Hello everyone, I think I have another puzzler on my hands.
I’m running a Goblin 500 Sport, with ArduCopter 3.4.4. Today, I ran up the chopper to my takeoff RPM in Altitude Hold mode, and then switched to Loiter mode right before takeoff. After that, it took off and loitered at around a height of 3 meters for 15 seconds. It suddenly descended and hit the ground, lifted off again, triggered an EKF failsafe, and entered into LAND mode and hit the ground again. Until the first time the copter hit the ground, I moved nothing on the transmitter after stable loiter.
I observed that under the logs, CTUN.DAlt (desired altitude) is stable, but CTUN.DCRt (desired climb rate) suddenly becomes negative, indicating that that copter is being commanded to descend, and the actual descent rate (CTUN.CRt) follows what is commanded. I’m confused: why can the desired altitude and altitude be stable but at the same time (no real altitude error), but have a negative required climb rate?
I’m hoping someone could take a look at the log file and see if this makes any sense to them
Looks to me like your collective is bouncing up and down. What do you have ACCEL_Z_P set to? The default of 0.5 for multi’s will make the collective bounce on most helicopters.
Hello Chris, my ACCEL_Z_P is set to the default 0.5. Are you saying that my collective servos are bouncing based on the CH1/2/3 outputs? Looking at that does indicate to me that the collective is bouncing, and I think you’re spot on about the P being too high.
I’m not sure that this completely explains why it would make the heli drop out of the sky though. The copter did hover quite nicely for about 15 seconds before it fell.
I don’t know that I can fully explain that either. I would recommend setting ACCEL_Z_P to like .25 and try it again. I never take off or land in any of the altitude controlled flight modes. I prefer to take off in Acro, but Stabilize works good too. Then switch to Alt Hold or Loiter once you get airborne. You will have much more control over the helicopter if you take off in a flight mode that does not depend on altitude or GPS position control.
I took a look at your log again, and I still suspect just the ACCEL_Z_P param is causing it. Part of the reason I suspect that is experience with it. My 700 nitro had pretty much the same problem set at 0.5. And my Z vibes were way worse than yours. Difference was that I took off in Stabilize the first time, switched to Alt Hold and the heli proceeded to repeatedly slam itself into the ground and every time it hit it took off in a different direction. I was so busy on the cyclic I couldn’t get my thumb to the flight mode switch. Pretty much what I would describe as vertical tick tocks, totally out of control. But that is a very high power to weight ratio helicopter with a O.S 1.05 in it. I switched back to Stabilize and got back under control, landed it, and it took me a few hours to figure out that undocumented “feature” of the heli firmware is set to multirotor default, and not specific to helicopters.
