Airspeed and pitch correlated


I was testing my airspeed sensor installation by tapping my finger on the tube as advised. The airspeed increased and the variance decreased as expected. But each time I put my finger on the tube, the artificial horizon showed that the nose of the plane dipped, even though it was firmly on the table. This seems to happen with and without GPS lock, with and without GPS connected to APM. But it does not happen if I uncheck the box "use airspeed". Then the airspeed reading behaves as expected but no false estimation of pitch. It happens in manual and stabilize modes, I have not tried other modes.

3691094219?profile=originalThis is AP 2.76 running on HKpilot board, connected to a laptop using USB, JP1 removed. No other power supplied.

What could be the reason?

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  • Thanks Tridge!

    I was trying this indoors, and sometimes had a 3D lock, sometimes did not have a lock at all (as in the picture). Clearly I need to do this outside with a 3D lock to make sure that it goes away. 


  • Developer

    You should find this only happens when you don't have GPS lock.

    The reason it happens is that the APM needs a way to estimate inertial forces on the airframe to correct the accelerometers. When we have good GPS lock (at least 6 satellites and 3D lock) then we use the GPS velocity to do the corrections. If we don't have good GPS lock then we use the cross product of airspeed with the gyros, which gives the affect you are seeing.

    Cheers, Tridge

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