I was test flying today in anticipation of a trial for the T3 Contest entry tomorrow. The winds were pretty high, around 20-25 Mph, and the airplane was unable to make headway against them. So I reverted the throttle to manual control (plugged ESC straight into the RC) and made sure that the throttle was always high enough to make good headway. Night and day! The plane navigated great and did figure eights overhead for 15 minutes before I put it back into manual and landed it.
Once I've got all the other gains tweaked, I'll start tweaking the pressure sensor and throttle, but for now the best way to test is with a manual throttle.
I've added the following text to the manual:
"Our recommendation is that while you're first setting up ArduPilot for your aircraft, you do NOT use it to control the throttle. Rather than connecting the ESC through the ArduPilot, connect it straight to your RC receiver as usual. That way you can control the throttle manually while ArduPilot is controlling stabalization and navigation. The main reason for this is that it's very difficult to tune aircraft in the wind, especially if the throttle settings are not sufficient to make headway (when the aircraft is heading into the wind with a insufficient throttle setting, its ground speed may drop to zero, which will confuse the GPS navigation, which is based on ground speed). Manually keep the throttle high enough to make headway into the wind, and you'll find it much easier to observe and tweak stabalization and navigation gains."