Electric Stunt > Gettin all AMP'ed up!
What is the right size motor
Ken Culbertson:
Getting ready to start replenishing my motor inventory having lost all of them in the fire. My collection came from word of mouth and trial and error, not from knowing what I was doing. This time around I want a stable that is properly fitted to the planes I will have. The only line I am familiar with is the Cobra. I had three 2820/12s, one 2826/12 and two 3520/14 in my inventory. All of these seem to be popular models but are they the right models. I am uncertain which characteristics I should be looking for. Every one of these motors is capable of powering a full size PA. The 2820/12 would need a huge 4s battery but it would pull the ship. The characteristic I look for first when flying a new motor is how fast it recovers when a load is applied followed next by how much battery it uses. We use "rule of thumb" figures of about 700-900 RPM@Volt which I assume is because of our rather low RPM requirements. What if you go higher or lower?
I know that everything in electric is related, even more so than IC and I don't want to get off on tangents, I just want to get in the right ballpark. Assuming I stay with Cobra (I am leaning towards a Bad Ass) then I plan for now to go with a 3520/14 - Castle 75 amp ESC for my PA and a 3515/18 - Castle 50 amp ESC for my profile. Timers are still an unknown.
If there is something already written that answers this point me to it - Ken
Dennis Adamisin:
Hi Ken
Start with the prop the airplane will need to fly on. Larger diameters are "thrusty" but smaller diameters fly better.
I usually pick a prop (lean toward smallish diameter) that flies the airplane then pick a motor that balances the airplane. I usually end up with oversize motors that way, but then I can try to minimize the battery. In a perfect universe this results in a light power system...
Current bird (720 squares, 67 oz) uses a Brodak 3520 with 4Sx3500 turning a 12x6EP. Also a Brodak F2B Hornet 60A & Hubin FM-9. It has WAY to much motor for that prop. Thus, have option (if I wanted to bother trying) for lighter motor and moving pack forward up to 2" to compensate...
Brodak site has a chart showing their products matched to props. Can use as a guide for "brand X" products too...
Dennis Toth:
I like Dennis's approach to sizing the motor by picking the prop. I like to use the smallest diameter prop that will do the job. This approach is not new but was used back when flaps first came out and the ships had shorter tail moments. Large diameter props slow the turn rate and also add lots of load (for e power that's amps). A small well designed prop can give lots of pulling power at a lower amp draw and allows a smaller battery pack.
Best, DennisT
Ken Culbertson:
--- Quote from: Dennis Toth on November 14, 2020, 09:41:53 AM ---I like Dennis's approach to sizing the motor by picking the prop. I like to use the smallest diameter prop that will do the job. This approach is not new but was used back when flaps first came out and the ships had shorter tail moments. Large diameter props slow the turn rate and also add lots of load (for e power that's amps). A small well designed prop can give lots of pulling power at a lower amp draw and allows a smaller battery pack.
Best, DennisT
--- End quote ---
I have been planning on an 11 x 5.5 three blade. Should I start there?
Ken
Dennis Toth:
Ken,
You might want to check the List Your Setup for a similar size ship. I would think that something around a AXI 2826 -10 at 920kV or similar would be a starting place on a 4S - 3500 - 3800 mah pack.
Best, DennisT
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