Problem DGIV AoA Control Issue with Reentry AP and AerobrakeMFD

[PaulG]

I am noticing AoA control differences during reentry between the DGIV autopilot PRO104 and the aerobrake MFD AoA controller.

Assume I have the control mode set to auto on the DGIV. Aerobrake MFD often STRUGGLES to keep the correct AoA deep withing the atmosphere. I often have to have the AoA set AND KillRot on.

If I switch that off and turn on PRO104 (for the same AoA) it responds immediately....and beautifully.

Why is this happening?

Thanks,
Paul

 

[Hielor]

I am noticing AoA control differences during reentry between the DGIV autopilot PRO104 and the aerobrake MFD AoA controller.

Assume I have the control mode set to auto on the DGIV. Aerobrake MFD often STRUGGLES to keep the correct AoA deep withing the atmosphere. I often have to have the AoA set AND KillRot on.

If I switch that off and turn on PRO104 (for the same AoA) it responds immediately....and beautifully.

Why is this happening?

Thanks,
Paul

The DGIV built-in AOA autopilot also uses a CoG offset (I believe) in order to help with the AOA.

 

[Tommy]

AFAIK, the DGIV doesn't use COG shifting, but it's built in AP uses both RCS and airfoil controls while Aerobrake can only use RCS.

Also, I seem to recall hearing that the DGIV's AP could "boost" the RCS beyond it's normal strength, but don't know if that's true.

It doesn't hurt that the DGIV's AP was built specifically for the DGIV, and an external, more general AP like AerobrakeMFD won't be optimised for the DGIV's flight parameters.

 

[PaulG]

I forgot to mention one thing though....if I turn off the aerobrake MFD AoA autopilot, I have a very hard time controlling the AoA manually. The ship tends to pitch downwards, even if I am applying RCS and have the trim set.

But, if I turn on the DGIV autopilot, it immediately pitches properly. So, even forgetting about the AerobrakeMFD autopilot, what is so different between manual control and the DGIV reentry autopilot control modes/surfaces?

 

[Tommy]

In order to manually re-enter a DG-IV you have to have both RCS and Aerodynamic controls active. In 2-D Panel view, turn the control selector to Elev & rudder (not ATM auto). Then in Glass Cockpit mode (use <f8> to switch between the two), click on the ROT box on the upper left of the screen.

 

[PaulG]

Ah yes! I do remember reading this as a "fix" in one other posting. That would explain the problem! Thank you!

That might make aerobrake MFD AoA autopilot work better too! I'll give it a go.