*VIDEO* Crash Diagnostics Help Please! DJI WKM on Cinestar8

[volans_oculo]

I'm new to multirotors, as I just partnered in an aerial cinematography business. I familiarized myself with the DJI WKM system, with thorough assistance from the business' former partners, and had a few great flights (no issues whatsoever) before my first job. I ran through pre-flight checks, calibrated the GPS, and had a couple of smooth flights with great landings before this:

I was flying in Atti. mode, and flipped to GPS Atti mode when the trouble started, but you can see that it didn't help much. I'm not sure what I was doing with the sticks, but I wasn't trying to fight it, as I immediately knew something was wrong with the flight control. Thankfully, the Octo landed in a bush and not on the rocks, suffering no real damage. We pulled it out, dusted it off, did a hover test, and another reasonable-length flight with no issues. After about 5 more flights it happened again, with a fresh set of batteries in a more dangerous location (unfortunately, we didn't get this one on video). I got it down onto a tiny patch of grass surrounded by rocks, cracking a gimbal leg and one prop.

I've been flying helis for a few years (including single rotor 3d), and have never had a crash like this.

Any theories on what went wrong?

Thanks!

 

[Carapau]

That just looks like being too aggressive in your descent without the gains properly set. Multirotors do not like coming down too quickly as the motors slow down a great deal and they are moving into more turbulent air. Next time this happens just bump the throttle up, let it stabilize and then continue. At least that's my method and it works pretty well for me.

 

[volans_oculo]

Thanks Carapau, that was my thought too. I'm going to play with the gains on my next set of test flights. I've also ready that it's far better to descend on an angle, instead of straight down, so that the copter doesn't fly into turbulent air. My heart sank when this happened, I'm just glad that damage was minimal!

 

[DucktileMedia]

you sure you didnt flip into manual? manual on the wookong is the worst thing Ive ever flown. Not like a SRH manual, its like balancing on a beach ball in a swimming pool and basically impossible. There have been a few warnings about flying in GPS mode today. Solar shower or something. Dunnno what to say, but I see way too many DJI products doing this. AND..........we keep buying them!

 

[volans_oculo]

you sure you didnt flip into manual? manual on the wookong is the worst thing Ive ever flown. Not like a SRH manual, its like balancing on a beach ball in a swimming pool and basically impossible. There have been a few warnings about flying in GPS mode today. Solar shower or something. Dunnno what to say, but I see way too many DJI products doing this. AND..........we keep buying them!

Quite sure I didn't flip into Manual. When we got to the rig, it was in GPS Lock mode, and I hadn't touched any switches during the flight. As you can see by the LED, it was in Atti when I took off. When it started oscillating, I flipped to GPS mode, and it looked like it would stabilize, but it kept on its rocky descent. Plus, I was giving no stick input that would have caused those oscillations (in fact, I was giving none at all). I posted this in the DJI forum, and Carapau figured it was caused by descending too quickly into turbulent air, with the gains set incorrectly. This was my theory, and I've read a few ideas for how to avoid/counteract the issue (angular descent/throttle-up)...

 

[Carapau]

Descending on an angle certainly helps but if it happens again just give it a bit of juice on the throttle and you will get out of it quickly. This is one of the reasons I prefer doing AP from my single rotor, they just fly that much better but having said that my gimbal cant quite keep up with the tic-tocs or the piro-flips.

 

[volans_oculo]

Descending on an angle certainly helps but if it happens again just give it a bit of juice on the throttle and you will get out of it quickly. This is one of the reasons I prefer doing AP from my single rotor, they just fly that much better but having said that my gimbal cant quite keep up with the tic-tocs or the piro-flips.

Even with a 3d-capable gibal, I've heard that neither Piro-Flips nor Tic-tocs don't generate the best footage.

...something about lens-obstruction

 

[glacier51]

volans_oculo:

Just interested to know if you had any wind at all.

There's a helicopter phenomenon called settling with power, and this looks exactly like it. The helicopter descends in its own rotor wash, controls become sloppy, blades flapping around like your octo oscillating side to side. If you don't fly out of this descending column of air, hence everyone's comment on a descent with some forward motion and believe it or not more power can actually exacerbate the situation - it accelerates the air even faster downward, you end up on the ground as you did. Of course with a more expensive pile of trash......

Ron

 

[volans_oculo]

volans_oculo:

Just interested to know if you had any wind at all.

There's a helicopter phenomenon called settling with power, and this looks exactly like it. The helicopter descends in its own rotor wash, controls become sloppy, blades flapping around like your octo oscillating side to side. If you don't fly out of this descending column of air, hence everyone's comment on a descent with some forward motion and believe it or not more power can actually exacerbate the situation - it accelerates the air even faster downward, you end up on the ground as you did. Of course with a more expensive pile of trash......

Ron

Hi Ron. There was a bit of wind, but it wasn't strong; maybe 5-7kts. I figured it was something like descending into rotor wash, and causing vortex rings which I discovered do not respond well to increased power. I'm guessing the basic gain settings weren't aggressive enough to regain attitude control automatically. Does this look right to you, or does it appear that the gains were so aggressive that they produced that oscillating effect as a result of overcompensation?

In either case, I'll be much more careful on big descents in the future!

Also, if it's windy, does that help move the column of descending air out from under the aircraft?

 

[glacier51]

Vortex Ring State and settling with power are slightly different phenomenon, but the results are the same.

>Also, if it's windy, does that help move the column of descending air out from under the aircraft?

Exactly. You can do this by flying forward or wind does it for you if you're in a hover.

I've been trying to do more manual flying. I've got a F550 with NAZA. With normal gains it seems I get way behind it and the last major crash was in manual. I now have a mix in the radio that drops the gains 50% when I switch to manual. Seems like it's a bit more stable for me.

 

[Roger Ramjet]

An almost identical incident happened to me in a smaller scale (F550 flying over 2500g with stock motors). Fortunately, I apply full power, which is not natural when things are going shaky....and the octo came back under control. It is almost as if you are falling under the power curve. The MC senses an unbalance situation and sends a signal to correct BUT the low power setting associated with the descent does not correct the condition immediately, it applies the correction late and weak which in turns, create a worse situation. Sort of a vicious circle. The MR seemes to be more vulnerable to this condition when very heavy....

My question is simple: were you maybe over the recommanded MTOW or very near?

 

[janoots2]

If I had to take a guess you descended too quickly, and for some reason your IMU could not catch up with the oscillation, most likely due to incorrect gains. I've wasted a lot of battery and video time thinking that descending is a waste with WKM, but, once you figure out you can still fly around while descending (and not straight down) you can actually get some great movement.
I could tell in your voice you were nervous with the height and wanted to descend. Nervousness usually equals over reaction. Strengthen your nerves by flying more grasshopper.

(and tune in your gains)

 

[Carapau]

Fortunately, I apply full power, which is not natural when things are going shaky....and the octo came back under control. It is almost as if you are falling under the power curve. The MC senses an unbalance situation and sends a signal to correct BUT the low power setting associated with the descent does not correct the condition immediately, it applies the correction late and weak which in turns, create a worse situation.

This has been exactly my experience. But having said that, it has been very interesting to hear the points about the vortex rings and the fact that power doesn't always go well in these situations. I will certainly bear that in mind for the future although I always try and avoid the situation in the first place.

 

[volans_oculo]

If I had to take a guess you descended too quickly, and for some reason your IMU could not catch up with the oscillation, most likely due to incorrect gains. I've wasted a lot of battery and video time thinking that descending is a waste with WKM, but, once you figure out you can still fly around while descending (and not straight down) you can actually get some great movement.
I could tell in your voice you were nervous with the height and wanted to descend. Nervousness usually equals over reaction. Strengthen your nerves by flying more grasshopper.

(and tune in your gains)

Thanks for the tip, sensei! I was a bit nervous. Do you have any specific tips for stable descents, or the best way to set gains?

 

[janoots2]

When I was first starting I would always just descend straight down too. But don't waste that battery time, instead throttle down so you are descending slowly and continue to fly around and get footage. The secret is coming down at an angle and understanding that it takes time to descend. It doesn't have to be a consistent angle in the same direction, just don't come straight down.

As for gains, I've found that using remote gains on a TX knob to dial them in is the most efficient way. I do throttle first, then apply pitch and roll together, then attitude, then pitch and roll again (to fine tune), then attitude again (to fine tune). The best way to find the values is just like the manual states - increase until you get oscillation, then back it down a little until you don't get oscillation. When fine tuning, you'll want the craft flying close around you so you can see any little wobbles as it passes.

Also, I would quadruple check your IMU and GPS measurements from your COG just to make sure they are correct.

 

[Electro 2]

Also, if it's windy, does that help move the column of descending air out from under the aircraft?

Absolutely. My first thought on watching this. "Yeah buddy, classic SUP, no wind and he tries to elevator it down" If you had flown forward, backward, or sideways, while decending, all would probably have been OK. I will say prime axis gains could be higher, if so, you get a sort of "shudder" when SUP occurs and you can fly out of it. "Drunken sailor" wobbles like these are typically low gains.

 

[R6Media]

If you experience settling with power the best thing you can do to escape it is to fly in any other direction quickly to regain control.
Forward/back/left/or right. whichever is safest for your given environment.
Adding more power will only make the situation worse (in real helicopters). This may or may not be the case in these light multi-rotors.
As reference, real helicopter descent rate shouldn't be greater than 300' per minute if flying below effective translational lift airspeed to avoid settling.

Fly safe.

 

[Carapau]

I wonder if there is much of a difference between real helis and rc models with regard to scaling. I ask this for two reasons, firstly whilst we can 'scale down the heli' you cannot scale down air but also, when doing aggressive descents whilst flying 3D on my single rotor, I have never suffered settling. Genuine question by the way and not a dig LOL

 

[R6Media]

That's an interesting point about the scale of air Carapau. curious.
I'm still not really sure if rc models behave similarly to real aircraft outside of basic aerodynamics. Something about the light weight makes me feel like there would be significant differences but someone much smarter than me would have to share their theories about it. This is nothing but gut feeling to me. I still have a tendency to fly my MR like a real aircraft though. old habits
As far as flying 3D on your single rotor, since the pitch is variable I would consider it more like rapidly "climbing downward" since it's still flying - if that makes sense. Flying faster than the disturbed air vortices' are descending would, I think, nullify the problem. Choppy air for sure but these things are pretty small. I just don't know. Interesting to think about though.

 

[Carapau]

Good point about the neg pitch on the SR. Curtis Youngblood has just released a variable pitch quad and looking at the videos of that doing aerobatics, it seems to handle pretty well- I haven't seen it do a vertical drop yet though.