I am fairly new to this fascinating hobby and looking for some help with ALT_HOLD. I have a QAV500 style quad, 850 motors, 1045's props, FrSky TRF8, and MB1200XL sonar. Nothing fancy. No GPS, video, telemetry, or other RF equipment.
Sometimes ALT-HOLD works great, usually on my first few minutes of flying for the day. Then after some flight time the quad decides to change it's mind and either slowly descend and land, or endlessly rise until in desperation I put it back in STAB mode.
My first thoughts were that the sonar is reporting bad data, but the logs would indicate otherwise. Here is a typical log file which leaves me with more questions than answers.
Blue = Throttle Out
White = Throttle In
You can see where I initially switched from STAB to ALT_HOLD at line #1500. From here all the way to line #4900 the quad 'more or less' maintained altitude. Not nearly as tight as I would expect, but reasonable. I was just hovering in the yard, not doing anything crazy, testing this feature. At line #4900the quad started to decent. At line #5100 I cranked up the throttle (still in ALT_HOLD) in an attempt to keep it aloft. At line #5200 it simply plopped itself on the ground.
What's going on? What am I doing wrong???
What should be the effect of the throttle stick when in ALT-HOLD mode? Should I be able to make manual adjustments to the "held" altitude?
Complete log attached.