I am running into a very strange issue and my only thought right now is that placing an autopilot below center of gravity versus above may make a difference. Could anyone comment on this?
In the past all of my single copters had autopilot placed above CG. This time I made a single copter with a very heavy battery placed high, so the autopilot ended up below center of gravity. No matter what I do, what the PID settings are I cannot eliminate huge oscillations and immediate crashes right after taking off (in Loiter, Alt-Hold or Stabilize).
What’s strange is that on a PID tunning stand it works fine. Just like my previous single copters.
The stand has a horizontal shaft that goes through the center of gravity of the copter. I push the top of the copter by hand or I apply flaps to get it off balance and it returns to the upright position right away without any oscillation. With some PID settings it may swing a bit past upright once, but that’s it. No oscillations whatsoever.
Unfortunately the moment it takes off, it starts swinging like crazy and quickly crashes. So far I cannot find any other explanation except that placing an autopilot below or above CG may make a huge difference. Please note - this time payload (battery) is really heavy.
One log below:
Single Copter Crash - small PIDs