[Sid]

Hi everybody
I have this 450 size home built wooden Quadcopter that is prone to wobbles on descent (Prop wash). I have tried to lower pids or increase with no success. The only thing has helped so far was to use smaller size props which I had a power lost and it hoverd around 70 percent.
At this point I'm running out of clues and decided to ask you guys a few question if you could answer me.

What increases Prop wash?
Size of frame and material used
Motor size
Stiff or soft Propellers
Esc speed
Type of the frame Like X,+, H. Symmetrical or semi Symmetrical
Location of battery and camera. On the top of underneath 
And what ever can cause wobbles on descent
 
Have a great day
Cheers
Sid

[NicholasDavid]

I'm not an expert but I have some thing to try.  TPA.  Usually u use it to get rid of oscillation at high throttle. Well you can also set it so it increases PIDs at lower throttle(like going through prop wash) so try increasing TPA when at lower throttle so you have more PID control in descent. start with say 10% and test and see if that helps.

I set mine to have 18% reduction in PID at full throttle trailing to 0% at half throttle and positive TPA(about 10%) at lower throttle. It gives me good control at low throttle for flips and course corrections while lowering my altitude or falling through wash. And less PID at high throttle for fast flight and punchouts without shaking crazy.

An easy one to try first through if you fly in rate mode is to get the PIDs to where you like how it flys and try adding more I gain.

As far as what causes it... Not enough I gain control at lower motor RPM...big floaty props that push a lot of air, and aerodynamics I suppose. Everybody deals with prop wash. Some just more than others.

[Sid]

Thank you so much for the info about TPA and I'm going to try it out on my next fly and report how it went.
But how about the other facts that some machines are prone to suffer from Prop wash?

[NicholasDavid]

I think the bigger the machine the more it suffers from its own wash.  Wider arms create more drag. Wider bodies do the same.  Bigger props at a lower KV need higher PID control at lower throttle conditions because their frames catch and drag in the air more.  Center of gravity issues can usually be completely resolved with higher/proper I gain tuning. I think it's mostly the larger frames that just interact with the air more.  Large propellers move tons of air and create more turbulence, it's just a drag thing. Aerodynamics. A propeller spinning has just as much drag as a solid disc the same diameter.

Proper I gain tuning and TPA should sort out wash situations.

[Lob0426]

Here is a video that explains why the multi-copters tend to wobble on decent. It is called Vortex Ring State.



I am sure that some settings can help, but the real problem is the turbulence. This means your props efficiency is constantly changing while in ring state. A faster FC would help also. Maintaining some forward flight while descending helps reduce or eliminate the wobble.

My Yuneec has the same problem and its FC is pretty close to a revolution (STM32 not sure of the actual chip). Coming in at an angle means a smooth descent. The Yuneec definitely has more wobble than my 250 size copter does. Lower RPM and bigger props.

[sasquad]

Can't come straight down with the 450 size. I built the DJI flamewheel 450 and if i try to desent with just dropping the throttle it will wobble like crazy. You have to "fly" it down, do small circles as you decent. No more wobbles.

[Sid]

Thank you all guys for all your replies. Today its been such a educating day and tomorrow I'm going to put it in test to see how it goes.
I don't have this problem with my 650 size Tricopter or 250 size Quad or even Qav400. But this 450 Quadcopter sometimes
scares the heck out of me because it wobbles so bad.

[Sid]

Setting the TPS did the trick. Thank you very much.
Sid

[NicholasDavid]

Your welcome.  Glad that took care of it. Everybody just uses the TPA for high throttle shakes, but it does so much more. I had another more knowledgeable guy at this forum give me that tip. Cliff I believe. Or the other cliff. I can't remember. Happy flying