Cause for violent roll oscillation ?

[1dr1]

Have had the PV for about three weeks with only minor bad landings. Now with some confidence under my belt, I tried for an altitude "record" and pushed it to about 415 ft. Down range was about 2-300 ft. As I was bringing it down and closer in, I experienced a very violent right to left roll oscillation. I was still able to throttle down, and regained some control and continued descent. At about 50 feet up, the oscillations started again then it hit the ground. Fortunately, it was over a vacant lot, so only two props were damaged.

Prior to this flight, I had switched over to NAZA mode and it seemed to respond to the Home Lock and Atti modes. I also calibrated the sticks before flight. All other gain settings are at default except for the gimbal, which is set to miss the stops. Pretty certain I was in Home Lock at the time of the crash. During the oscillations, I tried switching to fail safe, but got no response.

After a minor check out and prop replacement, all seems fine, but can't be certain since the winds are kicking up and bouncing the PV around a bit.

May have had some RF interference since I was in a residential area, but no overhead power lines around.

The roll oscillations wouldn't seem to a symptom of compass issues. Very little surface winds at the time of the crash. Could this be an ESC issue? Any way to diagnose?

Appreciate your comments.

 

[EMCSQUAR]

It almost sounds like prop wash. It's common if you're descending at a good pace (and usually straight down) you can get some major oscillations. The heavier the craft the worse.

 

[1dr1]

Thanks for the reply. I'll try another flight this pm after batteries are charged and see if the flight is stable.

 

[Nvr2fst]

It almost sounds like prop wash. It's common if you're descending at a good pace (and usually straight down) you can get some major oscillations. The heavier the craft the worse.

+1
Try going a little forward or back as you descend. It constantly drives you out of the wash. (For the most part). Especially when you need to come down fast. Not perfect but it helps.

 

[coloradosky]

It almost sounds like prop wash. It's common if you're descending at a good pace (and usually straight down) you can get some major oscillations. The heavier the craft the worse.

+1
Try going a little forward or back as you descend. It constantly drives you out of the wash. (For the most part). Especially when you need to come down fast. Not perfect but it helps.

Same diagnosis here, in addition to those accurate solutions I have used a wide circular descent pattern when the ascent has been straight up essentially- in order to keep the PV relatively close to where I want to land and in sight versus straight forward descent pattern or the reverse. This also lets you 'set' or hold a constant position with the control sticks once you find the sweet spot and the video effect is pretty cool taking a wide sweeping view as you get the job done of coming home.

 

[cb1]

I've noticed some instability during rapid decent but nothing as violent as you indicated. I guess it's understandable that the flight control algorithims are much easier to define for ascent where the lift is more dependent directly motor speed whereas descent appears to have a more chaotic fluctuation in lift due to the complexities of the airflow through the rotors.

Every time I see my Phantom fly I am still amazed that it is actually airborne and very responsive to my commands. Before computer controls nothing like this would even be able to fly. If you think about the complexities of multi-rotor flight and how DJI have developed flight control software with this degree of stability and has made it available to the general public is incredible. 10 years ago, the technology packed into the Phantom would have been a state secret. I'm sure they will continue to improve the stability in the future but I'm pretty happy with the performance and capability that I am getting now.


Here is some technical info on vortex ring state.

http://en.wikipedia.org/wiki/Vortex_ring_state

 

[1dr1]

Thanks all for the quick replies. I had never thought about prop wash. Well, off to the park now for a few test flights. Will let you know how it turns out.

 

[1dr1]

Two successful stable flights with only two modifications (2 new props and a better informed pilot). The vortex ring state (aka prop wash) is an interesting event to watch out for. While I limited my altitudes to less than 100 ft, I also coordinated my descents by adding pitch (toward me), which is easy to do in Home Lock. This has the added benefit of maintaing a constant return approach angle to minimize any RF control issues.

While I was at it, I experimented with Atti mode. Got another scare when I thought I was in GPS mode and flipped to Atti. I was already in Atti and really switched into Fail Safe. Of course, I had no control and the PV was rising and heading toward me, as it should in F/S. Quickly flipped back to Atti then to GPS and all was well.

 

[srandall25]

Have had the PV for about three weeks with only minor bad landings. Now with some confidence under my belt, I tried for an altitude "record" and pushed it to about 415 ft. Down range was about 2-300 ft. As I was bringing it down and closer in, I experienced a very violent right to left roll oscillation. I was still able to throttle down, and regained some control and continued descent. At about 50 feet up, the oscillations started again then it hit the ground. Fortunately, it was over a vacant lot, so only two props were damaged.

Prior to this flight, I had switched over to NAZA mode and it seemed to respond to the Home Lock and Atti modes. I also calibrated the sticks before flight. All other gain settings are at default except for the gimbal, which is set to miss the stops. Pretty certain I was in Home Lock at the time of the crash. During the oscillations, I tried switching to fail safe, but got no response.

After a minor check out and prop replacement, all seems fine, but can't be certain since the winds are kicking up and bouncing the PV around a bit.

May have had some RF interference since I was in a residential area, but no overhead power lines around.

The roll oscillations wouldn't seem to a symptom of compass issues. Very little surface winds at the time of the crash. Could this be an ESC issue? Any way to diagnose?

Appreciate your comments.

Was there ever any resolution to this? Did you find out what caused your violent oscillation? Same thing happened to me with my Vision+. It was also after changing to NAZA mode and following an RTH.

 

[JimDE]

Yeah when I saw this thread I was going to post that there are two threads goingbon in the Plus forum about a similar condition going on with their plus's running NAZA and ver 3.

I only paid attention at first because I saw the word NAZA and I recently switched to that on my PV. Reading the threads it seems a couple people have similar issues and one of them had it with ver 3 in phantom mode.

Just thought I would add that as before reading those posts I would of though of prop wash too ... Now I wonder ...