I finally put in the 360kv motor, with a Phoenix EDGE 100 ESC and took datalogs on flights with both old (300kv) and new (360kv) motors, using the standard prop. Some observations:
1. Castle-link is pretty cool. You can download it and run in demo mode to see the graphing/logging capability.
2. The difference in speed was not overwhelming. Since I am used to this plane, I could tell the difference, but one observer at the field commented “It’s still slow.” This engine change alone is not going to make this plane into a 3D aerobat. Where I noticed the most difference was on vertical…it climbs better. But on a fast pass, since 360 vs 300 is only 20%, and since wind resistance increases exponentially with speed, a bit more power doesn’t really buy you a ton of speed. The difference in the “fast pass” was noticeable to me, but it is not a fundamentally different plane.
3. I didn’t attempt to fly exactly the same set of maneuvers on both flights, but flew similarly. As an average over the entire flight, I did burn more current by just over 20% more current – almost in line with the difference in kv. The battery was noticeably hotter too.
4. The motor sucks the most current at takeoff, by a large margin…much more current than in a steady climb. I believe this is because the prop has mass and momentum, and takes some time to ramp up to speed. While it is ramping up, it cannot instantly generate sufficient back-emf to match the input voltage. Therefore, the battery dumps in massive current. I saw a “takeoff spike” significantly higher in the 360kv motor (94A vs 55A), but it could be because I was only polling at 1Hz and didn’t really catch the peak in both logs. In any case, this was very brief and I don’t think a significant concern in sizing your ESC.
5. Apart from this takeoff spike, I found the power and current about – no surprise – 20% more. For example, if I look at the portions at full throttle, the power depends on what maneuver is happening at that moment. For the 300kv, I would see valleys (during ~30 degree dive), in the vicinity of 450-600 watts. I saw peaks (climb over the top of a loop) of about 950-1100 watts. For the 360kv, with similar maneuvers, the valleys were 700-850watts, and the peaks 1330-1380watts.
6. Temperatures with the Phoenix EDGE 100 were very low – I only got up to 99degF maximum with the 300Kv, and up to 113degF maximum with the 360kv. Heck, it gets hotter than that in the places where some of you live! The airflow in this P-51 design is not great, but the Phoenix EDGE 100 is large and has a large heat sink.
7. Outside of the takeoff spike, I didn’t see anything that would challenge an 85A ESC. The 300kv had a max in the range of 50-55Amps over the entire flight. The 360kv showed a max in the range of 70-75Amps over the entire flight. These maxima differ by more than 20%, but I am talking about the absolute max, which is an outlier. Looking at the entire histogram of currents at full throttle, it is shifted about 20% higher.
OK after that long winded explanation, some conclusions:
A. I think an 85A ESC might have trouble delivering all the required current at takeoff, but this is a very transient situation and unlikely to instantly overheat. Outside of this ramp-up transient, I didn’t see anything that would challenge an 85A ESC (assuming it really can provide 85A as advertised, and has the airflow to support that).
B. However, changing 300kv->360kv by itself is not going to fundamentally change this plane. It only becomes a “somewhat faster” slow plane.
C. Flight times will be about 20% lower, unless you alter your throttle management (which you should do).
D. I’m going to leave the 360kv in there, but I don’t think FMS “missed the mark” with the 300kv. It is a reasonable choice also for this model and this prop.
Yes, very interesting. The ESC statement's were well "qualified," which I like. That said, as a choice, I feel safer with an ESC that can handle the peak load. Especially since it's about 105 F where I am. I'm happy with the upgrade, and it helped me just barely rationalize away the notion of buying a gas warbird for the time being.
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