Any brushless experts out there? - RC Groups
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Nov 25, 2002, 07:59 PM
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Any brushless experts out there?

It's no secret the Hacker 15L gives great hover performance for a 16-19 ounce plane. I have a Mega 7 turn motor that I want to get the same performance from. I have a Miniflash with a 9x6 prop pulls about 13 amps off the charger. The Miniflash is suppose to be a slow plane but with this prop it moves!

Now the 7 turn has a KV of 1300 RPM the Hacker has a 3700 KV and with a 4:1 gearbox it gets down to 7400 rpm with 8 cells. I'm going to disregard the wind loading for the time being. My 7 turn Mega with 8 cells generates 10400 rpm's. I propose going to 6 cells on the Mega and turning a bigger prop. I believe the Mega with the extra poles and bigger size will create more torque (thrust) than the Hacker if it didn't have the gearbox. By law higher pole motors create more torque than low pole motors i.e. 6 pole motors vs 2 pole motors.

6 cells on my Mega 7 turn will give me 7800 rpm more in line with the Hacker. I ran the numbers through P-calc, yea I know it's not very good, and the answers didn't make any sense. According to P-calc just changing from a 9x6 to a 10 x 4.7 will give less static thrust on 8 cells. That doesn't make any sense.

I know some of you guys have made thrust scales. Anybody done any tests like this?

Anybody have an opinion?

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Nov 25, 2002, 09:18 PM
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Expert? Nope, not here. But I ran a quick comparison on Motocalc. I compared the B20-15L geared 4:1 with the Mega 16/15/7 direct drive, 8x600AE cells, 10x4.7 prop. I got;

15L, 10.6 amps, 92 watts input, 85.2% motor efficiency, 7346 rpm, 22.0 ounces thrust.

M7, 14.3 amps, 119 watts input, 64.9% efficiency, 7323 rpm, 21.9 ounces thrust.

RPM and thrust/pitchspeed are nearly the same, but the higher kv and gearing of the Hacker are much more efficient, doing the same job at lower amps. Of course, these numbers are all off somewhat, and dependent on prop constants, etc., but are OK for a rough baseline comparison. Not as efficient as the Hacker, but still, none too shabby for a $69 direct drive motor!

Nov 25, 2002, 10:11 PM
Registered User
Thanks Marcus,

That's amazing the Hacker loses only 50 rpm due to wind loading. 3700x8=29600/4 = 7400rpm theoretical, 7346 Motorcalc.
But the Mega loses 3100 rpm, wow.

1300 x 8=10400 rpm; Motorcalc = 7323 rpm

What would 7 cells and a bigger prop due to static thrust?

Nov 25, 2002, 10:54 PM
Registered User
Originally posted by Parkflyer
What would 7 cells and a bigger prop due to static thrust?Grant
Well, the same M7 setup as above, but drop the battery to 7 cells, and increase the prop to an 11x4.7, gets you;

14.2 amps, 104 input watts, 61.1% motor efficiency, 6030 rpm, 19.1 ounces thrust. Still a good bit of power for the size of the system, but efficiency is dropping off rapidly. I think the 7t is at its best on 8 cells and a 9 inch prop. Of course, you could paint the Mega motor purple, and see if that helps any!
Nov 26, 2002, 10:13 AM
Registered User

Well maybe the 7 cell thing wouldn't work. Almost 3 ounces less thrust and only lose about .5 ounce in weight.

I think I'll try the 10 x 4.7 even though the efficiency looks pretty bad. That's one reason I went brushless to improve the efficiency. That efficiency is like a brushed motor.

It looks like the Mega is best suited for pattern, aerobatic planes and not the slow 3D type.


Your pretty familiar with the Megas, any thoughts?


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