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Old Feb 09, 2006, 11:49 AM
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Joined Jan 2006
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Question
Prop size and rpm on brushless

Hi

I've tried to search on the subject to no avail, so I'm asking straight out:

Brushless engines have recommended prop sizes depending on voltage it seems. I've just bought a motor with a KV value of 1200. The same motor comes in 950 and 750 versions, the 1200 having the highest current draw and efficiency. For 3S LiPo "my" engine is recommended with a 9x5 prop, while the 950 should use a 11x4.7.

What happens when I put a 11x4.7 on the 1200 motor. It will turn slower at any given current draw, but will it turn slower than the 950, and will it top out earlier? Will it overheat?

General guidlines on the subject are very welcome...

bo[R]ed
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Old Feb 09, 2006, 12:04 PM
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Chattanooga, Tennessee, United States
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Quote:
Originally Posted by bardolho
Hi

I've tried to search on the subject to no avail, so I'm asking straight out:

Brushless engines have recommended prop sizes depending on voltage it seems. I've just bought a motor with a KV value of 1200. The same motor comes in 950 and 750 versions, the 1200 having the highest current draw and efficiency. For 3S LiPo "my" engine is recommended with a 9x5 prop, while the 950 should use a 11x4.7.

What happens when I put a 11x4.7 on the 1200 motor. It will turn slower at any given current draw, but will it turn slower than the 950, and will it top out earlier? Will it overheat?

General guidlines on the subject are very welcome...

bo[R]ed

You'll fry it very quickly! The higher the Kv the smaller the prop one can use, to keep the amp draw within the motor's limits.

If your 1200 is safe with a 9x5 don't go beyond that - an 11x4.7 might try to pull twice as much current.
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Old Feb 09, 2006, 12:17 PM
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Beaverdam Creek, VA
Joined Aug 2005
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If you use the big prop at high voltage it will most likely overheat.

A motor is a generator too. With no load it will spin faster until the voltage generated inside equals the voltage coming in. Of course there will always be some loss to friction. When you put a prop on a motor it keeps it from reaching the no-load speed. The lack of internal voltage generated lets amperage flow.

A high Kv motor has fewer windings to generate voltage (back EMF is the term for it) and usually heavier wire which accomodates higher current before overheating. A low Kv motor generates more back EMF per revolution so it runs slower but with more turns of wire it makes more torque at the same amperage.

So a low Kv motor is suited for a big prop, Higher Kv for smaller props. If you put the big prop on the high Kv motor, it doesn't have the torque to turn it very fast without drawing a lot of amps. If it's way overloaded it won't even turn it fast, just overheat.

Good Luck!

Also, a low Kv motor on high voltage (and low current) will do about the same work with the same prop as a higher Kv motor on lower voltage (and higher current). There is a slight efficiency edge to higher voltage, current produces more losses.
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Last edited by BeavrdamElectric; Feb 09, 2006 at 12:28 PM.
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Old Feb 09, 2006, 01:35 PM
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Enlightening...

So what it all boils down to is this: Don't go overboard on the prop size, but as long as I stay within the voltage/current limit of my motor it's safe to test different propellers to optimize thrust. I have a power analyzer so I can experiment to see what props give the best thrust in the motor's optimum current range. I usually only use full throttle for very short bursts anyway...

Thanks,
bo[R]ed
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Old Feb 09, 2006, 04:30 PM
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Read page 4 of this CC document.

David
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Old Feb 09, 2006, 04:43 PM
BEC
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Auburn, Washington USA
Joined Jan 2001
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Quote:
Originally Posted by bardolho
Enlightening...

So what it all boils down to is this: Don't go overboard on the prop size, but as long as I stay within the voltage/current limit of my motor it's safe to test different propellers to optimize thrust. I have a power analyzer so I can experiment to see what props give the best thrust in the motor's optimum current range. I usually only use full throttle for very short bursts anyway...

Thanks,
bo[R]ed
Yes. And since you have a power analyzer and are willing to use it you are far ahead of the game - or can learn what works for you in your particular combinations of equipment - much faster than just asking for others' experiences.

Current (and the resulting heat from losses) is really the key parameter. It's a bit simplistic for me to say this, but the voltage is just about irrelevant. It is power losses, which go up with the SQUARE of the current, that is the most important parameter. Some motor vendors (for example Medusa) rate their motors by power input, which is a better way to describe the real limits than quoting a voltage limit.

Other components are more likely to have true voltage limits - notably ESCs.
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Old Feb 10, 2006, 06:26 PM
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Thanks everyone.

I've got somewhere to start from then. This will be my first brushless setup.

And to Ham2405: That's interresting reading. I knew that was the case for ESC's for brushed motors, I just didn't realize it would be the same with a brushless setup. I also realize I've expressed myself badly as I never intended to go beyond the motors max recommended current draw at WOT, just that to get long flights I'd like to get the prop that gives the most thrust at the motors optimum efficiency range. My experience from IC engines is that not all 9x5 props are the same. Or any other sizes for that matter. An APC would for instance give way more power than a master airscrew of the same size, and at the same time create less drag so the engine would rev faster.

Cheers,
Bawrd
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Old Feb 10, 2006, 06:45 PM
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Joined Mar 2005
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You will find that static thrust is optimized when turning the largest diameter prop the motor can handle.

However, except for 3D flight, static thrust is not always the primary goal.

It would be wrong to put the long low pitch prop on a pylon racer... and wrong to put a short high pitch prop on an indoor slow flying Cub.

This is one reason that the motor you list has 3 versions, a low rpm high torque one for long low pitch props, a medium one and a high rpm version for short high pitch props (or geared drives)
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