Ok, I suspect I'm up against the laws of aerodynamics here, but wondered if the APM can help?

Essentially I'm run big props on my quad - normal flight is perfectly ok, but it "fights" descending; wobbles, fluctuating rpms etc. For video reasons I'd like descents to be smoother.

As I said - it's probably an unavoidable consequence of my config. But an improvement would be great.

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It should be noted that I "cheat" at the moment - I stick it in alt hold (actually, I use it a lot), then do "throttle stick down". It gives me a very controlled rate of descent, and the "fighting" is reduced. It works...but I don't get the descent speed I want for the shot I have in mind. I can bump up the rate, but the "resistance" increases.

 

Normally, I just accept it as a fundamental property of big propped quads, but now I need to get a specific shot, it becomes a (small) issue.

 

I could go smaller props, but that will affect my load carrying and flight time...or I could go hex or octo? Never flow them...do they descend well?

 

I have a octo and no it does not descend well at all,,, as a matter of fact if done to fast it will fall on its side and come shooting down to earth and if you're in Alt Hold your not gonna keep it from smashing into the ground....

ooooh, that doesn't sound good! For some reason I had it in my mind that more props = more stable everywhere.

Apart from in ground effect - I imagine it's worse given the volume of air being moved by 6/8 props.

 

I found with my Y6 the only thing that helps is going to smaller props at higher RPMs. Also, some forward movement helps prevent the oscillation as you descend.

Basically the reason it does this is because as you are descending, one of your propellers stalls while at low RPM and as that side drop, it tries to compensate and ends of stalling the other side and so in, inducing the oscillation. I have also read aout this happening in forward flight because of the retreating blade stalling and creating assymetric thrust.

Use ESC's with "SimonK" firmware to increase the propeller responsiveness (thus allowing more aggressive P gains), and get smoother descents. Also smaller propellers/higher rpm is better for stability, but has less efficiency.

The problem is simple but difficult to correct. In a vertical descent the aircraft is moving through the dirty air caused by the propellers.

 

There is no good solution other than a slow descent rate, or some horizontal movement while descending to move out of the prop wash.

Actually, this is a good point - I found a slight, constant forward motion is enough to stabilise it, and get the props back into "clean air".

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