I am building a large VTOL with Quad Motors and I have ran into issue of stability. So far I have built the central wing section on which the motors go and started flying it “as is” using ardupilot copter, Speedybee F405 with 4x60 A ESC. The motor frame is 100 cm wide and 78 cm long, with T-motors V-3115, kv 1050, whole thing powered by Liions, 4S. The flying weight I started with was 3.500 and I cranked it up to 6000 grams now. The max weight should be 9500 grams.
Now the problem:
I started with 12 x 8 props, fairly quickly I got the more or less correct PIDs, I had one issue which I was not able to solve well, which was the Yaw control (Yaw was constantly overshooting, and it took about 3 cycles and 6…8 seconds to stabilize after strong yaw input), but pitch and roll were pretty good.
I increased flying weight from 3.5 kg to 6 kg, changed from small Lipo to Liions, and still was getting pretty descent behaviour.
Then I changed to 15 x 4 props, and that is where it went sour: basically I was not able to get a stable hovering, the wing began progressive oscillations in pitch axis, usually it took 3 to 5 cycles before I had to cut power.
With 12 x 8 I was using PID something like 0.16, 0.16, 0.03 for roll and pitch.
With 15 x 4 to get hovering, I tried all the range of PIDs more or less with the same result, I tried the following:
0.27 0.27 0.04
0.16 0.16 0.04
0.12 0.12 0.02
0.07 0.07
0.05 0.05 0.001
0.04 0.04
0.02 0.02
Also, I even tried PI of 0.03 0.06
With higher PI values the oscillations were very quick to develop, but with low PI, they always appeared, the wing felt very mushy, but still began to oscillate…
I am not getting why changing from 12 x 8 to 15 x 4 resulted in such a drastic change of behaviour. I need bigger prop size and lower pitch for better hovering efficiency…
Does some one has an idea what maybe going on? I am at complete loss…