I fly my F450 with APM 2.6 AC3.2. This month I had 2 similar incidents where the quad just fell during mid flight in Position Hold mode. The motors slowed down for no reason. First time I had a crash with broken props and GPS mast. The second time I have raised throttle to max during the fall and it recovered. It was slightly windy during the time of flight. Below are the specs of the quad and attached are the logs. Could anyone figure it out what might be the cause. I flew the same quad number of times without any issues.
Frame: F450
Motors, ESC, Props: 930KV 2212, DYS 30A SimonK with 10x4.5 props
Battery: Zippy 3S 2200mAh
FC: APM 2.6 AC3.2 firmware, Ublox 6 GPS Compass
Replies
Since you have no replies so far, I will see if I can help. There is undoubtedly better advice available, but I dont see it replying yet.
I have been using ardupilot about 8-10 months. I have tuned 2 helis, 2 airplanes, and 2 quads. On the helis and quads, I have had to tune altitude hold. I have seen symptoms where they do not hold well, I believe early on I had some drop out of the sky....you need to have your finger on the switch assigned to go back to stab when you test alt hold.
My belief is you should increase the P and the I term in alt hold. I believe that is called throttle velocity ....without hooking one up to see....also there is a gain term, may be called P, just for alt hold on you tuning screen. So do searches for how to tune alt hold, I think that is all you prob need. I suspect it is marginal, and looses lock and drops. I would just increase the terms associated with alt hold and throttle. I think the I term will help in particular. I may have seen advice to keep the ratio of P and I constant on the throttle.
There are procedures for examining telemetry to determine if you tuning is optimum, you can google those or check the "manual". Or just be ready for adverse alt hold performance as you turn it on.....I have had helicopters oscillate up and down rapidly, and I only allow that to happen for a brief second with my finger on the switch. (That is a symptom of instability caused by terms being too high. If it seems twitchy as you change altitude or otherwise, it may be a little high. I think yours is a little low. It might benefit from 50-100% increase in those terms, possibly less, but you dont want it to ever loose lock, so use as much as you can without adverse results. Fly high!)
Hope this helps. Jim.
Thanks for the reply. Initially I had issues with Alt hold mode due to excessive vibrations, incorrect Thr_Mid settings but everything is resolved and I have been flying the same quad for several months without any problem. Default Alt Hold PID worked for me & holding the altitude well (not nearer the surface due to ground effect). This is the first time the quad crashed due these strange reasons (Baro & GPS error). I don't understand what exactly happened. If GPS signal is lost then the quad should land (this is set in MP).