Why wobble on descent ?

[Robert Alexander 1]

Dear friends,

today I finally had ideal conditions and flew my 3 batteries in a blue windless sky without any accidents

Having almost no wind made me notice a repeatable behaviour. My little quad has so much power compared to it's mass that I can liftoff like a rocket When it climbs (never more than 3-4 seconds max or it'll become too small for me to handle) it shoots straight as an arrow but when I want to bring it down quickly it does wobble noticeably (while hovering or descending gently this does not happen).

Can anyone guess why does this happen ?

Thanks !

[Essjay]

Hi Robert, what you're experiencing is what is termed the "vortex ring", for which you can find an explanation here

**LINK**

In order to avoid the wobbles, try to descend at a slight angle.

Steve

[Shaunie]

I'm not a quad expert, I'm more of a fixed wing type of person, but:-

Think how a Quad stabilises itself in the air, it has no aerodynamics as such but is standing on four columns of thrust. The gyro then adjusts the relative magnitude of these four columns for stability. In the extreme case of you shutting off all thrust the Quad just becomes a ballistic object, no stabilisation whatsoever. If you reduce thrust so that it is markedly less than the weight the gyros have nothing much to work with and the Quad will wobble as the rotors one one side have to speed up markedly to correct a drop, they will then be going too fast when it levels and so that side may go too high, thus a wobble starts.

There are other effects at work here too. Prop stalling is a possibility, I know My Hubsan X4 does it you can hear the props whooshing when they stall and you have to add a great deal of power to stop the sink rate. Vortex ring state (VRS) is also a possibility, this is where air displaced downwards circulates upwards through the hub and round the outside of a rotor disk causing the rotor to effectively recirculate its own air, plenty out there if you Google it.

I expect the effect could be tuned out to a point on higher end Quads but the answer I have found is don't descend too quickly, learn fine control of the throttle stick. (Or slide down sideways).

Waiting now to be shot down by a Quad Guru!

Shaunie.

[Robert Alexander 1]

Wow this is so interesting. Having a background as a sailplane pilot made me think I knew everything about aerodynamics but these propellers seem to have a complex life !

Now that you mention if I descended AND also drifted towards me (so not vertically) the effect was markedly less or even absent (I'll check it next time now that I know what to look for).

I have to confess that shooting up and bombing down was a way to get some thrills after a long session practicing just overing at wanted heights. Silly but a diversion from serious practice (and practicing the most varied flight manouvers could always come out handy in special situations)

Thank you all for your kind and clear explanations !

Robert

[Peter Christy]

I have no experience with quads, but I do have a lot of experience with fixed-pitch helicopters, going right back to the days of the early 70's. What you are describing sounds suspiciously like "over-pitching".

If a fixed pitch helicopter is flown with too much pitch, it will go "up like a rocket" - but when it comes time to descend, it has to be throttled back so far that the rotor rpm is insufficient to either maintain proper control, or to stop it wobbling - just like a gyroscope winding down. Back in the early days, this led to a pronounced "porpoising" on the landing approach - sometimes violent enough to cause a boom-strike!

I would suggest fitting lower pitch propellors, to produce a more moderate rate of climb. That way, you will not have to reduce the RPM so much to descend, and the internal gyroscopes will not have to work so hard to maintain stability!

--

Pete

[Biggles' Elder Brother - Moderator]

I think Essjay has the explaination - but maybe it could be put a bit simpler for non rotary wing types

Basically what's happening is this: the quad props each create a wash beneath them of swirling and very turbulent air. When you descend slowly this turbulence has a chance to be pushed out of the way by the "fresh" wash still coming from the rotors. But if you descend very quickly this doesn't get a chance to happen, and so before you know it the quad is effectively sitting in its own prop wash! - and that is very "dirty" air. Its a bit like the effect a fixed wing model experiences when it flies through the wake close behind a much bigger model - shake, rattle n roll time!

The way to avoid it is exactly as Essjay says - descend diagonally that way you avoid flying through your own downward wake!

BEB

[Robert Alexander 1]

Cool ! As I said I have 200hrs flying sailplanes and so know very well what to fly in a prop wash means (during aerotow) The only shakiest flight experiences I had was flying in "rotors" during wave flight conditions !

I have tried my quad with both 8045 and 9047 props. Cannot say why but I like the flight dynamics of the 9047 more (but as I've said I know near nothing about propellers).

Thank you all for the great explanations.

[Essjay]

Thanks BEB, you put that much better than I could!

Steve