probably already old thread but i'm new to APM
and I had this same issue with NAZA v1:
when i fly in alt-hold, and i push the pitch stick forward to almos the max the quad fly fast but it loses altitude like 1-2 meters, maybe more, but when i stop it, the quad gain altitude very fast, like it was taking a breath :)
and then it stabilizes
my point is, ti seems impossible to do aerial photography following a car for instance, if the altitude drops right?
if you use a canopy, (or anyway, based on the build) - leaning into "wind" generates a lower pressure "behind" - where APM is - this will make the barometric altimeter sense a pressure drop(= indicated altitude increases, and it will try to correct by reducing thrust.) - then you stop, the actual altitude is sensed, and it goes up.
If you want to fly precisely fast or in strong wind, you need to have a frame/chassis that does not influence pressure too much.
interesting... so you suggest to use a canopy? where is the baro sensor on the APM? i do have the APM plastic case anyway... shoudln't be enough?
sorry... or do you mean it's the case/canopy that create this problem and the APM should let be "breathe" outside the case?
depending on the case, connectors used/open - you can imagine that under certain flight angle, air will "blow" into bigger connector openings, giving a pressure increase, or the connectors can be on the rear of the airflow, giving a pressure drop inside the case.
It all depends on the way case,canopy, and frame is designed.
another simple solution is to put tape over any open connector spaces, leaving only a 1mm gap on front and back. - this will minimize the pressure change and air that get "packed" into or sucked out of the case.
of course, reducing autopilots altitude compensation can help, but it's just a workaround, it does this because it
's influenced, otherwise it would do so all the time.
Finally - vibrations can influence Z position too, (make it think it falls/climbs) - check logs for that, but I don't have reason to say that's the problem.