title_body: Why did a quadcopter in vertical descent appear to lose thrust and descend rapidly?
upvoted_answer: It sounds like you've encountered what's called "vortex ring state", when a rotorcraft descends into its own wake -- essentially, a self-created downdraft.
  
  There are two ways to escape from such a situation:
  
  
  Power out of it.  Climbing out of the craft's wake vortex requires about twice as much power as hovering, so while most full-sized helicopters can't do this, most quadcopters and other small-scale craft can.
  Move forwards (or backward, or sideways) out of the craft's wake.  This is the standard procedure for helicopters.
  
  
  You can also avoid it entirely by not descending too fast, or by not descending straight down.