On the Altitude loss in forward flight from aerodynamic pressure bubble.

Hi Folks.

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?

cheers 

Jim

htps://www.youtube.com/watch?v=Vn-Fg53Or4c

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Thanks.

i defently have this issue with my tarot hexa and 680pro cannopy. interesting stuff. im gonna block some unused conection openings in pixhawk and see if that make´s any difference. After that, if necessary, change pid on alt hold and see if that help.

Thanks. 

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