, airflow is independant of wheelspeed on a plane

TSC.  

[EGNX]

[quote]
Just remember, airflow is independant of wheelspeed on a plane
TSC.

[expat]

Just remember, airflow is independant of wheelspeed on a plane

TSC.  

Correct, if the conveyor counteracts the aircraft thrust, then we will have ground speed (put a tacho on the wheels, rolling road), but no air speed because the aircraft is not moving forward.

Of course, when you build in the factor that there will be lag in both the conveyor belt control system, and in the motor system itself it becomes all the more complicated. Depending on that I believe, will be the take off distance required, which will be quite a long way.

Now this is definitely something we need to take into account.....  ;)

(Remember, dont worry about aircraft limitations just yet!)

Ahhh, so now you are introducing a conveyor belt limitation. Up until now I have assumed that the conveyor belt could match the thrust, as you said it counteracted the aircraft.
So we are to ignore aircraft limitations but have to factor in conveyor limitations. Bit of a goal post move is it not, because if you conveyor cannot match the aircraft due to a lag, then the aircraft with to superior engine acceleration will always be able to stay ahead of the conveyor. The only question then is how slow is the conveyor to accelerate and can the aircraft reach V2 before the conveyor catches up to counteract the aircrafts forward speed.


Matt

[EGNX]

Ahhh, so now you are introducing a conveyor belt limitation. Up until now I have assumed that the conveyor belt could match the thrust, as you said it counteracted the aircraft.
So we are to ignore aircraft limitations but have to factor in conveyor limitations. Bit of a goal post move is it not, because if you conveyor cannot match the aircraft due to a lag, then the aircraft with to superior engine acceleration will always be able to stay ahead of the conveyor. The only question then is how slow is the conveyor to accelerate and can the aircraft reach V2 before the conveyor catches up to counteract the aircrafts forward speed.


Matt

Now someones thinking laterally!  8-)

[Jakemaster]

This question has so many wholes it is impossible to find a correct answer.

Assuming a PERFECT situation, the conveyer matches the aircrafts ground speed precisely with no lag and the aircraft takes no damage, no, it wont take off because it wont move because no matter how much thrust you give the conveyer won't allow the plane to ROLL forward.

If you put a giant fan infront of the plane blowing at takeoff velocity, the plane will takeoff without rolling.  But in a dead calm situation, the plane needs to roll.  

[Chris_F]

So the answer to the question is that it is unanswerable.  The only way to answer it is to make assumptions.  Assume the wheels are ideal (frictionless) and the conveyor is powerless to prevent the airplane from taking off.  Assume the wheels are not ideal but the conveyor is idea then the conveyor is able to prevent the airplane from even moving.  Assume anything is real-world and you need data on the whole system in order to make a judgement.  It hinges on its assumptions, which makes for a very poorly stated probem.  Devise such an experiment in the real world and what would likely happen is this:

The company that insures the 747 calls up and says they'll cancel your policy if you try the experiment.  The company that financed the 747 will call soon after saying their lein on the airframe prevents you from operating it without insurance.  So the whole thing ends before it begins.  In other words, it's LIABILITY and not PHYSICS that prevents the plane from taking off.  :)

But assuming you own the plane outright I'm sure the FAA would step in before you have a chance to give it a go.  So it's REGULATION and not PHYSICS that prevents the plane from taking off.

Assuming you've managed to bribe the FAA in to letting you give it a try, you line the plane up on your conveyor, advance the throttles, and the conveyor lunges to life, speeding along to try to keep your plane motionless.  Then the tires fail at speeds beyond their rating.  The bare wheels now dig in to the fast moving conveyor and rip the gear clean off.  The airplane collapses on to the fast moving conveyor and is quickly accelerated backwards.  The engines dig in as well and blow apart.  Now the entire mess, aluminum, carbon fiber, flaming jet fuel, gets chucked at high speed off the back of the conveyor and does, briefly and for a very short distance, take flight.

So yes, the plane will fly.

[DizZa]

One: Planes aren't permanantly attached to the ground
Two: Planes take off on airspeed not ground speed
Three: Planes get propulsion by pushing air, not turning wheels.

So it WILL take off, all that would happen would be a very high wheelspeed.

[quote]+1

[BFMF]

[quote]So the answer to the question is that it is unanswerable.

[Brett_Henderson]

Quote:

If the wheels are truly just "free-wheeling".. you could turn the conveyor on (any direction) while the engines were shut down and the plane would stay put while the wheels just spun. We know that wouldn't happen.



If you did that the plane would not match the speed of the conveyor belt. The wheels would just freewheel and then the drag accociated with the wheels would pull the plane back.

That was my point..

[Chris_F]

Who says you HAVE to do an experiment with a REAL 747? You could easily test the theory on a much, much smaller scale. Couldn't someone test the theory with RC planes?

Accurately modeling the phenomena on a small scale would be a huge technical challenge.  You'd have to accurately model the friction and mechanics of an actual 747.  An RC model won't tell you anything except whether or not an RC model can take off under those conditions.  And even then it will only answer the question for one specific RC model and one specific treadmill.  Other combinations may perform totally differently.

Whether or not the plane will take off depends on a huge quantity of factors, any one of which can influence the ultimate result.  Thrust, friction, treadmill speed, etc.  All will change the result.  Can a 747 wheel bearing exert enough friction force to counteract its engines?  I doubt it.  The quantity of heat would be enormous and the whole thing would fail or melt or burn up.  Can the tires exert enough force before they skid?  Doubtful.  If you go full bore with the engines on a 747 and the brakes locked I'd think you'd leave some big black smears on the pavement.  So the tires can't create enough friciton.  Can the tires spin fast enough without coming apart?  I doubt it.  They'd probably shred.  But, any one of these factors could be totally different in a scale model.

[zeberdee]

Somebody asked me this question today, and then eventually told me the answer... lets see what you can come up with...

Will it or wont it take-off???

Heres the situation:

For ease, lets say that a Boeing 747 is lined up a very long (many, many miles) conveyor belt type device, that counteracts the movement of the aircrafts wheels as they move. But as the aircraft speeds up will it ever take-off? Yes or no and why?

Artist impression: But will that really happen?
(This is just something to represent the concept, don't let it distract you from the question)


Diagram:

The runway is going the wrong way to counteract the wheel movement. The wheels will turn at twice the indicated airspeed as per your diagram. Yes the plane will take off, the wheels will not be turning.

[EGNX]

The runway is going the wrong way to counteract the wheel movement. The wheels will turn at twice the indicated airspeed as per your diagram. Yes the plane will take off, the wheels will not be turning.

Sure on that???  ::)  :P  ;D

[zeberdee]

[/quote]
Sure on that???