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spacestout -> RE: Hobbico Electristar Select EP Trainer RTF (5/4/2007 5:50 PM)
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There could be a lot of reasons for 33 vs 35A. Inaccuracies in watt meters, differences in density altitude (airplanes don't care about the absolute altitude, they care about the air density presented to the prop and wings. Density is usually lower at high altitudes and is affected by temperature and humidity. When I fly my full scale plane I have to realize that the instruments can be in error and can tell a different story than absolute altitude and speed). The only way you'd know the reason for our difference for sure is to test them side by side with the same equipment. With that all said, I wouldn't be surprised if the battery types made a big difference. I'm no expert. I just started R/C in January and have most experience with LiPos. I am an electrical engineer so that helps some with the V and A. And I'm a private pilot so that helps and hurts.
I think the stock prop is great. I'd still have it on the plane if I hadn't broken it. I was pulling 28A with it (and that can vary depending on if your battery is peaked or not). It was the 11x7, so I tried that in an APC. What a surprise to find it only pulled 22A. Lesson number 1: All props are not created equal. Some of the calculation programs out there such as Aveox and Motocalc base their work on theoretical, but then test actual props to refine the model. I like to use such programs to get me in the ball park, but ultimately you have to experiment. The APC 12x6 gave me 28 amps and better (over stock) climb. The 13x6.5 APC has been the best. Full throttle on takeoff. Back off, set up for another manuever, then full throttle in the climb, then back off in descent. I land and all components are warm but not hot.
The bottom line is not burning out your components. Our problem has been that the specs for our motor have been hard to find. I noticed on my Great Planes Realflight simulator that the electristar specs said the motor was an electrifly Rimfire 42-50-600 Outrunner. 42mm diameter, 50mm length, and 600Kv (RPM per volt). I looked that motor up at http://electrifly.com/powersystem/motors/motors-rimfire-42-50.htm. There is a table with all the things you need to (such as internal resistance, no load current, etc) to enter into these calculation programs such as Aveox.com. It seems likely that our motor is this Rimfire motor. The simulator is made by Great Planes, who owns Electrifly, so it all ties together. My test data supports the specs as well.
So, our motor rating appears to be 35A continuous and 50A surge. That means with proper cooling you can run the motor at 35A, with occasional bursts up to 50A (then back off for cooling - throttle management). The ESC is rated at 45A continuous and 50A burst. So, drawing 35A with my prop at full throttle seems to be safe. Warm, not hot, components seem to bear this out. The Rimfire table shows using even larger props with this motor, all with current near 35A. Of course, it will all vary depending on battery, density altitude, humidity. Some day I may try a prop with more thrust as I get bored with trainer flight. I'll experiment on the ground to know the current draws at various throttle settings, then use full throttle for vertical manuevers and back off to safe settings.
A single blade prop is most efficient. It requires a counterweight. There are always tradeoffs in which parameter you are trying to maximize.
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