Additional insights and notes regarding operation in X-Plane 12 vs Real-World:
(the following was written as a reply in the VSKYLABS SR-71-TB support forum at the .org)
Auto-throttle (no...):
There is no auto-throttle in the SR-71, and the out-of-the-box capability of the VSKYLABS SR-71-TB should not allow you to utilize this feature.
One can always experiment with direct assignments to various modes that does not exist in the given panel, but the results may be unexpected or 'buggy'; If you are overriding the auto-throttle or engaging this from an other end - then unwanted stuff will occur such as the use of after-burner to maintain speed. Other implications may occur such as unintentional engines shutoff, since the SR-71 throttle will go to the 'cut-off' virtual detent once the autopilot will try to set it to idle, in some conditions. The VNAV option uses *only* the nose-pitch axis.
up/down sway when engaging the autopilot:
As advised in the manual, you must engage the autopilot in a well-trimmed aircraft. Including acceleration (we forget that trim is for the 'z' axis as well, with speed, especially if you are accelerating). If you have a pitch-sway, simply hit the CSC/NSW button and stabilize the aircraft. Then, re-engage it. Like the real SR-71, the VSKYLABD SR-71-TB incorporates an auto pitch-trim loads so it should smooth the sway almost instantly.
Pitch-prediction:
The SR-71 autopilot envelope is extremely wide; it controls the aircraft from low-speeds to Mach 3.5. Setting it to lower constant might look the right thing to do to cope with the aspects mentioned above, however it reduces the VNAV capability to maintain climb-outs and descent via the FMS.
Pitch-wheel feels backwards:
When you are engaging the SR-71 autopilot in the '1st level' (that is, engaging the autopilot without any other secondary modes), the autopilot pitch trim-wheel works intuitively. That is - you scroll back, the nose goes up and vice versa. However, if you are flying in KEAS/MACH hold mode being engaged, since the aircraft is not equipped with auto-throttle, the pitch trim wheel is not used, as the autopilot 'records' the speed at the moment of engagement.
What happens in the VSKYLABS SR-71-TB is the same; the autopilot records the speed at the moment of engagement. However, the pitch trim wheel is not disabled nor will override (disconnect) the speed hold mode. What it does is as follows:
It works as a speed-bug, allowing you to *increase speed* (scrolling 'up' - that is 'forward') or to decrease speed (or scrolling 'down' - that is 'backwards'). This mode is experimental and was used extensively during development (if you go to full-screen HUD mode you will notice the speed-bug moving accordingly).
PS: what happens with the roll-wheel when you are in heading mode is quite similar - in the real SR-71 you can't set a new heading once the autopilot was engaged in a certain heading. The VSKYLABS SR-71-TB roll-wheel would not work in the roll axis, once you are flying using the heading-hold mode.
Back to the SR-71 (both real and VSKYLABS) - if you wish to change your speed during the speed-hold engagement: you can simply push the CSC/NSW button, then fly the aircraft to the new speed, then depress the button and the new speed will be set. Alternatively, you can disengage the speed-hold mode, set the aircraft to a new speed by flying the autopilot with the pitch trim (scrolling back will get the nose up and vice versa, as explained above), then depress the speed-hold mode.
Some final words:
In practice, the VSKYLABS SR-71-TB has been flown dozens (literally, to say the least) full mission profiles that includes 'traditional' FMS/GPSS flights, as well as full mission profiles to Mach 3.2 - 3.5 at above 80,000 feet.
The FMS integration is not fully suitable to hold a fully automatic mission profile, as it has its limitations, including modes intervention (like KEAS bleed which is triggered automatically once intercepting the bleed speed line).
The best practice that was found to work exceptionally well is the one which is directed in the SR-71A manual (both in Section I and Section II Normal Procedure).
The SR-71 autopilot, in many ways, reminds me of the old Sperry autopilot (to be found in the DC-3/C-47). It doesn't support heading change (the VSKYLABS have this mode available out of extending the usability) and the state of mind of the pilot does not include the use of 'FMS' in the light of modern fully capable autopilots.
On the personal level - I think that for the old-fashioned pilots (or the ones who are not flying modern airliners using fully automated flights), it feels more natural. My personal flying experience involves the use of really 'old' autopilots as well as modern FMS equipped avionics. It might take a while to adjust but once you get the hang of it, then it really works flawlessly.
In general - keeping some 'extended' features available may be confusing (for example the use of the pitch trim wheel during speed-hold mode). This is still under evaluation.
