OK. What!?!
Vortex Ring State!
It's the helicopter variant of a stalled wing.
I had not heard of it yet, when I fist encountered the effect. It comes down to this: if you descend vertically, and you do that relatively fast, you get into the downwash of you own propellers. That will cause your props to loose lift, and you descend even faster. Your natural reaction would be to throttle up, because you are now descending more rapidly than you planned, but that makes the downwash - and therewith your predicament - only worse! The only thing you can do is pitch or roll to move out of that vertical column of air, not necessarily with more throttle. Only: you have to think of that, and act on that, before you hit the ground... I didn't.
It caused my quad to "land" on one arm, and that was a touch too much...
So now I had to go shopping for a new frame arm. I decided that buying a new frame would be a better option, because it is not that much more expensive, and I could immediately go for a slightly larger frame, which could hold a camera and gimbal underneeth. The current frame would be just a touch too low, and the legs proved to be slightly under-equiped to deal with moderately traversing landings, so to say (which means I broke a couple because I cannot properly land straight).
I decided to go for an S-500 frame, also because the arms tilt up 5 degrees. That does two things: it reduces the chance of Vortex Ring State, because the props don't "blow" exactly vertically downward, and it provides extra stability in flight (look up "dihedral", if you want to know more)
Again, I ordered mine in China, so had a couple of weeks to mentally prepare the re-build...
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