PFD unstable when airspeed is enabled

Hi everyone, This is my first Forum/ APM Experience... I am trying to have my X8 fly with the airspeed sensor but I am having a really strange behavior. 

I have the APM 2.5 directly from 3Drobotics, Came totally assembled with their Airspeed sensor. I loaded the version 2.73 of Arduplane and basicly trew the aircraft in the air after a couple of ground tests. 

On the groundm what bugs me the most is the PFD moving up and down a couple of degrees (+- 5) at about 1hz This happens only when the ''use airspeed'' fonction is enabled otherwise the PFD is rock solid. (no need to mention that the plane is level and not moving. )

The Airspeed sensor is also quite erratic itself when on the ground but I thought no big deal since the sensor usually is not accurate for small pressures diferences. 

Flew OK but there are a lot of strange behaviors in the air as well... First one is that It would slow the throttle down when in downwind... Usually if it would be flying with a true airspeed it should increase throtle to compensate the altitude loss of a more agressive bank turn... Seems tol me like it is in fact flying Ground speed while in Airspeed mode and the reverse is true, It would increase throtle when in downwind if the option is disabled... 

It might sound quite confusing. So I would like to know if there are solution to theses problems : 

1. Why is the PFD is ossiclating when ''use airspeed'' is enabled

2. Is there a way other then the ''use airspeed'' variable to know if the APM is using and thrusting the airspeed sensor. 

Thanks! 

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Replies

  • Developer

    Hi Michael,

    The airspeed being erratic on the ground is normal, as the airspeed goes as square root of pressure difference. It will settle down once you above about 5m/s, and get more and more accurate as it gets faster. Do make sure you calibrate it though, by plotting airspeed and groundspeed from a flight pattern and adjusting ARSPD_RATIO.

    The PFD (which we usually call the HUD in this forum) should only be affected by the airspeed if you don't have GPS lock. Once you have 3D GPS lock the APM will use the GPS velocity numbers to do centripetal correction. If you don't have GPS lock it will use the cross-product of airspeed vector with the gyros, which is why it affects attitude at very low speeds.

    To know why it is lowering the throttle downwind I'd need to see a tlog (telemetry log). If flying with ALT_CTRL_ALG=0 and ARPSD_USE=1 then it should be adjusting throttle to maintain airspeed.

    You might also like to try the new 2.74beta3 release with ALT_CTRL_ALG=2 to use the new TECS speed/height controller. That is a much more sophisticated airspeed controller, and will completely replace the existing controller in a future release. Have a look at the tuning guide for more information.

    Also have a look at the tuning guide for the new attitude controllers. Those include better roll compensation factors.

    If you post a tlog then we can go into a more detailed analysis of the flight.

    Cheers, Tridge

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