RE: Motor - Prop Selection ???
An update here… Have learned TON's since last postings, and I noticed I never posted the results from the tests on the motor selection.
First, the Maxi 09 from [link=http://www.rc-dymond.com/order_motor_brushless.htm]Dymond Modelsports[/link] was not the best selection for 3D performance, but it does perform very well. We started with an APC 9x6, 3S 2100 12C Lipoly, and 25A ESC. This setup drew 26 Amps WOT, and we decided it was overloading a bit too much. Backed it down to a 10x5E, and now pulls 21 Amps. Performance was very good, but since then we have learned a few more things... hope this helps others to get past all the mud and fog to come to some level of understanding on this stuff.
Anyone, please chime in if you find error in my understanding.
Propellers:
Slow Fly Props... The APC "SF" props (GWS, etc) are VERY different from the "E" series props. The SF props have very broad blades all the way to the end and skimpy connections to the hub in the middle. The E props are more like the big props for nitro planes, where they taper to the tip and have thick healthy sections at the center. SF props are for “slow fly” planes, where 3D is probably the primary flying mode and lots of prop wash is desirable over the control surfaces. They are designed to carry small loads and turn relatively low speeds (generally well below 10,000 rpm). Slow-fly props are apparently not very efficient compared to E props (power in to power out ratio).
E props are designed for high speed, and better efficiency. They generally are smaller in diameter and applications tend to best use them turning really fast for sport/speed flying. They are a poor choice for 3D applications. They don’t produce a big wide blast of air. The faster they turn, the more efficient they become, until the blade tips approach supersonic velocity.
Motor Selection:
The Maxi 09 motor we selected is a really strong little workhorse. But, we learned something about the “Kv Rating” I kept asking about. Kv translates to RPM’s per Volt (RPM/V). So, that means that 1200 Kv motor, running at say 10.2 volts, will be turning at 1200 x 10.2 = 12,240 rpm. Other definitions need to be clear to understand all this…
Brushed Motor: A conventional motor, where brushes ride on a commutator on the moving stator with windings and stationary permanent magnets on the outer shell. Typically runs on conventional single phase DC voltage.
Brushless Motor: A unique adaptation of giant industrial electric motors, where there are no brushes. The motor runs on three-phase AC power. There are 3 leads to these motors, and the Electronic Speed Controller (ESC) digitally produces this variable low-voltage 3-phase AC power from a DC battery power source. These motors run at far greater efficiency than brushed motors.
InRunner: A motor, brushed or brushless, running thru a gear reduction assembly to reduce RPM and increase torque. Allows use of larger diameter props with greater pitch turning slower to provide better prop-wash over control surfaces. Also provides the opportunity to alter gear ratio.
OutRunner: A “direct-drive” (typically) brushless motor design, where the prop is mounted directly on the motor shaft and the magnets turn about a stationary stator. Outrunners have typically far lower Kv ratings than Inrunners.
So, for a typical “OutRunner” setup, you look to match the Kv rating to the type of aircraft or flying style. A lower Kv means greater torque and slower rpm to turn big fat low-pitch props for lots of air movement to wash over control surfaces, obviously that’s for 3D flying. A high Kv is selected to turn smaller high-pitch props fast for high-speed flight, where you rarely transition from sub-stall speed flight to WOT high-speed flight.
For an InRunner setup, the same applies, except you use a higher Kv motor, and use the gear reduction to reduce the relatively high rpm to the desired revs for the application. The downside to InRunners is the inherent loss of efficiency with the extra bearings and gear friction. The upside, as Matt explained, is the wider prop selections you have available by changing the gear ratio. The InRunner also allows use of relatively inefficient and very inexpensive brushed motors. The biggest benefit to the InRunner in my mind is that if you choose the wrong Kv motor, you can correct it to an ideal setup with gear ratios.
Continued…