A couple months ago I was testing ArduPlane and some of the different modes. I was not getting on very well in FBW-B* mode. If you look at the image below. You will see the purple track. I was loosing altitude and I couldn't understand why. You can see just before the path goes red, I'd lost a lot of altitude and was heading for some pesky trees. I switched to FBW-A and pulled up out of this 'death dive'. :-o
The problem was that FBWB_ELEV_REV was set to default of 0. This means 'up elevator' (pulling back in the elev stick) means go to a lower altitude. Setting this to 1, means that pulling back on the stick will put the plane at a higher elevation. This just makes much more sense to me.
see http://code.google.com/p/ardupilot-mega/wiki/FlightModesFlyByWire (And I will update the doc ;-) )
*I'd stopped using the airspeed sensor and FBW-B requires you have one, so haven't been using this mode since.
Comments
Thanks for the heads up. One more quirk to add to the pre-flight checklist.
It seems holding altitude nice until you changed direction and then you got the decent.
I had the same: barometers are light sensitive, read this The mysterious AltHold problem .... solved
After I stopped the light reaching the baro through the window in the box, I still got soft descends, because the dim light could still reach the baro form the other side. Now I covert the baro on the board with a tiny black plastic cover, and now I have no problems at all.
Hope this helps...