Paul Bradshaw’s - “E” Power Set-ups That Work

E-Flite the Horizon Hobbies house brand did themselves a big favor when they started labeling their electric motors with the good old glow engine sizes, most all of us can get a good idea of the potential that way.  E-Flite also does a nice job on the actual box of  the motor showing the suggested max amp load, suggested voltage or lipo cell count (S) and even a suggested propeller.  Because of my altitude (6,000’) I find myself usually going about one inch bigger in diameter on the propellers

Once we’ve looked at the E-Flite offering we find that the 110 size should do the job nicely, and it will, but who can resist all of that extra horsepower on tap with the 160, not me!  If my memory serves me right the 160 is listed at about 2500 watts and sustained loads of about 70 amps with a 10S lipo pack.  If we do the math a 10S lipo pack under load is good for about 3.5 volts per cell or 10 X 3.5 = 35 volts X 70 amps = 2450 watts.  If we divide the 2450 X 10 for the weight of the plane we have almost 250 watts per pound.  That’s almost like flying your 40 size Ugly Stik with a O.S. 120.  We should back it down a little to create that headroom we talked about.

I’ve decided that I’m going fly this plane with a 9S lipo pack because I have a lot of 3S packs that I can connect in series to get the 9S voltage that I need, and it creates the safety margin I like.  Additionally if I reduce the voltage a little I can run a bigger prop which is much more efficient especially at my altitudes.  The specs on the box suggested a 20 X 10 APC E prop on 10S, but for a 9S set-up and the altitude I’m going to start with a 22 X 10.  This should put us in the 65 amp territory while static on the bench. Keep in mind that when the plane is flying depending on the drag of the airframe, it could unload as much as 20% or more, dropping the full throttle amp load down to about 50-55 amps.  The only time you should see the high bench amp draw again is pulling out of a hover or other high-alpha maneuver, and that shouldn’t be for more than a few seconds.

The last important decision is the ESC, (electronic speed controller) based on what we think we know I want an ESC that can sustain about 75 amps with a 10S lipo pack, remember it’s that headroom thing again.  I like the Jeti Opto 77 because it’s small, light, and it’s never let me down.  Others could be the Castle HV 85, Hyperion 90, or even the Jeti 90.  Just make sure it’s a 10S controller for this application.

Last but not least is the battery pack size.  For some reason I’ve ended up with a number of 3200 mah 3S packs.  Some may think these are a bit small for some purposes but I really like the weight advantage over the more common 5000 mah packs.  The 3200’s have a 20C discharge rating so pulling 64-70 amps is no sweat and when you finish this set-up with the 3200’s it saves more than an entire pound of weight on a 10 pound plane.(9.2 vs. 10.5) It make s a difference.  As far as flying time I set my timer at about 7 minutes and I’m usually down by 8.  The 5000’s are good for about 10-12, but I have a short attention span of about five minutes so the 3200’s work for me.