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Prop size for brushless setup

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Old 02-09-2007, 05:43 AM
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CaseySP
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Default Prop size for brushless setup

I'm very new to electric flight - but have been flying i/c models for nearly 20 years now.

I bought a (exact) copy of a GWS Formosa that came ready built with a GWS brushed motor, brushed ESC, 650mA 9.6v NMHi battery and GWS servos/receiver and a cheap 4ch 27MHz TX. I'm ditching the receiver and TX to allow me to use my 36MHz JR set up (I intend to fly miles from my regular field at a reserve accross the road).

After hearing about the "gentle" performance of the Formosa with the std powerplant and short running time, I went to town and bought a 1800 3 cell LiPoly pack, automatic charger, new (brushless) 30A ESC and an outrunner brushless motor. The only problem was I got the wrong motor (probably much bigger then needed [sm=angel_smile.gif]) and it's slightly too big in diameter to fit the GWS stick mount gearbox.

The motor's specs are: 850rpm/v; 11.1v; current 9.2A; pull(?)550; rpm 5600; G/A(?) 59; Max output power 150W. Note - the specs with a question mark mean I have no idea what it means - can anyone enlighten me?

Anyway I was thinking of making a mount for the motor and use a "direct drive" (ie no gearbox). The motor came with nicely turned aluminium prop drivers for either the can or the shaft, so I presume it is meant for direct drive applications. Is this motor suitable for my Formosa in this configuration and if so what size prop should I be using?

Link to the motor is here.
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Old 02-09-2007, 08:46 AM
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Default RE: Prop size for brushless setup

Ton's of gobbly gook to filter through, not necessary though.

Start with the motor, get the mfg. specs on it and look for the middle of the road prop for general good performance, a pusher style configuration will want a smaller speed prop compared to a tractor which likes something with larger diameter and more radical of a twist.

Combine this with your power supply and plane properties and you'll get it dialed in. Having one prop you put less juice into, to keep it afloat and put into glider mode works for some parties and a smaller prop you have in full power, less resistance for some nice long powered flights for others and of course, dead on for maximum performance to keep it airborn to the best of the power system's ability. You'll likely mess around with 3 props for testing those, 3 varing sizes and pitches of those to take it furhter and at that point, the one you settle upon will be entirely due to your tastes. The science of this gig, software and phd cannot possibly encompase, you still gotta just get hands on about it all and hope you don't burn out your esc's if you are pushing their limits experiementing.
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Old 02-09-2007, 09:22 AM
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Default RE: Prop size for brushless setup

Thanks ....

The specs I gave are all I have. Its a Chinese Ebay special so documentation apart from the electrical specs are a bit scarce and I was probably lucky to get what I have. No mention of prop size or if I need to run a gearbox.

The Formosa is a conventional layout - looks a bit like how pattern ships used to about 10 years ago only smaller. Something I would call a .10 to .20 size if it were i/c. Maybe I'll start with a 9x6 on direct drive and see how many amps it pulls.

Ideally, should this motor be running at 9.2A at full throttle?
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Old 02-09-2007, 09:31 AM
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Default RE: Prop size for brushless setup

You will not need a gearbox with that motor. It's RPM rating is pretty low to begin with. Which is something you need to look at when selecting a prop. The bigger the prop the less RPM's you wind up with. And already starting relatively low, you are going to want to keep the RPM's up. I would not be surprised if a 9x6 pushes you over the 9.2 amp mark. You might want to try and look at a 9x4.5 or 8x5. But as you stated, you won't really know until you throw it on there and do some bench tests to see how many amps it is pulling.
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Old 02-09-2007, 09:42 AM
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Default RE: Prop size for brushless setup

That's great thanks fliprob17. I though the rpm seemed about right without needing a reduction drive - plus it has the prop drivers bundled with it.

I might just use the GWS props that came with the plane in that case and take some measurements before I bother with a trip to the model shop. They are about the sizes you suggested.

Cheers.
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Old 02-09-2007, 09:58 AM
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Default RE: Prop size for brushless setup

how much does the plane weigh rtf? If it's under 20 oz, the 9/6 is a good first choice, and in fact, find it to be a good all around prop for most birds if you just want to get them aloft without stressing the esc.
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Old 02-09-2007, 09:59 AM
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Default RE: Prop size for brushless setup

ORIGINAL: CaseySP

That's great thanks fliprob17. I though the rpm seemed about right without needing a reduction drive - plus it has the prop drivers bundled with it.

I might just use the GWS props that came with the plane in that case and take some measurements before I bother with a trip to the model shop. They are about the sizes you suggested.

Cheers.
That is probably the best bet on figuring this out.
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Old 02-10-2007, 07:07 AM
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Default RE: Prop size for brushless setup

Right - so I did a bit of testing (all figures at full power):

With the 8" GWS prop it was drawing around 5.2A.
With the 9" GWS prop it was drawing around 7.5A.
With a 9x4.5 APC prop it was drawing around 6.7A.

It seems the GWS 9" prop must have a pitch in excess of 5" - maybe it is a 9x6.

The speed controller and Lipoly pack were both warm to touch but not much above ambient after a full power run.

The BEC cut the power to the motor when the battery dropped to 9.10 volts. I would have thought 8.25v would be right for a 3 cell pack (ie 2.75v per cell)???

Saucerguy - the Formosa should weigh around 400g (just over 14 US oz) ready to fly. Haven't weighed mine though, but it will probably be slightly heavier with a more substantive motor mount instead of the stick and larger ESC.

Edit - I didn't even think of using my tacho until now. Will have to repeat the testing in the 'morrow and see what rpms each prop liberates.
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Old 02-10-2007, 08:39 AM
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Default RE: Prop size for brushless setup

GWS props usually hafe the size marked on them. The one on my E-Starter seems to be marked "9070", meaning it is 9X7. They are pretty soft, so sometimes they tend to flatten under load.

Sounds like an APC 9X6 Slow Flyer prop might work. You are still below 100W, which is better than the stock system but a little more should be nice.
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Old 02-12-2007, 12:00 PM
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Default RE: Prop size for brushless setup


ORIGINAL: CaseySP

Right - so I did a bit of testing (all figures at full power):

With the 8" GWS prop it was drawing around 5.2A.
With the 9" GWS prop it was drawing around 7.5A.
With a 9x4.5 APC prop it was drawing around 6.7A.

It seems the GWS 9" prop must have a pitch in excess of 5" - maybe it is a 9x6.

The speed controller and Lipoly pack were both warm to touch but not much above ambient after a full power run.

The BEC cut the power to the motor when the battery dropped to 9.10 volts. I would have thought 8.25v would be right for a 3 cell pack (ie 2.75v per cell)???

Saucerguy - the Formosa should weigh around 400g (just over 14 US oz) ready to fly. Haven't weighed mine though, but it will probably be slightly heavier with a more substantive motor mount instead of the stick and larger ESC.

Edit - I didn't even think of using my tacho until now. Will have to repeat the testing in the 'morrow and see what rpms each prop liberates.
WOW!! It stayed much lower. So the 9x6 looks good after seeing those numbers. The low voltage cutoff should be at 3.0 volts per cell, so the 9.10 volts was probably dead on. Once you stop the motor, the pack bounces back up a little to it's resting voltage.
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