On occasion when hovering I find my quad descends all the way to ground without being commanded to do so. Nothing spectacular, it just sinks until it touches the ground then sometimes comes back up a bit. Increasing the throttle doesn't necessarily stop it. This also occurs in Alt-Hold and Loiter modes.
I've had this issue with my APM 1 and now APM 2 so I figure the issue may lie with my motors/props. I'm using 1050kv motors with 10x6 props to lift around 1.5kg. These are driven by 18A plush ESCs. Does this sound like it should be adequate?
Apologies for bringing up this old thread but I thought I should update it with my latest findings.
I took the plunge and replaced the motors with the 850kv ones from the DIYdrones store. This didn't make much of a difference.
However, I then replaced the 10x6 props with 10x4.5 and suddenly the quad is like a completely different beast. No more unrecoverable sinking!
I'm an electrical guy, so I don't really understand the significance of what I changed. Perhaps someone with more knowledge in this field could fill me in.
Does this sinking happen after the batteries are getting depleted? I have found that moter thust at a given throttle setting is directly proportional to battery voltage. Your batery voltage could be sagging quicker than the altitude controler can compensate.
I was having this problem. I was using 25C 2700mah batteries and they just weren't strong enough (and my copter was only ~1200g).
I went up to 30C 3400Mah battery and 30a ESC's. Now I'm flying with my throttle end point at 60%, and it flies like a dream. No more sinking to the ground.
You don't mention what software your using.
I am finding this happens with v2.7.1, amongst other things.
There is definitely a bug with the Alt hold.
Do you have the wobbles as well?