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Old Apr 03, 2012, 04:00 PM
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-james-'s Avatar
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Low rev big prop, high rev small prop?

Hi guys,
I was just wondering what the advantages to each of these scenarios would be, due to a problem with a self-wound suppo motor I've ended up with a 2700kv motor on 3s turning a rather small prop, 8x4 cut down to 5 inches but no idea on the pitch now, I just know it has worked in the past
So what are the disadvantages of using a smaller prop and higher rpm as opposed to a larger prop spinning more slowly?
I understand that the large prop creates more torque, would this just allow a higher angle of climb as opposed to a faster shallower climb?

Thanks and any thoughts would be appreciated
James
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Old Apr 03, 2012, 06:42 PM
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Joined Feb 2006
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A larger diameter prop is more efficient than a smaller one. For the same input power, the larger diameter prop provides more thrust. Before outrunner brushless motors came along, a lot of folks used gear boxes to take the output of a high rpm motor and drop the rpm to turn a larger prop. The great advantage of outrunners is that they generally have a low KV. They provide higher torque at low rpm thus making the motors well matched to larger-diameter props (without the use of a gear box).
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Old Apr 03, 2012, 07:34 PM
Out of helis, sanity returning
Tallahassee, FL
Joined Feb 2009
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My general understanding of this very murky subject is that for planes that fly fast you want high KV and smaller props. Slow flyers, like gliders, want lower KV and bigger props.

Rick
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Old Apr 03, 2012, 08:10 PM
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Andy W's Avatar
Marietta, GA
Joined Jun 1999
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Originally Posted by 2400RDR View Post
My general understanding of this very murky subject is that for planes that fly fast you want high KV and smaller props. Slow flyers, like gliders, want lower KV and bigger props.

Rick
That is correct. Large props generate more thrust at a given RPM than small ones, but might have the same pitch speed. So slippery models with little drag can use more of the power for higher RPM (which means more pitch speed) to fly faster. Draggy models like sailplanes sacrifices speed for thrust. There is a balance, a speed where drag increases so much that for the power available, more pitch speed won't increase climb rate..
..a
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Old Apr 05, 2012, 03:26 AM
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Ah ok that makes a lot of sense, I guess that the 5x5 prop ive got spinning at 29000rpm isnt ideal for an easy pigeon 2m glider then :P
I'll let you know how I go on with testing it
Thanks,
James
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Old Apr 09, 2012, 06:23 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Originally Posted by -james- View Post
Ah ok that makes a lot of sense, I guess that the 5x5 prop ive got spinning at 29000rpm isnt ideal for an easy pigeon 2m glider then :P
I'll let you know how I go on with testing it
Thanks,
James
What motor is it? Maybe you can change the terminaton on it to lower the Kv. Or even consider rewinding it.

Jack
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Old Apr 11, 2012, 04:31 PM
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It's a no brand inrunner :/ therefore messing with the windings isn't an option :/
I do however have a 2217 motor that needs a good rewind :P
What's the optimal kv and prop for a 2m glider?
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Old Apr 11, 2012, 09:16 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Originally Posted by -james- View Post
It's a no brand inrunner :/ therefore messing with the windings isn't an option :/
I do however have a 2217 motor that needs a good rewind :P
What's the optimal kv and prop for a 2m glider?
The 2217 probably means a 22mm stator that is 17mm long. The motor would be 28mm or so O.D. and maybe 26mm or so long. All of that combines to insinuate how much copper there is in the winding and the size of the magnets. And all that boils down to how much input power it will handle.

But one of the key things to know about the motor is the weight. That is because average quality motor will handle 2.5-3 Watts of input power for each gram of their weight. So if you know the weight you know about what power level you can operate it at.

I recently put a motor in a R2 Hobbies Raptor 2000 (2 meter span) and used an older Atlas 2317/20 motor. That plane flew at about 984 grams/34.7 oz.

The motor specs are:

880 Kv
8-14A (18A/60s)
75 grams/2.6 oz.
28.5mm O.D. x 33.5mm long

That motor was tested on 3S with two folding props, a Graupner 10 x 6 and a Haoye 12 x 8 and was able to run both props without over heating. Results of static tests were

10 x 6 12A 115W (1.5W per gram)
12 x 8 18A 184W (2.5W per gram)

So it could be flown with either of those props, the 12 x 8 would give faster and steeper climbs of course.

So motors of that weight and Kv and props of that size are probably about what you want.

Some glider power systems are optimized for a 30 second or so full throttle run time. On those the motors will usually overheat if they are run at full throttle for much longer than that.

For more mellow or recreational flying, lower power levels, slower climb rates, and longer run times are likely to be possible. A lot of it boils down to how you want to use the plane and how you fly.

The prop size and cell count (i.e., voltage) of the battery combine to determine the KV you need. And the Kv has to be right to get the prop up to a useful RPM.

If you change from 3S to 4S you need a different Kv or a smaller prop. If you change to a smaller prop you may need a higher Kv motor, if you change to a bigger prop you may need a lower Kv motor. A lot of this just has to be figured out through prop testing results, power calculators, and the like.

Jack
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