[Flightgear-devel] autopilots, adverse aileron yaw and jsbsim questions

[dave p. <ski...@mi...>]

While optimizing the aitopilot config files for the Century IIB and III 
autopilots for the pa24 and the Altimatic IIIc for the SenecaII, a 
significant difference between the values of parameters (gains in 
particular) that give non oscillatory behavior in yasim and jsbsim 
became very apparent.  I had to completely turn off turbulence to get 
stability without significant overshoot in 
SenecaII/Systems/ALTIMATICIII.xml.  With the values submitted to cvs, 
the Seneca still has a wing rock in LOC REV and LOC modes with the 
default weather that has some turbulence.  The pa24 with the same values 
is as solid as a rock with the default turbulence all the way down the 
glide slope.  With the turbulence zero, both behave the same.

The SenecaII wing rock with light turbulence appears to result from a 
very exaggerated adverse aileron yaw.  So I did the same experiment with 
the c172p and pa28-140 which both use the kap140.  With the default 
turbulence, the c172p oscillates so bad that you cannot complete the 
approach with the LOC needle going full stop to full stop near the 
runway.  The pa28-161 (also yasim) is as solid as a rock all the way 
down the glide slope with light to moderate turbulence.

If you watch the oscillation for either jsbsim model, you should note 
that when the yoke is rotating counter clockwise, the nose is yawing 
right and then finally swings back left, as would be expected with 
extreme adverse aileron yaw.

 Most high performance AC show very little AAY except in significant 
slow flight.  I would not expect that small aileron deflections should 
move the nose counter to the roll in a SenecaII or pa24.

Two questions:
1.  Have others noticed this difference between jsbsim and yasim?
2.  Can this adverse aileron yaw be toned down in jsbsim?

Regards,
Dave Perry