Debate!

[EGNX]

[quote][/quote]
Sure on that???

[eno]

One has to ask .......... is the conveyor motorised or driven by the friction of the vehicle above?  ;)

[EGNX]

[quote]One has to ask .......... is the conveyor motorised or driven by the friction of the vehicle above?

[expat]

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.

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?

I did, see page two, however as we now have to factor in conveyor belt limitations, it would appear that my constant speed experiment is no longer valid.


Matt

[TSC.]

that counteracts the movement of the aircrafts wheels as they move.

The original question clearly states that the conveyor belt matches the speed of the wheels, how fast the wheels are spinning has stuff all to do with airspeed over the wings.

TSC.

[zeberdee]

So if the wheels arn't turning how can the aircraft generate airspeed to lift off?[/quote]
The wheels are stationary because the runway/convayor belt is moving at the same speed as the plane, the plane is not moving in relation to the runway as they are both moving at the same speed, until the plane becomes airborne

[expat]

Ok, I am taking one last stab at this.

Without the rolling motion of the wheels the aircraft cannot gain speed to take off, unless it is on skids and snow or floats and water or the conveyor is traveling with the aircraft, which it is not, it is against it and counteracting it.

Now there must be a link between wheel speed and air speed. Without wheel speed, we have no air speed, just as a normal runway and if the wheels do not turn then we will have one of two things..........a levitating aircraft or some big black skid marks down the conveyor runway.

Now take a nice big engined race car, put it on a rolling road and open up the throttle. Does the car shoot through the wall or does it stay on the same position. The fact that the wheels are turning the circular conveyor belt (rolling road) is irrelevant because to gain airspeed we need wheel speed. After all, the tyres of the 737's that I work on are rated at 250 mph, and if we did not need wheel speed for take off to translate to airspeed, then I could save a whole lot of time and my back replacing them, they weigh in at 187 kg each!!
Now it we put wings on our car on the circular conveyor belt (rolling road) will it suddenly produce lift and fly.

The problem is in one statement from EGNX we are told that the conveyor belt counteracts the thrust of the aircraft and in other cryptic statements we are told that the conveyor belt has limitations, but the aircraft does not...............I am really beginning to lose the plot :-/  

Matt

[Hagar]

Ok, I am taking one last stab at this.

Without the rolling motion of the wheels the aircraft cannot gain speed to take off, unless it is on skids and snow or floats and water or the conveyor is traveling with the aircraft, which it is not, it is against it and counteracting it.

Unless I'm completely missing the point here I fail to see how the rotation of freely spinning wheels affects anything.

[expat]

Put you aircraft on the conveyor with the engines turned off. Turn on the conveyor. The aircraft travels backwards on the conveyor just like your suitcase in the baggage hall. Now start the engines and increase the thrust to stop the backward motion of the aircraft. It is now sitting on the conveyor belt and staying in one position thanks to the rolling effect of the wheels.
Now if we increase the thrust and at the same time the conveyor belt speed so we always have this equilibrium (stated the conveyor counteracts the thrust) up to the max thrust of the aircraft, will the aircraft will remain in the same position? and if it does, it will have no forward motion to produce lift.

Matt

[Woodlouse2002]

Lets say that the aeroplane is on ski's and the conveyorbelt has snow/ice on it. Will it take off then?

[Hagar]

Put you aircraft on the conveyor with the engines turned off. Turn on the conveyor. The aircraft travels backwards on the conveyor just like your suitcase in the baggage hall. Now start the engines and increase the thrust to stop the backward motion of the aircraft. It is now sitting on the conveyor belt and staying in one position thanks to the rolling effect of the wheels.
Now if we increase the thrust and at the same time the conveyor belt speed so we always have this equilibrium (stated the conveyor counteracts the thrust) up to the max thrust of the aircraft, will the aircraft will remain in the same position? and if it does, it will have no forward motion to produce lift.

I think that's debatable. As I pointed out earlier, the speed of the conveyer belt must be relative to the speed of the aircraft. (Airspeed or groundspeed relative to the ground, not the belt.) If the aircraft is not moving the belt will stop. The rpm of the wheels cannot affect the motion of the aircraft as they are in no way connected with the thrust of the engines.

[Mobius]

Alright, assuming no friction inside the landing gear (wheel bearings, etc...), the aircraft will take-off.  As more and more friction is factored in, more and more thrust is required for take-off.  Assuming there is enough friction in the wheels to completely counteract the thrust produced by the engines, then no, it will not take off.  But that is the point of wheels, to drastically decrease the amount of friction.  So, as the question was stated (disregarding limitations) the aircraft will take off.


Here's my new analogy:  Assume you have a conveyor belt, and a perfect lubricant.  You cover the bottom of a wooden block with this perfect lubricant (no friction between the block and the belt (u = coefficient of friction = 0)), and place it on the belt.  The belt is turned on, and what happens?  Friction is the only force that can move the block on the conveyor belt, and the formula to find the friction force is F (friction force) = u (coefficient of friction) * N (the normal force, or the force due to the mass of the block and gravity).  Because we already said a perfect lubricant was used, the coefficient of friction is 0, therefore the friction force is 0, which means that no matter how fast the conveyor moves, the block will always stay in the same position, stationary.  Now, assume there is no lubricant used, and the coefficient of friction between the block and the belt is 1, using the equations from before, the friction force would be equal to the normal force, meaning the block would move with the conveyor belt, just as it logically would, and does in the real world.  Now, going back to the original situation with the perfect lubricant.  If the conveyor is moving and the block is stationary, and you push the block with your hand, the block will move wherever you push it.  Now, if you imagine the perfect lubricant is the set of perfectly frictionless wheels on our airplane, and your hand providing a movement force is thrust of the jet engines, you can see from the last situation, that the aircraft will move.  So the aircraft will take-off, as long as the force applied will overcome the friction force in the wheels.

[Stormtropper]

Put you aircraft on the conveyor with the engines turned off. Turn on the conveyor. The aircraft travels backwards on the conveyor just like your suitcase in the baggage hall...

...thats the point, with free rolling wheels, the aircraft DOESN'T move...the wheels will spin to counter act the motion of the conveyor, but the aircraft is stationary...

...and the other thing...the heavier the aircraft, the more the friction between the rubber on the wheels and the conveyor (the friction between the aircraft and the conveyor will be a number close to zero as the aircraft itself and the conveyor aren't touching...and what ever friction you have is between the wheel and the aircraft...)....which would mean the larger the difference between the amount of friction between the aircraft and the conveyor and the wheels and the conveyor...so the plane will move LESS.

[EGNX]

I'll tell you now....
That if were tried in real life, with a real 747.............. It wouldn't take off. Due to its limitations....
Now i've only said don't think about aircraft limitations just yet... nothing about the conveyor...
So... what 'problems' could we have with the conveyor that would allow an 'ideal' aircraft to take-off?
(Ideal, meaning - maximum endurance etc...)

[expat]

So... what 'problems' could we have with the conveyor that would allow an 'ideal' aircraft to take-off?

Apart from building a rubber belt that is twice as long as the runway required.............

So for the ideal take off........
Here is one, the size of the bow wave that the nose gear would produce would probably "throw" the belt.
In a previous life I was on a Harrier field deployment squadron. When the aircraft took off on the "tin strip", they would produce a small bow wave in the metal planking, sometimes if the planls had become loose up to about 6 inches high. Now transpose that to a bit of floppy rubber.

Matt