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?

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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?

On my Naza I just increased the vertical gains until it stopped doing that as much. Switching to a 4S battery helped a lot too. I'm not as familiar with adjusting the same in APM though.

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.

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