Different speed with different altitude in position mode

Hi all!
We have a quadrotor without GPS, but it has optical flow + IMU + range finder. We can flight in offboard mode but we found that it flies slower if the altitude is low, even when we use a remote controller and the drone is in position mode.
Anyone knows why?

Here a log if it can help: https://review.px4.io/plot_app?log=e936d712-6b40-4a67-88de-e87f75c3dae6


I know from my experience that at low altitude, part of the power is spent not on movement, but on compensating for the ground effect. So you give full pitch, but the autopilot doesn’t have enough power for the speed you want, or does not allow high speed, so as not to lose stability.

ground effect is not nice :grimacing:

P.S. It is better to analyze the log recorded in offboard mode.

We fixed setting SENS_FLOW_MAXR from 8 to 16.

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