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AXI Brushless

In article , The Natural Philosopher wrote:

| > I stuck a needle into a straw, and put a GWS electric 10x8 prop (those | > ugly orange things) on it. The straw didn't bend much, so I used | > three more straws to make it much taller, and held the straws upright | > by sticking them between some stuff on the floor. | > | > I then blew a large fan on it from many feet away (trying to reduce | > the effect of the uneven flow from the fan), and noted how much it | > bent -- a few inches. I then put a rubber band on the prop so it | > couldn't spin, and backed off again, leaving everything else alone. It | > deflected a little more than it did before, indicating more drag. I | > tried a few different variations of angle and distance from the fan, | > and got the same general results. | > | > So, there you have it. A real world test. Feel free to repeat it | > yourself -- you probably already have the equipment you need. | > | > This does *not* mean that a windmilling prop will always have less | > drag than a stopped one -- only that in the extreme case of a prop | > that can spin freely with no drag, it seems to have less drag, at | > least with this prop. In the real world, an unconnected (or unpowered | > with no ESC brake) electric motor might come pretty close to this | > ideal (especially if it's a high quality motor with ball bearings and | > with no gearbox involved) but an IC engine isn't likely to come close | > at all. | | This is of course why helicopters regularly arrange to stop their rotors | when the engine fails, so that the higher drag of a fixed blade rotor | will slow them down more than the old fashioned 'autorotation' that you | have so conculisvely proved would be inferior to slowing their descent....

You're being silly. I have not conculisvely proved anything, and if you believe I think I have, then your reading comprehension is very poor. However, I did try a few real world tests, and in those limited tests, the wind milling prop did have less drag. If you doubt this, do it yourself.

Still, if you really do want to make that argument, you'll have to explain to us how a helicopter, full sized or R/C, can perform an autorotation landing *without the pilot touching the collective at all*. After all, my 10x8 GWS prop doesn't have a collective control. so why should you get one?

Since the pictch of a R/C plane prop is fixed, to make this fair, your helicopter pilot will have to leave the collective control at the same place as he has it set for normal, powered and right-side-up flight.

Also, props are usually designed to blow air in one direction and not the other. If the rotation is reversed, they will blow air in the opposite direction, but not very efficiently.

So ... to make your helicopter analogy complete, your pilot should perform his autoration landing ... inverted. Upside-down. With the collective adjusted appropriately beforehand for right-side-up flight. (After all, when a plane has a windmilling prop, it's not usually flying backwards.)

Good luck!

The GWS prop-on-a-stick test certainly isn't perfect, but it's a relatively close approximation of a direct drive prop on a brushless motor (which has really low drag) with no ESC brake on a plane.

Reply to
Doug McLaren
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True :-)

So does a sycamore seed, connected to no engine at all. I wonder why nature has evolved those?

:-)

That would work. The pitch has to change on the rotor for autorotation to work. Whether that is done magically without touching the collective I cannnot say. Not being a chopper pilot. But for a rotor to act as a brake, the picth needs to go into reverse somehow. Otherwise the blades will stop and start to turn backwards...

Mmm. Jury still out on that one I am afraid. At low speeds it doesn't make much difference.

Reply to
The Natural Philosopher

If a helicopter's engine fails, the pilot must reduce collective immediately to prevent rotor RPM loss. The machine will drop alarmingly and will glide steeply, as the air is now moving upward through the rotor to keep it turning and generating lift. The inboard area of the rotor generates the autorotative forces, and the outboard area produces much of the lift. Forward speed is essential to autorotation, and an engine failure in a hover is big trouble unless there is plenty of altitude to establish the glide. There is no way any pilot is going to stop a rotor to improve glide. If you believe that, you'll believe in Santa, too. The rotor's strength for lifting is in the centrifugal (centripetal) forces generated by weighted tips, and if rotor RPM is allowed to drop out of the green arc the blades will "cone" upward and drag will increase, lift decrease, and coning will get worse until the helicopter is falling out of control. Rotor brakes are for stopping the rotor on the ground.

Dan

Reply to
Dan Thomas

| So does a sycamore seed, connected to no engine at all. I wonder why | nature has evolved those?

Interesting. Yes, they do fall slowly. I suspect that it's because the tip goes through the air much faster than it falls downwards ... maybe. I'm sure somebody's made a disseration or something out of explaining why :)

I had a Pico stick wing fold a few hundred feet up once. This caused a rotation that ripped off both halves of the wing and then the horizontal stabilizer, leaving only the fuselage stick and vertical stabilizer, with the other three pieces fluttering down.

I figured this would make a lawn dart, but it didn't. It started rotating down like one of your seeds -- doing two or three revolutions per second, but going down very slowly. No futher damage was done when it finally hit the ground a minute or so later :)

(I wish I'd gotten it on film. It was neat!)

In any event, I don't doubt that windmilling props create more drag than stopped props under certain conditions. Perhaps under most conditions. But I don't believe it's always the case anymore -- there's a lot of variables involved.

| > So ... to make your helicopter analogy complete, your pilot should ... | That would work. The pitch has to change on the rotor for autorotation | to work. Whether that is done magically without touching the collective | I cannnot say.

That's what the collective is -- it adjusts the pitch over the entire rotor.

Reply to
Doug McLaren

I once stopped the engine in Cessna 150 (a REAL one, with lots of altitude) to see what the difference might be in the glide. The glide was steeper for the same airspeed with the prop stopped. Thinking of the prop's turning as a disc is inaccurate. The prop cannot be everywhere in the disc at once. The increased drag comes from the 90 degree negative angle of attack on the prop blade when it's stopped, as opposed to the much lower negative AOA when it's windmilling. There is less drag windmilling for that reason alone, though the difference is not that great. In both cases the prop blades are badly stalled in an inverted condition. A helicopter glides in an autorotation because the blades are turning, but their pitch is reduced so that the AOA is within a flyable range, and forward speed figures into it, to. It can't autorotate straight down.

Dan

Reply to
Dan Thomas

It is obvious you do not know how a heli works in auto-rotation! Think auto-gyro. Forward airspeed causes the rotor to turn. In an auto-rotation, altitude is traded for forward airspeed which is used to keep the rotor spun up. At the last possible moment, rotor speed is traded for vertical velocity, allowing the heli to settle gently.

David

Reply to
David AMA40795 / KC5UH

That's because even with the engine idling its producing SOME power.

>
Reply to
The Natural Philosopher

Its obvious you missed the obvious sarcasm.

Yes, *I* know that...

Reply to
The Natural Philosopher

Not an appreciable amount. I've had two engine failures, and in both cases the prop RPM was the same as if the engine had been idling. The windmilling force will keep the prop turning at around 1300 RPM whether the engine is alive or dead, unless it's seized. The theory still stands: With the prop stopped, the AOA on the prop blades is 90 degrees or so, and that will generate huge drag. Turning, it's much less, and drag will drop somewhat.

Dan

Reply to
Dan Thomas

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