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Permalink Reply by Jan Schermer on March 20, 2012 at 12:01pm

You can use alt hold. Tune and try it where you can see it, and make a note how much throttle you need to descent at the wanted rate. The same goes for Loiter mode. You can set your transmitter to trigger the predefined throttle at a press of a button if you want. But for the sake of god do _not_ try to land while in alt hold :-) I learned the very hard and bloody way.

Btw, I also test stability of my setup by killing the throttle completely and then recovering after a while of it falling - good stress testing (both the copter and the pilot ;)). So far, it always recovered but you should NOT do that regularly.

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Permalink Reply by Brad Hughey on March 20, 2012 at 4:37pm

Descending vertically in a VTOL aircraft at too high a rate can induce a phenomenon called "vortex ring state".  Regular helicopter pilots call it "settling with power", and it's the unfortunate phenomenon of inducing a large doughnut-shaped airflow feedback loop around your rotor blades by falling into your own downwash.  If that weren't bad enough, the high pressure differential generated above and below the rotor (propeller) disk in a rapid vertical descent can reduce your relative inflow velocity to the point of causing blade section stalling.  And just like the fixed-wing example of a stall, the best solution is to pitch forward (lower the angle of attack) and apply power.  Just applying power might not, in severe cases, do more than exacerbate the problem.  Since the vast majority of quads employ fixed-pitch blades, the best recovery technique (if you had enough altitude to respond) would be to try to get some lateral speed to fly out of the downwash column.

So, yeah, NOT something to do regularly.  :-)

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DeveloperPermalink Reply by R_Lefebvre on March 22, 2012 at 8:27am

So to break this down, what Brad is saying is that the safest way to get down is to make the quad fly a pattern, square, zig-zag, or spiral if you can, with successively lower altitude on each waypoint.

This would actually be a pretty good flight command for somebody to program. A spiral descent, with settable diameter and descent rate.

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Permalink Reply by Chris on March 21, 2012 at 4:36pm

Btw, I also test stability of my setup by killing the throttle completely and then recovering after a while of it falling - good stress testing (both the copter and the pilot ;)). So far, it always recovered but you should NOT do that regularly. 

you have got to post some video of this

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Permalink Reply by Jan Schermer on March 22, 2012 at 1:09am

http://www.youtube.com/watch?feature=player_detailpage&v=1uj__z...

there you go... :) this is soft, but it's fun to see it stabilise when nearly upside down... :)

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Permalink Reply by Chris on March 21, 2012 at 4:33pm

when I do a fast decent I pitch the quad full forward and reduce power till it starts to buffet around.. then increase throttle slightly till flight gets stable...  then I fly a downward spiral till I get to the altitude I wish.

the other option is get telemetry and learn to fly on instruments.   

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