How does control effectiveness change with density altitude?
Some of you test-flight bods may be able to help me here:
I'm playing about with high-altitude flight simulation in X-plane and an external autopilot controller to control the simulated aircraft, but I'm coming up with a problem when tuning the autopilot at high altitudes (>FL500), where the gains that work at lower altitudes are increasingly unstable.
Now, as far as I understand, C_L is a function of alpha with an added component to account for control deflection that is approximately linear with deflection. For level flight, the 1/2.rho.v^2 and wing area components of the lift equation are consistent, even though TAS is through the roof at the higher flight levels. I can't see then why X-plane, which uses some form of physical aerodynamic modelling, would be simulating a higher system gain at higher TAS/Mach/FL. Is there a compressibility effect coming into play here, or have I misread the lift equation?
What's going on here? Anyone wise to this?
Thanks in advance for your help.
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