I'm flying a 450 with APM2.6 running the latest (last, v.3.2.1) firmware. In most scenarios the quad handles superbly, but I do suffer noticeable altitude loss *during* forward flight in Alt controlled modes (principally loiter/PosHold)
During forward flight, my multicopter will loose height steadily over the course of the forward movement until I stop, and then it recovers. I don't have a lot of evidence to share right now about this, but you can see it in this on-board video between 15-40secs. https://www.youtube.com/watch?v=tFrwcHgheY4
In researching this, I've read a lot about the problems of a high-pressure bubble forming which affects altitude solution, due to the barometer reading an ascent and then the FC compensating with a descent.
When viewing examples of this on youtube, it is apparent that the descent happens at the end of the forward flight, as the multicopter corrects. (see Randy's example here: https://www.youtube.com/watch?v=Vn-Fg53Or4c). This isn't exactly what I'm seeing.
My question is, does this look like the same aerodynamic high-pressure bubble issue that has been quite widely discussed, and I should look to those threads for possible mediation, or is it something else that I should investigate further and report back with logs and ground based video?