ORIGINAL: cwharper
What I've been doing is to limit the current to 35 amps by setting the upper throttle endpoint lower than 100% so that I won't accidently over throttle it during flight. I've been very pleased with this setup, but I don't know if this is the most efficient combination.
I believe that electric motors don't work this way. You haven't limited the current to any specific amps by just limiting your TX. You have reduced what the TX will tell the RX/ESC, but electric motors simply draw according to their load. Load that model beyond the prop/motor's ability and that motor is still going to draw what amps it can get. And if the pack can supply more than 35amp, it will supply it and the motor will get it. Now, I'm not much of an Electric guru, but I help a guy fly his electrics and we have burned up one motor and ruined a couple of batteries flying around at half throttle. The guy simply doesn't want to pay for a wattmeter and has little idea what his prop/motor combinations are going to be pulling, and tries all manner of props.
I've heard to put the largest diameter prop the plane can handle considering ground clearance for the best efficiency but what about pitch?
Some things we hear are true. Some are not. The idea that efficiency is the function of diameter is not entirely true. Actually, it's not very true. OK, it's basically wrong. It leaves out so many things that affect efficiency that it's really just hot air. If you heard that from a good source, then I'd go ask them to go over the idea with you again, because you missed something he said. Honest, sound bytes like
"largest diameter prop the plane can handle considering ground clearance for the best efficiency" are way too simple to have a hope of being true.
If I change from a 13.6.5E to a 13x8E will the decrease in RPM while maintaning 35 amps be more efficient without loss of thrust for my lazy high wing floater? I don't fly this thing at full throttle all the time. All it takes is about half throttle to keep it flying comfortably.
With an electric, about the only way to answer that question safely is with a watt meter. And remember, there really is nothing that "maintains 35amps" in that power system.
Your problem is really a lot more of an electric model power system problem than it is an aerodynamics or prop efficiency problem. The electrics forums should provide you with lots better help.