On the wings of the wind, hold that thought. Did you have wind at the altitude you were flying?
With no disrespect intended, I think his concern may have been dismissed to quickly. Sure, everyone knows that in the absence of GPS the bird will go with the flow, but I personally had a similar issue where my bird was hauling but in an eastern direction every time I went to atti mode. It was actually kind of fun, fly west, let go, and watch it straight-line back to me, but unless I has found a Funnel Gust, (like John Candy in Summer Rental), something was wrong. My 20 foot flag pole agreed with local weather reports, no wind that day. I even rotated the bird thru the return path and it followed the same "straight-line" path. That evening I concluded that I needed to recalibrate the IMU and the compass. Like post before said I could, I calibrated the IMU indoors on a 100% level table and the compass was done outdoors in an area free of ferris objects.
Following these recalibration's, I tested the atti mode again, but this time the bird did follow the light breeze, drifting in a directions that were less than straight-line as was observed before. It is also notable that prior to performing an IMU calibration the system said it was not needed, but something changed.
I 100% agree that understanding how each mode will affect you bird is a must, and until you do, I would prevent accidental mode changes by turning off "Multiple Flight Mode", but I would also recalibrate.
Be sure to pick wide open space for any testing of atti mode. Your drift should not be any greater than the forces surrounding them. A 3 to 5 mph wind does not cause a 10 mph drift. I have also noted that wind is not commonly sustained in a straight-line direction, neither should a atti mode drift.
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