Definitely NOT just the XR2. I can't change the reference AoA within the stock DeltaGlider either.
I doubt you'll have much luck using AerobrakeMFD's Attitude hold for an inverted re-entry - you'll likely get a lot a lot of "wobble" that get's progressively worse as dynamic pressure increases.
If you give the COG shift a couple quick taps to move the COG towards the rear, elevator trim can be used to maintain AoA while inverted. An example can be found in the Home Direct tutorial found at OH (this tutorial is also included in the IMFD Full Manual/Playbacks tutorial, also at OH. While the tutorial uses IMFD for a Moon-Earth transfer, you can still use the flight recording without having IMFD installed.
Thanks for the tip about the COG.
However, I can obtain satisfactory results with the AoA control loop. If you didn't know, the controller is a PID controller whose parameters you can specify in a config file somewhere. Since I have taken courses in feedback control, it's not hard for me to figure out the right parameters.
For anyone who's interested in the PID tuning, the plant transfer function is of the form G(s) = A/s^2, and you really only need PD control D(s) = K(T_d*s+1). So, K_p = K, and K_d = K*T_d. Obviously, you are placing a zero at -1/T_d which pulls the two poles at 0 back into stability. However, be aware that the farther the zero is placed in the left-half plane, the larger the gain K you will need for stability, which is bad for two reasons: 1) disturbance inputs and 2) Uses more fuel.
