I have followed a few recent discussions on the optimal motor selection (which left quite a few questions in my mind), and faced with a need to design an electric system for next year, I decided to perform an instrumented comparison test (i.e.: where multiple parameters are measured, recorded and analyzed. No guessing or speculation, or assumptions).
NOTE 1: it is NOT my purpose to start a heated discussion on the validity of my test methodology, my conclusions from it, or how my results correlate with your own choices of a power system. I am sharing the results of these tests here for the benefit of those that are at the same point in the learning curve as I am, and to save others time and energy in seeking the same answers.
NOTE 2: My focus was on motor selection. I will address ESC and battery selection very briefly, as it was not my focus.
NOTE 3: This is not intended as a complete tutorial on control line electric systems. There are several threads out there, and other resources. If you are not familiar with certain terns, such as Kv or mAh, seek info elsewhere.
NOTE 4: No animals were hurt in the course of these experiments.
The purpose of the test was to select the most optimal motor for a conventional .60-size model (~680 in^2 wing, ~62 oz weight, ~ 68 ft lines, ~5.1" lap time). There are several considerations that go into my definition of "optimal", roughly in the order of importance:
1) Energy efficiency (i.e. minimal losses when converting electricity into mechanical energy). Because this determines how big a battery my plane needs to haul, and batteries are heavy!
2) Motor's own weight and weight distribution. Because why haul unnecessary weight? Also, larger diameter motors will create more gyro effect, which is undesirable, so I would prefer "slimmer" motors all else being equal (i.e smaller outer diameter).
3) Construction and materials quality (esp. bearings). Because I want the motors to run reliably and require minimal maintenance or rebuild intervals.
4) Price - because it matters! (To most of us).
The questions I was particularly curious about:
1) Are bigger, heavier motors that much more efficient that carrying an extra 1-2 oz in the motor saves me more than that in the battery weight?
2) Are more expensive motors more efficient than the cheaper motors of the same weight to make appreciable difference?
3) Side test: is a 6S setup more energy efficient than a 5S setup, due to lower average current?
Methodology:
- First, I determined the approximate propeller size that I want to be using. Something like APC 12x6 EP.
- The propeller pitch and RPM determine airplane speed. I'm guessing RPM in the 9000-9500 range.
- The target RPM determined that I'm looking for motors in the 650-780 kV range. This narrowed the choice of motors. I had a few, and borrowed a few.
- Based on flight data (Castle Edge Lite 50 data log) on a Top Flite Score (680 in^2, 64 oz), I figured approximate average power required ~ 460 W. Note this is averaged over the entire flight, and during actual gflight will vary up and down by as much as +/-80%, but both the motor and ESC can handle short spikes. They can't handle sustained overload, however.
- Based on the 460W power level, I determined that an APC 10x6E prop at 10400 RPM on a test stand represents an equivalent load to a APC 12x6E in flight. So I used it as a motor load in all my tests.
- All other variables were controlled for: same atmospheric conditions (room temp 25C, humidity, altitude, etc), same ESC (Castle Edge Lite 75), same timer (FM-9), same battery (Zippy Compact 2700 5S), same RPM setting. *(Note: the Plettenberg Orbit did not want to run in a Governed mode, so I had to use Set RPM mode. As a result, the actual RPM were about 100 less than on other motors).
- Each motor was mounted on the test stand, fresh battery used every time; data was recoded for a 3-min run and downloaded from the ESC through Castle link. I chose 3 min to be easy on the battery and save test time. Later I extrapolate the battery consumption to the full 5'30" flight time.
- I also checked motor case temperature after each run using laser thermometer. It gave me an idea of how much energy got converted to heat (waste) instead of mechanical energy (RPM), and how thermally stressed the motor was.
The results are attached. Seven motors were tested, ranging in weight from 6 oz to almost 8.5 oz, including some popular choices. (Look at the 7 motors with actual test data. Other motors's stats were entered for comparison purposes).
Conclusions:
- The larger, heavier motors were indeed more efficient... but... not enough to matter. Why? Because, even though they required 100-200 mAh less of a battery (5S), I can't take advantage of that, because I can't find 2500 mAh 5S batteries on the market! I can find 2700-2800 mAh 5S packs, but that covers all motors in the 6-8.5 oz range! And since I can't cut off a piece of battery to make it 100 mAh smaller and 1 oz lighter, I can't take advantage of this difference. As a result, I vote for the 6 oz motors.
- The more expensive, "elite" brand motors like Plettenberg, Scoprion and AXi were slightly more efficient than, say, the 6-oz Turnigy, which got hotter and would need every bit of 2700 mAh to complete a pattern on a normal day. This does mean, though, that if I need a spare 200 mAh in the pack for a faster flight on a windy day, a better motor will allow me that, while a Turnigy does not leave that margin, requiring to step up to a bigger heavier battery to leave some spare "fuel in the tank". Vote for the more expensive motors.
- I also ran a test of 5S vs. 6S setup using the Cobra 2826-12 motor (everything else same - prop, ESC, etc). Strangely, the 5S setup consumed 21.46 Watt*hr for the 3 min run, while the 6S setup consumed 23.3 Watt*hr. This is contrary to what I expected. The only explanation I have is that due to higher voltage and fixed given Kv rating of the motor, the ESC had to throttle down so much that the overall setup became less efficient. Since this was only one test and not the main focus of the experiment, I would not give this too much weight, other than to conclude that there was not enough convincing evidence for me to switch to 6S at this point. There are very few 6S 2200 mAh batteries on the market to choose from, and they are not any lighter than the 5S 2700 mAh batteries. No tangible gain to be had here.
- This concludes this series of experiments. I've narrowed my choice down to either Cobra 2826-12 or AXi 2826/13 with a 5S 2700 mAh battery.
- Other areas to study would be: propeller efficiency (what are the best props on the market for electrics) - my preliminary take is that APC is great for starters, but not good enough for Experts; ESC efficiency (is 75A ESC an overkill for the 23A average current?) - I think so; battery choice (is ThunderPower worth the price, or is Turnigy / Zippy a great deal for the price - my internal resistance measurements show that Turnigy and Zippy have lower R than more expensive brands).