Electric Stunt > Gettin all AMP'ed up!
What is the right size motor
Ken Culbertson:
--- Quote from: Dennis Toth on November 14, 2020, 05:54:55 PM ---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
--- End quote ---
That will probably work for the profile but it seems a bit small for the PA which will be about 700 sq" and 65oz. Will the AXI take a 5s? The Cobra 2826 will. I have had luck with a 5s 3000 mah in a 3520/14. My new PA will be about the same as a MaxBee with a slightly larger wing. (my Avatar is the one I am going to build). What prop would you select to pull such a beast?
Ken
Dennis Toth:
Yes, the AXI will take 5S packs. You should trim the prop diameter for a ground amps of around 33 amps with the 4S pack should be good with a 3500 mah. For the larger ship go with the motor size (could be a different brand) recommended by Dennis.
Best, DennisT
ericrule:
Years ago I had a similar conversation with the folks who used to distribute Hacker motors. Larry Renger was also involved since he knows a whole lot about model airplanes. After listening to a lot of technical stuff Larry and I went for a coke to try to come up with something reasonable that non technical people like me could use in real life. We determined that in order to fly well we needed to have 100 watts of power for every 1 pound of weight. After a number of field tests we upped that to 150 watts of power for each pound of model weight to give us "contest level" power. That number seems to have worked very well.
A simple way of determining prop size and pitch is to use the prop chart Cobra has developed. That chart gives you a prop size and pitch that is very close to perfect. Of course it is only close but at least it gives you a good place to start. The only way to make sure is to flight test it.
Another simple table to use is motor KV to battery. 3 cell batteries need a motor kv of 1100 - 1300, 4 cells need kv of 850-950 while 5 cells need kv of 600 - 700. 3 and 4 cell motors use about 450 mAh per minute of flight while 5 cells use about 368 mAh assuming proper prop loading and leaving 25% battery capacity after the flight.
All of the above data are approximations. Like everything else in life theory dissolves when it comes head to head with reality. Get as close as possible then flight test and adjust.
Ken Culbertson:
--- Quote from: ericrule on November 17, 2020, 11:27:03 AM ---Years ago I had a similar conversation with the folks who used to distribute Hacker motors. Larry Renger was also involved since he knows a whole lot about model airplanes. After listening to a lot of technical stuff Larry and I went for a coke to try to come up with something reasonable that non technical people like me could use in real life. We determined that in order to fly well we needed to have 100 watts of power for every 1 pound of weight. After a number of field tests we upped that to 150 watts of power for each pound of model weight to give us "contest level" power. That number seems to have worked very well.
A simple way of determining prop size and pitch is to use the prop chart Cobra has developed. That chart gives you a prop size and pitch that is very close to perfect. Of course it is only close but at least it gives you a good place to start. The only way to make sure is to flight test it.
Another simple table to use is motor KV to battery. 3 cell batteries need a motor kv of 1100 - 1300, 4 cells need kv of 850-950 while 5 cells need kv of 600 - 700. 3 and 4 cell motors use about 450 mAh per minute of flight while 5 cells use about 368 mAh assuming proper prop loading and leaving 25% battery capacity after the flight.
All of the above data are approximations. Like everything else in life theory dissolves when it comes head to head with reality. Get as close as possible then flight test and adjust.
--- End quote ---
This was VERY helpful and exactly what I was looking for. The technical jargon puts me to sleep even if I do understand it!
Ken
GerryG:
My personal experience would support the numbers that Eric has put forward. These numbers (the Mah used) actually explain why my battery drainage is higher than I expected. Out of this I have a question. If I were to go to the flying field with 2 4S batteries and 2 5S batteries, can I use both types on the same plane (ignoring the slight weight difference) and assuming that the ESC and the motor can handle 5S? Or is there something else that I need to know?
Thanks
Gerry
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