I've recently started using LibrePilot, and have extensive previous experience using and testing PX4, and APM, and wanted to bring up an issue I've found with the PID control in LibrePilot.
Believe that for many situations, the "I" term in PID is not necessary, but have been unable to zero it without getting undefined behavior from the multirotor. In PID theory, if I set the I-term to zero, I should have a PD loop that will always go to level position. What actually happens if I set the I term to zero, is the copter will fly level for about 15 seconds, then it will roll/pitch in a random direction, as if it lost it's level/trim, and this will persist until switched back to a flight mode with an I-term.
Also the PID in the new release feels much nicer than 15.9, if this issue is fixed, then it'll be the best of all the Open Source FC's that I've tried!
Looking forward to reading your replies.
Have tested this and gotten same results on Revo, and CC3D