The Ultimate Aviation Question (+extensive belt solutions by Creaking Door)

The Split function is broke 🙁 Sorry I had to take a decision on the majority of posts. If it becomes functional again, I will remember and have the necessary done for this thread.

Is it possible to split this into two threads?

The original “The Ultimate Aviation Question” thread in Historic Aviation and another thread “Will an Aircraft Take-Off from a Conveyor Belt?” in General Discussion.

Post #40 started the conveyor question and after that only posts #54, #59, #60, #65, #70, #73, #84, #87 and half #93 are anything to do with the original thread.

I wouldn’t want to be responsible for having the original thread evicted. 🙂

Bounced over to GD from the Historic forum following very little discussion of a historical nature 😉

BR

Right…this is going to be my last post on this thread since it seems that everybody else has moved on. 😮

I’ve thought of a third scenario…and I think this is why some are convinced the aircraft will not fly.

Scenario C:

“…the conveyor is trying to prevent the aircraft from flying…”

The only way the conveyor can stop the aircraft from reaching take-off speed is to increase the resistance (force) it applies to the aircraft…

…and the only way it can do that is to increase the speed that the conveyor moves.

Assuming the conveyor can accelerate quickly to a very high speed, the various resistances (rolling resistance, inertia of the wheels and air-resistance against the rotating wheels) will build-up so that…

…they will be equal-and-opposite to the thrust generated by the propeller.

When this is so the aircraft will stop accelerating, stop moving (relative to a fixed datum and the stationary air) and so will never be able to take-off.

Of course before this conveyor speed is reached the aircraft tyres will have exploded and the wheel-bearings will have vaporised! 😀

But does the original question state that the conveyor is trying to prevent the aircraft from flying?

No, I don’t think it did.

I think the problem here is the (lack of) definition of how the conveyor belt behaves.

As I see it there are two possible scenarios:

Scenario A:

“…conveyor belt that rotates in the opposite direction to the wheels, at the exact same speed as the wheels…”

The aircraft wheels cannot move unless the aircraft moves (relative to a fixed datum)…however to start with the aircraft wheels will be turning slowly.

The conveyor will be moving in the opposite direction at the same speed as the aircraft wheels are turning but the aircraft will still move forward (relative to a fixed datum) since at low speeds the various resistances (wheel-bearing friction, rolling resistance, inertia of the wheels and air-resistance against the rotating wheels) will be insufficient to balance the thrust of the propeller pulling the aircraft forward.

The aircraft will continue to accelerate, the aircraft wheels will rotate faster, the conveyor will move faster, and eventually the aircraft will reach take-off speed (relative to the stationary air) and fly.

Even at take-off speed the conveyor will only be turning the aircraft wheels twice as fast as normal which will not be fast enough for the various resistances to prevent take-off. 😀

Scenario B:

This is closer I think to the ‘spirit’ of the question.

“…conveyor belt that moves in the opposite direction but at an identical speed to the direction and speed that the aircraft (airframe) moves from a fixed datum.”

Again in this case all the various resistances (wheel-bearing friction, rolling resistance, inertia of the wheels and air-resistance against the rotating wheels) will be kept within reasonable limits (twice the normal take-off speed for the aircraft) and so will be insufficient to balance the thrust of the propeller pulling the aircraft forward.

As a result the aircraft will continue to accelerate and eventually reach take-off speed (relative to the stationary air) and fly. 😀

So which scenario is correct? Personally, I don’t think it matters.

As far as I am concerned, either way, the aircraft will take-off pretty much as normal…and I think I can prove it. 😉

Hang on, the wheels are free to turn yes, but they are attached to the aircraft, therefore the motion (motion is an applied force) of the belt is being applied to the not just the free spinning wheels but the aircraft that they are attached to.

Motion isn’t an applied force…I’d agree that motion can apply a force…but there has to be another ‘mechanism’ to cause the force to be applied.

In this example the ‘mechanisms’ that will apply a force to the airframe as the wheels rotate will be friction in the wheel-bearings, rolling resistance of the wheels, inertia of the wheels (which I don’t think anybody has mentioned yet) and air-resistance against the (top of the) rotating wheels.

And if that sounds like a contradiction, then yes, it is… :confused:

…but I think it is important that people can’t just state that the belt ‘pulls the aircraft backwards’…the belt exerts a force on the aircraft…the question is how…and is that force strong enough to prevent the aircraft from taking-off? 🙂

…like a person standing at the back of a treadmill holding the handles of a wheelbarrow.

Yep, that’s a pretty good analogy.

If an aircraft is sitting on a conveyor belt.. that rotates in the opposite direction to the wheels, at the exact same speed as the wheels, will the plane take off?

I’m 100% with Creaking Door. Who gives a toss about the relative motion of the landscape? For one thing the earth is already spinning anyway, for another the analogy of taking off upstream on a river is also perfect. The issue is undoubtedly all about gaining airspeed and as the aircraft pushes or pulls itself relative to the air all is well. It will accelerate and take off.

But there is a factor that intrigues me, and I seem to be alone in this. The conveyor belt rotates at the same speed as the wheels. It must therefore be the same diameter as the wheels for the wheels and conveyor to be stationary relative to each other. Just picture two cogs, one bigger, one smaller. If the conveyor had a different diameter than the wheel, the wheel would have to skid on the conveyor or begin to rotate around it. Because the conveyor’s rotation is at the wheel speed.

So, if the conveyor is the size of the wheels, a wee bit of a push or pull external to the conveyor (ie. against the air) would see the aircraft roll off the conveyor and it can then do its own thing. Alternatively if the conveyor is of larger diameter than the wheel, all you need to do is rotate the wheels backwards (fit a little electric motor or whatever), and the conveyor will spin, at wheel speed, forwards. Given the conveyor’s larger diameter the aircraft will be accelerated vis-à-vis the air and in due course lift of.

Up, up and away …

Why does a wheelbarrow in a river need brakes :confused:

Try appying the brakes on the wheelbarrow.;)

The motor driving the movement of the belt is providing the applying force against the imparted force of the aircraft that’s being applied to make the wheels turn.

:confused:

I think you are confusing force and motion…and the belt cannot apply a force to the aircraft, the airframe that is, but only to the wheels which are free to turn.

Hang on, the wheels are free to turn yes, but they are attached to the aircraft, therefore the motion (motion is an applied force) of the belt is being applied to the not just the free spinning wheels but the aircraft that they are attached to.

A scale model would be something like a person standing at the back of a treadmill holding the handles of a wheelbarrow. The freespinning wheels of the wheel barrow is on the belt of the treadmill.
The person is the engine, the wheelbarrow is the airframe, the wheel is obviously the wheels.

ok A person can push a wheel barrow off the front of the treadmill as long as he pushed the wheelbarrow with sufficient force that the wheel rotates faster than the speed of the treadmill.
If like the original question the treadmill is able to instantaneously match the speed of the wheel of the wheelbarrow no matter how hard the person pushes he won’t be able to push it off the front.

Scale that up no matter how much thrust against the air the engine generates and imparts through the airframe to make the wheels rotate, as long as the conveyor belt is able to match that speed of the aircraft wheels ithe aircraft will remain stationary.
No forward movement, no lift, no take-off

But the fast flowing river doesn’t increase in speed instantly, infinitely, and the two steps in the floats are already there to help the A/C break free.

ok, with the aircraft sitting stationary on the belt thrust is applied by the engine, this would normally be imparted through the wheels in order to create ground speed.

The problem – or rather the clever bit of the question – is that from an early age riding bicycles, being driven round in cars and buses, we understand that the driving force goes through the wheel. We rely on “intuition” that we learn early in life but the question catches us because an aircraft applies the force to the air. The wheels are passive.

I actually clarified that in a subsequent post:

The propellor generates the “thrust”. Whilst the aircraft is on the ground that thrust must be imparted against the ground. The point on contact being the wheels.
The forward thrust makes the wheels turn. I took that as read but never mind.

Why am I talking about friction? See my previous post. 🙂

And you explained it better than I could – ta. 🙂

ok, with the aircraft sitting stationary on the belt thrust is applied by the engine, this would normally be imparted through the wheels in order to create ground speed.

The problem – or rather the clever bit of the question – is that from an early age riding bicycles, being driven round in cars and buses, we understand that the driving force goes through the wheel. We rely on “intuition” that we learn early in life but the question catches us because an aircraft applies the force to the air. The wheels are passive.

Try thinking about an aircraft on skis, or, and this one I like, a floatplane.
If we are allowed to write our own question then how about an aircraft floating on a fast flowing river going in the opposite direction to the aircraft – how does it get airborne? At least it won’t have overheated wheel bearings but then there is the issue of surface tension creating water drag. :diablo:

Thread realignment:

Why is my aircraft more unreliable than my 17 year old, 344000 mile Land Rover even though all the aircraft parts, genuine and PMA, cost at least 10 times as much as genuine Land Rover parts???????

AAHHHHHHH

Re-engagement of thread drift:

Does the handrail travel at the same speed as the conveyor?

camlobe

Yes, but not in the same direction….

Dont forget drag, the belt is made (of rubber or similar) and the tyres, of a similar material, and of course deflection, there will be more contact between two similar surfaces, creating more drag; the wheel bearing issue is only one part of the jigsaw.

The thing to do, is to imagine that you are at the end of the mythical conveyor (however long) and with the A/C in front of you (whichever way round) the belt starts to move towards you, bringing the stationary A/C into contact with yourself, you might be able to hold it back, depending on size,(for this purpose we’ll assume its an average spam can) at very low speeds, but as the speed of the conveyor increases so will the force, and I believe you will not.

Why are we talking about friction?

Why am I talking about friction? See my previous post. 🙂

I think we should be talking about positive feedback loops.

Yes, that aspect of the ‘mythical’ belt bothered me too.

It would have been better if the question had said that the belt…

“moves in the opposite direction but at an identical speed to the direction and speed that the aircraft (airframe) moves from a fixed datum.”

Still, that would have made it a bit easy! 😀

That funny I always thought that the Atmosphere was a Gas in it’s free state.

Yes, it is a gas…but when dealing with moving air (or things moving through it) it is referred to as a fluid…sorry, poor attempt at a joke…hence the smiley face. 😮

If it were true what you are saying about bearings then they would last forever, which is just laughable, i’ll say it again the more friction on a bearing [speed] the hotter it gets…

The point I was making (or trying to make) about friction is that when most people talk about ‘friction’ they are actually talking about two distinct, related but separate effects.

Friction that resists relative movement of objects in contact with each other; this is a force (a vector) and does not increase as the relative speed of the objects increase.

Heat that is generated as a result of relative movement of objects in contact with each other; this heat (a scalar) does increase as the relative speed of the objects increase.

When I said…

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.

…I was only referring to the friction force not increasing with speed.

Since this force, derived from the friction in the bearings, is the only force that could possibly prevent the aircraft from moving it is important to show that it will be less than the thrust produced by the propeller irrespective of how fast the belt goes.

Yes, the faster the belt goes the more heat will be generated in the wheel-bearings…

…but the force will remain constant and will be identical to the force, derived from the friction in the wheel-bearings, if the belt was stationary.

So if the plane can take-off from a stationary belt it can take-off from a moving belt…the only difference being the temperature of the wheel-bearings. 🙂

The motor driving the movement of the belt is providing the applying force against the imparted force of the aircraft that’s being applied to make the wheels turn.

:confused:

I think you are confusing force and motion…and the belt cannot apply a force to the aircraft, the airframe that is, but only to the wheels which are free to turn.

The propellor generates the “thrust”. Whilst the aircraft is on the ground that thrust must be imparted against the ground. The point on contact being the wheels.
The forward thrust makes the wheels turn. I took that as read but never mind.

The motor driving the movement of the belt is providing the applying force against the imparted force of the aircraft that’s being applied to make the wheels turn.

we are told these balance since the belt can match the speed of the wheels

Therefore the forces are balance, and the aircraft isn’t going anywhere

Here’s my take on the aircraft and the conveyor belt.

ok, with the aircraft sitting stationary on the belt thrust is applied by the engine, this would normally be imparted through the wheels in order to create ground speed.

You’ve lost me there…the propeller pushes on the air.

The wheels turn because the aircraft moves…the aircraft doesn’t move because the wheels turn.

Newtons Laws … equal and opposite forces means either stationary or at a constant speed…

Yes, Newton’s Laws apply but how does the belt apply a force to the aircraft?

The belt can only apply a force to the bottom of the aircraft wheels, and the wheels are free to turn so the forces are not equal-and-opposite…

…therefore according to another of Newton’s Laws the aircraft must accelerate. 🙂