• **
  • 66
Output mixing for coaxial props?
« on: October 20, 2016, 11:51:23 pm »

Question background:
I'm using a CC3D controller to play around with optimizing coaxial propeller efficiency, currently on a test bench.  I'm using a higher Kv motor with a somewhat smaller-diameter & higher-pitch prop for prop-2 since this is supposed to get the best thrust per power efficiency results.  If you're interested I can go into details, but I'm basing these experiments on this http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550.pdf So far my bench testing generally confirms these findings.

I built a test stand specifically to be able to see the operating torque balance of motor-1 to motor-2 (since this is generally the most efficient situation).

Right now I'm trying to find balanced torque throttle settings at a given motor-1 throttle setting by adjusting throttle on motor-2 until the stand shows balance and noting the value.  The idea here is to find a throttle-1 to throttle-2 bias curve that generally delivers balanced torque at any throttle-1 setting.

I was hoping, at least to start, to be able to just set max-output of motor-2 in Configuration / Output / Max, but it's not behaving like I expect.  (I'm setting max on motor-2 because it's a higher-Kv motor, thus it runs faster at a given PWM pulse rate).  Motor1 is set at max=1900 and Motor2=1500.  When I enable 'Test outputs' and set Link on the motors, the sliders behave the opposite of what I was expecting.

I was hoping to see Motor2 ramp up proportionally to Motor1, so Motor2 would be at 1500 when Motor1 reaches 1900 @100% throttle.  But instead I'm seeing Motor1=1500 @~50% throttle while Motor2 is at 1500 @100% throttle and of course the sliders won't exceed 100%.

I'm confused.  It looks like some sort of min/max clamping & scaling is going on internally.  Is there any way to get the behavior I'm looking for?

Side question:
I was hoping to combine max-pulserate (above) with my coaxial test stand torque balance results and feed that into a custom throttle curve in the vehicle configuration in the hopes of achieving higher coaxial efficiency.  Is this a good or bad idea?

Thanks in advance for your any suggestions or help you guys can provide!

Re: Output mixing for coaxial props?
« Reply #1 on: November 10, 2016, 12:55:09 am »
As I recall the sliders in the GCS do this, that is ... match absolute numbers rather than relative positions on the sliders.

I wonder if the flight firmware does the same thing.  You could try controlling them with the transmitter stick and see if they behave more as desired.  You would probably have to configure something against the rules like manual control and then "disable sanity checks" might do it.

Also be aware that the mixer stuff is something you could look into.  Each output channel is a mixture of factors times input channels.  All quad motors have 127 times the throttle as one part of their mixture.  You could set your high kv motors to 63 to try to cut them in half.  You should probably do this on the System page in MixerSettings rather than using the GUI, or you could tell the GUI after setting it up as a quad or whatever that you now want to change it to Custom, at which point the last page will allow you to change the mixer settings directly.  Be warned that there is some stuff in the flight code (I forget what / where) that looks at the aircraft type (e.g. quad) so there may be some consequences?


  • **
  • 66
Re: Output mixing for coaxial props?
« Reply #2 on: November 10, 2016, 02:10:23 am »
Thanks for responding, Cliff.  I appreciate the help & will update this thread if I have further questions.