I posted this on the ardupilot forum, but wanted to be sure it's known here in case this is a common problem: http://ardupilot.com/forum/viewtopic.php?f=48&t=6776
Hi Guys - I'm new to the Iris and have been trying different flight modes. Tonight I switched from stabilize to Loiter and started the aircraft in a vertical climb. It was a couple hundred feet up (estimated) and I reduced throttle to keep it from going any higher. The Iris kept climbing, so I lowered the throttle further, and the climb continued. I pressed the right stick forward from center, and that finally stopped the climb, and it began to descend. The descent rate seemed pretty high, I added throttle and it started climbing again. Again, forward right stick got it out of the climb, so I didn't add any throttle until it got to a comfortable altitude. I've attached a screenshot of the seemingly pertinent CTUN values form Mission Planner, along with the log file.
From what I can tell, the aircraft was properly receiving my throttle input, but the autopilot was ignoring the stimulus - this based on the ThrOut graph. However, the ThrOut graph did get very noisy, and it looks like the AngBst parameter may have had a factor in this. The mission planner dataflash logs page gives this definition for AngBst:
AngBst: throttle increase (from 0 ~ 1000) as a result of the copter leaning over (automatically added to all pilot and autopilot throttle to reduce altitude loss while leaning)
Some conditions: I had a GoPro mounted on the front, and had to hold aft elevator in stabilize mode. It was apparent the COG was too far forward and I made no effort to trim it out with the SkyRC controller. I did have the datalink receiver hooked up to a Nexus 7, which seemed to be working OK, but I was too busy worrying to check it during the event.
This wasn't what I expected, and either I don't understand Loiter mode, or the root cause could have been the fore COG. What should I have done to prevent this? I wouldn't think trimming from the transmitter would help this situation. That would be just like holding the stick aft. Should an autopilot parameter be changed to prevent this from occurring again? I do have a tarot gimbal that would solve the COG issue, but I'm not ready to destroy that just yet 
I'm hoping somebody with more experience than I can explain this - I'm reluctant to use loiter again at this point.
Thanks in advance,
Wayne
Replies
The first time mine took off upward when in loiter it turned out that I was at about 5 meters and the battery failsafe RTL kicked in. The altitude of the RTL was 50 meters! It took off like a rocket but I left it alone and it came back down and landed. I was already over the landing spot.
The first time that happened I panicked, went to stabilize and cut the throttle too much. Cost me two props and a broken arm.
So does anybody know if there is a parameter that can compensate for "tilt" from the GoPro on front to prevent loss of control in loiter?