> I flew the rw29R ILS into Jeffco (KBJC) with the SenecaII Altimatic IIIc
> autopilot all the way to the runway slowly pulling back the power so the
> auto trim did the flare out (sort of anyway). Touch down was on the
> center stripe!
>
> But with the no turbulence, you could see the ball move off-center
I think you meant, with turbulence ON. ?
> every time the autopilot applied aileron to roll the AC. This off-center
is
> WAY more than one would see in the real SenecaII. Jsbsim has a very
> noticeable exaggerated adverse aileron yaw issue.
>
> -Dave Perry
"JSBSim" does not have any "adverse aileron yaw issues". JSBSim only models
an aircraft given the data that is fed to it. Let me elaborate. There are
two issues at work:
1) Turbulence is admittedly poorly modeled. Indeed, this has been an
experimental feature for quite a while, and has not gotten the attention it
has needed.
2) Adverse yaw is a function as defined in the aircraft model. You can
either have it, or not. It's up to you. The SenecaII adverse yaw is
described in the SenecaII config file beginning with these lines:
<function name="aero/coefficient/Cnp">
<description>
Yaw moment coefficient due to roll rate(per radian)
Reduces Dutch Roll damping. Positive value desirable.
</description>
<product>
<property>aero/qbar-area</property>
<property>metrics/bw-ft</property>
<property>aero/bi2vel</property>
<property>velocities/p-aero-rad_sec</property>
<table>
<independentVar>aero/alpha-rad</independentVar>
<tableData>
-.1396 .4890E-01
-.1047 .3126E-01
-.6981E-01 .1374E-01
-.3491E-01 -.3595E-02
.000 -.2134E-01
.3491E-01 -.3939E-01
.5236E-01 -.4852E-01
.6981E-01 -.5772E-01
.8727E-01 -.6699E-01
.1047 -.7633E-01
.1222 -.8576E-01
.1396 -.9525E-01
.1571 -.1070
.1745 -.1195
.1920 -.1304
.2094 -.1405
.2443 -.1597
.2793 -.1803
.3142 -.1857
</tableData>
</table>
</product>
</function>
I think it is conceivable that the adverse yaw is too high for the SenecaII.
That's easy to adjust. Add the line noted, below, to your SenecaII file for
the above-seen Cnp definition:
<function name="aero/coefficient/Cnp">
<description>
Yaw moment coefficient due to roll rate(per radian)
Reduces Dutch Roll damping. Positive value desirable.
</description>
<product>
<!-- ################## Add next line ################## -->
<value> 0.30 </value>
<!-- ################################################### -->
<property>aero/qbar-area</property>
<property>metrics/bw-ft</property>
<property>aero/bi2vel</property>
<property>velocities/p-aero-rad_sec</property>
<table>
<independentVar>aero/alpha-rad</independentVar>
<tableData>
-.1396 .4890E-01
-.1047 .3126E-01
-.6981E-01 .1374E-01
-.3491E-01 -.3595E-02
.000 -.2134E-01
.3491E-01 -.3939E-01
.5236E-01 -.4852E-01
.6981E-01 -.5772E-01
.8727E-01 -.6699E-01
.1047 -.7633E-01
.1222 -.8576E-01
.1396 -.9525E-01
.1571 -.1070
.1745 -.1195
.1920 -.1304
.2094 -.1405
.2443 -.1597
.2793 -.1803
.3142 -.1857
</tableData>
</table>
</product>
</function>
The above scale value (0.3) will make the scale factor only 30% of what it
originally was. You can play with that value as much as you want - even
scaling it to zero.
Let me know if that helps.
Jon
Jon S. Berndt
Development Coordinator
JSBSim Project
www.JSBSim.org
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