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   Aerodynamic model.
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   Form:
   g = ACAero( alpha, beta, x, d, qBar, control, flex, rho )
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   Inputs
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   alpha                 Angle of attack (deg)
   beta                  Sideslip (deg)
   x                     State
   d                     Aerodynamic data
   qBar                  Dynamic pressure
   control               Controls
   flex                  Flex data structure
   rho                   Atmospheric density

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   Outputs
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   g                   .torque  (3,1) Core torques
                       .force   (3,1) Core forces
                       .modal   (:,1) Modal forces
                       .inertia (3,3) Delta inertia due to aerodynamics
                       .mass    (3,3) Delta mass matrix due to aerodynamics

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