http://copter.ardupilot.com/wiki/autotrim/
Removed Trims applied at radio (during the first flight) and put everything back at 0 or neutral. I recommend you be directly behind the quad, facing the wind. Actually, there should be no wind.
No problem getting into AutoTrim calibration Mode (just hold stick a while longer than required for Arming). If it doesn't work in Loiter Mode, try Stabilize. Red (and tiny yellow) lights will dual-strobe-flash when you enter mode. Only catch is that you have to take-off (with a likely out-of-trim quad) and hold a nice hover for the next 20 seconds. Lights will stop flashing signaling the end of the test period. Land the Nova and disarm to save new generated AutoTrim settings. It's only my second flight, so it took a couple of tries, but I got it done. Takes off and flies much better now (even in a little wind).
Summary: With AutoTrim the roll and pitch trim are captured as you fly in a stable hover.
1. Find a wind free environment with sufficient space to fly your copter without crashing into something. Be sure radio trims are reset to zero or neutral.
2. On Mode-2 transmitter, hold throttle stick down and right for 15 seconds or until you see the small red and yellow leds (behind Arming lens) strobe-flash in a cyclic pattern. Action is like Arming except holding longer.
3. In Stabilize mode, fly your copter for about 25 seconds in a stable hover. IIRC, red/yellow leds stop flashing in flight.
4. Land and put your throttle to zero and wait for a few seconds (the trims are being saved to eeprom). Nova will dis-arm itself.
5. Take off again in Stabilize mode and check if your copter is flying level now. If not repeat steps.
Also, I got to try Loiter Mode for the first time. Worked fine. One of the coolest things I've ever seen one of my RC models do over the years. While Loitering, I quickly took this pic with my iPhone (I know, pretty blurry).
If you are having trouble arming quad, here is a summary: If you have GPS-3D Lock (rear green led is On-steady), AND hdop <= 2.0 ... you can (and should) Arm in Loiter switch mode. Then, switch to Stabilize Mode and take-off. Otherwise, you can just Arm in Stabilize Mode ... but don't expect RTL (pilot invoked or emergency) to work during that flight. Also, don't switch to and try to use any GPS dependent modes like Loiter during that flight.
It was getting dark, but was fairly easy to see whole quad (actual direction orientation ... not so much). Good flight and got in some good practice. Pretty boring flying, but I'm getting more confident with it.
AutoTrim Notes:
While flying normally, you can trim on radio sticks if you must (temporarily to keep from crashing). But to do a proper Auto-Trim, they should be neutralized or centered first. Also, remember that Trimming due to wind forces is counter-productive because you might be pointing in a different direction later in flight. Instead, that's why we have Loiter mode.
If you enable Auto-Trim and do it poorly, the next time you take-off, quad will be out-of-trim. You can either do it again until you get it right. Or, you can use Mission Planner to reset the Trim-Defaults with the following parameters:
AHRS_TRIM_X,0
AHRS_TRIM_Y,0
AHRS_TRIM_Z,0
Auto-Trim is often required because quad's CoG is slightly off, but I think there might be other reasons. Good pilots have been know to accomplish an AutoTrim procedure in their garages (because no-wind is important), but you will have to get it pretty high to avoid the "ground effect". Also remember that concrete and shelves are hard and cars scratch easy (you have been warned) :-)
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