[jcg1541]

Yes, good idea.

Looking at the picture, the vortex ring seems to be some kind of inverted ground effect that comes out of nowhere.

[karla]

Haha yes that's a way to put it I guess.
Its the number one dangerous flight conditionfor a full size heli and a common cause of crashes. It may happen when descending vertically (90, tops 70 degrees) and rate of descent is relatively high for a normal heli, 2-4 m/s. I don't know how this translates to our smaller helis, the tip of a rotor moves typically at 200 m/s, we have higher rpms but much shorter rotors. I just have a gut feeling our helis will need to have a higher rate of descent to get in to it. Anyway, the heli basically gets trapped in its own main rotor downwash. Descending faster or slower will move it out of the predicament.
I think you could expect to move through this state as you dive.
Other than that it could be useful to your ends.

[jcg1541]

Karla, My setup is closer to 200 size than 250. The rotor head is from the Blade 180 , and blades are Oxy2 210 blades that are barely over 200mm from bolt screw to the beginning of the sloped cut.
The rotor diameter is close to 490 mm . The weight is less than 250 grams including Raspberry Pi.
I do use the attitude mode to launch/land from/on hand .  The outer loop high gain is very helpful for catching.

I had a look at your PIDs. The only thing that sticks out is your I terms, never seen so high in 250 or 450 size heli. But I would not worry, if its working.
B t w your outer loop PIDs are very high but you only do rate mode so it is not used.

[karla]

Got it John,
Will try later to calculate at what decline rate such heli would expect a vortex ring state.
Meanwhile, I think you can really just experiment and try it out.
Now you know what to look for and how to recover from it.
symptoms
. heli vibration
. random yawing, rolling and pitching
. increasing rate of descend
. less cyclic stick respons
recover by rolling

What do you think?
 

[karla]

I have been asking around a bit but seems no readily available method to do this calculation.
Answers mostly like this:
 - VRS is a very turbulent state and therefor stochastic and hard to predict/calculate.
It happens, roughly, when rate of descend of downwash is equal to heli descend. But how to figure out difference between full scale heli compared to rc size heli - not clear.
So no real help for you.

[TheOtherCliff]

My experience with this is with quads.  I never had much issue with my unstabilized .30 to .50 sized nitro helis.

Falling straight down is part of the issue.  Descending at a 45 degree angle does not have the problem in my experience.

Also if you are falling quickly (straight down) there is no problem.

At least with a quad, well tuned PIDs (such as from AutoTune) change the problem almost into a non-problem.

Best to experiment in a controlled way to know where the issue is, to avoid it when needed.

[jcg1541]

Karla, I have cut 40% of the gains with much larger swash travel range to make the heli agile enough to flip.
Previously I tried to optimize the collective for long duration by stipulating the collective range in Librepilot PWM output tab.
Now the swash travel range is almost doubled, and the gains are all different now. I am still yet to cut the gains further down, maybe half of my previous.

I had a look at your PIDs. The only thing that sticks out is your I terms, never seen so high in 250 or 450 size heli. But I would not worry, if its working.

[jcg1541]

Just some thoughts:

You might try increasing pitch/roll PID "I" term.  Quads "blow out" (unstable pitch) at high speed if the "I" term is too low.

That was a valid idea. I had a Proprotional 35, high Integral 150 dive today. And it felt more controlled and not on the whim of heli physics pitch-up-down.
The dive is at the end of the video, 1:06 to 1:15 , sorry it took a minute to climb to the diving height,

I was able to do 120 degrees from 1:06 to 1:08 , then to aim for 90 degrees dive at 1:09 - 1:11 , then again to go at 120 degrees from 1:12 to 1:15 . I was in control.
Tank you my friend.

[TheOtherCliff]

 

[jcg1541]

OK, I have the leisure to do a aileron flip while diving straight down now with a even higher Integral .
The first 15 seconds is climbing, then the diving immediately follows at 0:16  ,

. I still have the bad habit of elevator pitch up trying to pull up the imaginary airplane at the end of
a dive that looks stupid. In helicopter, elevator pitch up doesn't really help any thing, and elevator
pitch up should be substituted with elevator pitch down for forward pull out.

[jcg1541]

I think it will not be difficult to fly just with yaw set to axis lock and all others manual. To test what Cliff suggests.

Karla, I have tested manual mode with this 210 sized heli now. The result is that it tilts and twists all directions.
(the heli is too small to be seen most of the time, and police cars zipping by not noticing what goes on there)

The rotor is too small to have stabilization by itself without flybar. This is a FBL setup. This is a FPV session.
0:01 tilt right and backward
0:02 attitude mode to rescue
0:03 tilt right and forward
0:04 to attitude mode
0:05 tilt left straight
0:06 attitude mode
0:08 tilt backward
0:09 attitude mode
0:10 tilt forward
0:11 attitude mode and stay in attitude mode until safe landing
0:25 landing approach too high and close to trees
0:35 backing off far and fast from the botched landing
0:40 good landing approach
0:53 touch down
Every second is an adventure of twists and turns. It needs a computer to stabilize it every second of the way.

[jcg1541]

Smoothest and stable dive so far,

[TheOtherCliff]

That is looking acceptable!  What, about 4-5 seconds?

Just a question: are you using TPS (Stabilization->Advanced->ThrustPIDScaling)?  Mathematically at least, whenever you double the head speed, you should halve the PIDs, etc.  So if you are using a throttle controlled variable head speed, this will keep the PIDs in sync with the head speed.

[jcg1541]

Interesting. I am not using the thrustpidscaling. My thrust ranges from 78% to 87%. And P is 15 now. The scaling will reduce 1.5 of P.
I will have my pid around this number for a while until my proximity dance skill leap to the next level.


The initial half flip overshot made it dive on the side, still straight down dive though. And then elevator erroneously used to correct it. The elevator pitch up was intended to turn the nose down after a half flip. But now the elevator only made it swing on the horizontal plane. And it ended up with piro correction. All this need to be fixed on the pilot's brain. And work on the brain starts now.

[jcg1541]

Got leveled pull off at the bottom of the dive now thanks to Sikorsky's research for turbulence solution with a canted tail rotor,

. The heli now has the same 20 degree slanted tail as the Blackhawk to fix previous ugly high-G-pullback at the closeup of the dive videos.

The swinging during diving is due to the high wind today at 15 MPH on the ground and even higher in the air at 120m altitude.  You can see how the wind blows at the tree at the end of the second video,. It is not PIDs problem, this is the view from the FPV goggles, transmitter is a generic tiny whoop 200mw VTX,

. No changes to PIDs since last 2 posts.