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Old 03-23-2020, 11:41 AM   #1
peterel
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Picking a motor


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Ok I'm stuck in the house, so I'm going to try my first foam board airplane. Down loaded some plans printed it out and started building. Got the plane almost finished, as I don't know sh*t about electric motors the plans call for a BL2515-09 920kv Emax motor with a 9 x 7 prop. Now I find out they don't make that motor any more. Can some help me with picking out a replacement motor? Planes a low wing 60 inch wing span about 1180 to 1400 grams ( that's what the plans say) I don't fly very fast I just like to cruse around.
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Old 03-23-2020, 03:23 PM   #2
michaely
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Re: Picking a motor

Emax BL2515-09 920kv motor?

Google search doesn't shows it and Ecalc, my go to electric flight predictor, doesn't either, but it shows an Emax BL2815-09 920kv in its library: 4oz/112grams, max watts of 391, which would match a 1400gram/60" wing span model.

Do provide as much more information on the model as you can: name/link to the plan, wing area; would be nice to know how the motor is mounted, etc.

The 9" diameter, 7" pitch prop is a bit unusual for a leisurely flying plane; typically with such a motor on a 3cell lipo I might expect a larger diameter and slighter lower pitch prop...… so more information on what you building would help clarify this
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Old 03-23-2020, 05:15 PM   #3
peterel
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Re: Picking a motor

Sorry BL2815-09

E-Max says to use a GT2815-07 motor with a 11 x 5 or 11 x 6 prop with a 40 amp esc running on 3s batteries, sound about right?

Last edited by peterel; 03-23-2020 at 05:35 PM. Reason: more info
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Old 03-23-2020, 06:49 PM   #4
michaely
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Re: Picking a motor

Quote:
Originally Posted by peterel View Post
Sorry BL2815-09

E-Max says to use a GT2815-07 motor with a 11 x 5 or 11 x 6 prop with a 40 amp esc running on 3s batteries, sound about right?
Yeah.... without knowing more about your model.....and assuming wing area of 600sqin (a 10" chord for 60" wing span is a 'reasonable' guess), the GT2815-07 (weighs 120gr., max watts of 422), picking a relatively small 2500mah 3s, 40amp esc, 11x5 prop...
Ecalc predicts a thrust of 59.7 oz, pitch speed of 46mph, drive wt. of 14oz, nice light wing loading of 11.8oz/sqft and a cubic wing loading of 5.8 (sport and some trainer category, according electric newsletter Ampeer), lots of power (121watts/lb, 1.22:1 thrust to weight ratio) and a mixed throttle flight time of 7 minutes.
You could go with a larger battery, extending flight time without affecting wing loading too badly.

Going to a 11x6 prop would push a 40amp esc to very close to its limit and you said you weren't looking for speed so I would stay with the 11x5

There are lots of other motors, with the same or differing kv and same/different prop size that would no doubt work. And leisurely flight performance doesn't need a 1.22:1 thrust to weight ratio typically so a smaller motor could very well also be fine. Depends on the full details of your plane/flight expectations.

"Scirocco" over at rcgroups provides a description for good method for choosing the right motor every so often in his answers to "what motor should I use?" queries and 'ron van sommeren' provides some concise key relationships between prop size, kv, number of battery cells.
And a subscription to something like Ecalc allows youto play with these factors so its an easy way to educate yourself, know what flight performance to expect, when you are overloading the motor or esc ….. as well as save money by making better buying decisions.

good luck
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Old 03-24-2020, 01:19 PM   #5
peterel
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Re: Picking a motor

Wow just doing some price checks. Are Eflite motors that much better than Emax? Just doing some checks on Banggood and Hobbyking and our big 2 Canadian guys.
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Old 03-25-2020, 08:51 PM   #6
michaely
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Re: Picking a motor

Quote:
Originally Posted by peterel View Post
Wow just doing some price checks. Are Eflite motors that much better than Emax? Just doing some checks on Banggood and Hobbyking and our big 2 Canadian guys.
I would get Emax GT2815-07.

E-flite motors carry price premium because of marketing: Horizon Hobby, the big supplier of things electric in North America, only sells Eflite motors for airplanes (I think from a glance at their website).
A new/retail bought Emax will likely be a bit less than half the price of an E-flite BL10.

And, according to Ecalc, the Emax Gt2815-07 is more efficient than the E-flite BL10.
With the prop/3s batteries you are using, Emax's efficiency is higher end of the 80%s while the E-flite is in the lower half of the 80%s, resulting in marginally more watts of output power (less input power consumed in heat, etc.) and a bit more thrust.
Not saying that E-flite BL10 wouldn't work fine for you (or that other E-flite motors are not as efficient as other brands)

I sent you a private message.
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