In this architecture, the three PI loops for pitch, alpha, beta are replaced by a 3-input, 3-
output PI controller that blends the pitch, alpha, and beta measurements to calculate the
inner-loop setpoints p_demand, q_demand, r_demand. Intuitively, this architecture should
be more successful at reducing cross-couplings between axes. Note that the P and I gains
are 3-by-3 matrices scheduled as a function of alpha and beta.
Attitude Control in the HL-20 Autopilot - MIMO Design