why Gears in some burshless motors?

Folks,
To a electric newbie, higher the propeller rpm, the more air the propeller is going to push and hence will result in pushing the plane
faster.
So, why are gears used to reduce the rpm of the propeller?
Can't I just use ESC to control the motor rpm, ie, to slow the rpm?
I'm talking in context of pusher models, replace push with pull in the above text for puller models :)
Thanks
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Geekay wrote:

At lower flight speeds a larger propeller that pushes more air more slowly is more efficient.
--

Tim Wescott
Wescott Design Services
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snipped-for-privacy@yahoo.com says...

General Rule 'O Thumb: A slow prop with a large pitch is more efficient than a fast prop with a small pitch.
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James Beck wrote:

At low speeds
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small pitch.
Actually that doesn't even begin to be true - A high pitch prop will be stalled at slow speed. I've never seen anyone put on a (say) 2:1 gearbox so they can go from an 8"x6" prop to an 8"x12" prop. the airflow out of the back (and therefore top speed) would be exactly the same but the initial acceleration of the 8"x12" pitch would be awful because it would be stalled. Only if the prop tip speed got near supersonic (well over 25,000 RPM in this diameter) would the higher reving prop perform worst.
It is larger diameter that is more efficient. an 6" prop at 20,000 RPM won't work anywhere near as well on most aircraft as a 10" prop at correspondingly lower RPM.
There is only so much power available from a motor so throttling back to give you a lower RPM doesn't help at all. Loading a motor up with a bigger prop just causes it to run badly and draw too much current so the only way to turn a bigger prop without too much loss of efficiency is with more torque i.e. a gearbox (or outrunner)
Electric motors and cells still don't have so much power that we can waste it as we do with IC engines. Gearboxes allow us to match the prop to the model better. if the prop is blowing air back on full power at more than 15% above the flying speed its just wasted energy. A gearbox (or outrunner brushless) will allow us to move a bigger chunk of air back at lower speed giving better acelaration and climb. Just like pulling away in first gear instead of 4th in your car.
Of course if you need the equivalent of 4th gear for speed such as in a clean, fast pylon racer the small high reving engine is better but the initial acceleration will be worst.

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Geekay wrote:

Same reason gears are used on a car. To make a slower heavier car go up hills.

Yes, but that doesn't get you up hills either.
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GEEKAY answered already, but here's a shorter answer: Friction and vibration concerns aside, electric motors are more efficient the faster they run. The trick is to run your motor at its most efficient speed while running the propellor at its most efficient speed; the difference between these two speeds is made up with gearing.
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Kel wrote:

Almost true.
For any given arrangement of poles, ironwork and magnetic strength and bearings, there is an optimum RPM for best power and efficiency.
If thats too high for the prop/airframe that is most efficient, a gearbox does the matching.
Its NOT true too say that the higher the RPM the greater the power and efficiency. It IS a parabolic curve, and has a peak somewhere..however for many cheaper motors, that peak lies beyond the RPM of the motor to withstand physically. This is NOT true of a quality brushless motor with good magnets. Optimum RPM may range from less than 10K for a big multipole out runner to over 60K for a hot two pole inrunner.
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| > GEEKAY answered already, but here's a shorter answer: Friction and | > vibration concerns aside, electric motors are more efficient the faster | > they run.
That's hardly true. At it's highest speed (attained with *no* load), a motor is 0% efficient. Same goes for it's lowest speed (completely stopped with a load it can't move.) The most efficient point is some where in between.
| For any given arrangement of poles, ironwork and magnetic strength and | bearings, there is an optimum RPM for best power and efficiency.
To be more precise, there's two optimum RPMs -- one for best (highest emitted) power and one for highest efficiency. They're not usually the same.
I doubt that TNP needs it, but the Astroflight Electric Motor Handbook --
http://www.hobbylinc.com/htm/ast/ast600.htm
is pretty good (though very dry) reading. It's a bit dated -- only talking about brushless motors in passing, for example -- but it still does a good job of explaining how things work and such.
(And to be fair, brushless motors may be built and controlled differently, but they still obey the same rules as the brushed motors. The book could benefit greatly from some updating, but it wouldn't change that much.)
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Doug McLaren, snipped-for-privacy@frenzy.com
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Doug McLaren wrote:

I always arrange to make them coincide ;-)
That's why I sad there is ONE RPM for best power AND efficiency.
The algorithm is simple:
Work out max heat *dissipation*, in watts (non trivial, but its about 35W for a speed 400, 45W for a speed 480, and 60W for a speed 600 sized 'cans'*)
Divide by two.
Divide by idle current to get optimum WOT voltage.
Take the other half of the losses and divide by motor resistance**
Take its square root.
That's optimum current.
That puts the max efficiency point coincident with the max power point. You can get the RPM by taking the current, multiplying it by the resistance, subtracting the result from the input voltage and multiplying that by the KV.

Bob Boucher is one of the people who know their stuff. sure. I worked it pout from basic electrical motor theory, and guess what, came up with the same answers as Bob. Who probably did exactly the same.
*Temperature rise is both a variable depending on the magnet material, with neodymiums being the most fragile, then ferrites, and then Cobalts. Cobalt can take alarmingly high temperatures - well over 100C. Neodymium magnets are the most efficient (tends to go with stronger magnets, for complex reasons), and cobalts the toughest and intermediate in efficiency. Ferrites are the least efficient, but still able to achieve better than 75% efficiency.
** always remembering that as the winding temperature rises, so does the resistance. A motor that will take 40A for 3 minutes, may not be able to take 30A at extended flight times with larger LIPO batteries.
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