If friction does not increase with the increase of speed, why do we put grease in the wheel bearings…

One of the reasons we put grease into wheel-bearings is to reduce friction but that doesn’t prove that friction increases with speed. With wheel-bearings (roller bearings) the friction is very low to begin with.

If friction is a function of speed why don’t your (friction) brakes on your car stop working when the car stops? :confused:

…and oil in the engines?

Engine bearings (plain pressure lubricated) work in an entirely different way since the crankshaft (for example) doesn’t touch the main bearings when the engine is running so we would be dealing with fluid friction here.

One thing that is more than obvious, is that almost everybody that has contributed to this, has a different idea what they mean by an “Aircraft”…

The type of aircraft is irrelevant; it wouldn’t matter if it was a microlight or a Boeing 747, the result would be the same. 🙂

…and how long the conveyor belt is, and even which started to move first, the belt or the A/C…

Hence my earlier attempt to define the properties of the conveyor. With regard to which moved first; it doesn’t really matter, for the sake of argument the conveyor could move first if you like.

…to sumarise my view, if the A/C was able to move forward even 1″ then, it will fly because it has broken free of its opposing forces, however I think that they will be too great at the very moment when it would start to move, and so it won’t.

All aircraft manage to break free of these forces when on a stationary runway and at the moment the brakes are released the belt would be stationary too…so what is the difference?

Put it another way if the aircraft was rolling slowly backwards on a flat stationary runway (to simulate a slowly moving belt) what would happen when full power was applied?

Why dont we ask the man who started all this!

Good idea…but I’m not sure he knew the answer when he posted the question! 😀

What is the difference between an aircraft on a hypothetical powerful and long conveyor with the brakes on..and a steam catapult launch?

Effectively none…but I don’t see any aircraft-carriers being fitted with conveyor belts any time soon! 😀

So are we all basically in agreement then…that the aircraft will move along the conveyor and take-off pretty much as normal?

With respect to the ‘drag’ created by the aircraft tyres, brake shoes (brakes off) and wheel-bearings; these are small forces that every aircraft should easily overcome on a relatively flat runway.

The fact that the conveyor will be spinning the wheels faster than during a normal take-off will not affect these forces…as they are forces of friction and these forces do not increase as the speed of the wheel increases.

There will be other forces that do increase as the aircraft speed increases such as the drag created by the top of the wheels spinning rapidly into the direction of flight but this will be a small force and will only lengthen the take-off run slightly.

The only other factor I can think of is the inertia of the spinning wheel will be greater. 🙂

The length of the conveyor belt is irrelevant…

No, the length of the conveyor is irrelevant only if the aircraft doesn’t move…but it must move. I was only trying to eliminate any problems due to the ‘hypothetical’ nature of the conveyor.

I know you’re probably thinking that I don’t understand the question here but I do…I really do.

A man on a walking maching doesn’t need a longer treadmill because he starts to run ,because the speed increases pro rata with his (and he remains in the same place); if someone was to turn the speed up to his maximum running speed, could he move forward to turn it off….no

One thing this question absolutely is not like is a walking machine, or an airport conveyor, or a rolling road (see previous posts #44-55).

The reason that you can’t move forward on a walking machine (that hypothetically always runs backward at exactly the speed you walk) is because you are pushing against the belt in an attempt to move forward…the aircraft question is not the same.

The aircraft propeller pushes against the air, the stationary air, above the belt to generate the thrust…and the only part of the aircraft touching the belt are the wheels that are free to spin.

It is therefore impossible for the belt to balance the thrust generated by the propeller…so the aircraft will move relative to the stationary air and as it accelerates it will generate lift and fly. 😀

Physical laws state that the forces acting on the aircraft must balance (equal zero) if the aircraft is not to move (accelerate) relative to a fixed point (or the stationary air) and these forces must balance irrespective of the speed of the aircraft.

The absolute key to this is windspeed, which is either generated by the, er wind,(which we are assuming there isn’t any) or else the A/C’s forward speed, creating lift, and so flight.

Yes, at least we agree on that point.

If the A/C had retractable U/C, (and was at full takeoff power on the conveyor), and retracted the U/C would it fly…. no because it isn’t moving forward.

No, it wouldn’t be moving to start with but after a normal take-off run the aircraft would be way-off down the conveyor and flying as normal. 🙂

Don’t you just love hypotheticals…..if you accept that the conveyor belt can match the speed (full power thrust) that any given A/C can reach, then it will never move forward, and with zero windspeed, it will have zero lift, and so will not fly…

No, no, no…even if you had an infinitely long, zero weight conveyor that could accelerate as fast as the aircraft to the maximum speed of the aircraft, and that could react instantaneously to the movement of the aircraft wheels the aircraft would still take-off.

It would take-off because it moves…it has to…by the laws of physics! 😀

Congratulations to Art Nalls and his team! 🙂

A great pity that the RNHF couldn’t have operated one…but I do understand the funding wasn’t available. 🙁

Never thought a Sea Harrier would ever fly again…but as they say…never say never…RNHF? 😉

P.S. BAZV is XZ439 sporting ‘sharks teeth’ in that photo?

I’m assuming zero wind. 🙂

I think this is a difficult question because the answer isn’t intuitive…nice one ZRX61! 😀

Try it a different way:

If a model aircraft with rotating wheels was placed on a conveyor belt that behaved exactly as the one in the original question…and you used your hand to simulate the thrust produced by the propeller:

Would you be able to roll the aircraft forward (relative to a fixed point) on the conveyor?

According to those who say the full size aircraft could not fly it would be absolutely impossible to move it forward (relative to a fixed point)…no matter how hard you pushed. :confused:

What has propeller thrust have to do with generating lift?

Nothing, except that unless it is opposed by an equal and opposite force that thrust will cause the aircraft to accelerate…and as it accelerates through the air it will generate lift.

If I were to walk along one of those conveyor belts you find in an airport and maintain an overall net zero velocity (no back or forward displacement from a fixed relative position) will I feel any air passing my body? No?

Agreed. I understand that if the aircraft does not move it can’t generate lift…

…but my argument is that it does move…it has to…so long as the brakes are off!

How then will any air velocity pass the wings to generate lift ?

Ask yourself this:

How are the forces balanced with the aircraft on the belt?

Well, let us forget its weight for now, since it is not flying (yet)…that leaves the thrust produced by the propeller.

Now if the aircraft is not going to move that thrust must be balanced by an equal and opposite force, right?

The only part of the aircraft touching anything else is its wheels, on the belt, and the part of the wheels touching the belt is, well the part touching the belt, right?

And the wheel is free to turn (the brakes are off) so any force applied to the wheel at that point will simply cause the wheel to spin…so the thrust cannot be balanced…the aircraft will accelerate…and as it does it will generate lift and fly. 😀