Complete missions have been flown already (in X-Plane 12) using *as-much-as-possible* autopilot control (the new, upcoming SR-71 autopilot).
As expected, it requires practice and following of the precise advises which can be found in the SR-71A-1 manual (both in Section I and Section II).
Please refer to the SR-71A-1 manual, Section II, page 2-46: 'Autopilot Operation, Cruise or Climb'.
*pdf page #147 in the VSKYLABS Remastered manual.
Autopilot operation in the real SR-71A was a combination of manual-flying and auto-flying technique. In most of the cases, it required the pilot to stabilize the aircraft at the desired airspeed and/or heading before engaging the speed hold / heading mode (and in general - most flight attitudes).
Once the autopilot was engaged, it required constant pilot awareness and monitoring by using the trim wheels on the AP panel.
The pilot had to anticipate turn entries, and if required, to use manual stick inputs, to avoid excessive roll rates and bank angles during autopilot engagement. In some cases, to increase precision in turns or other course interception it required manual flight correction and stabilization before re-engaging the modes.
One noticeable aspects of the the SR-71 autopilot is the lack of Altitude Hold modes:
No ALT-HOLD, no Vertical speed-HOLD. This alone, increases the pilot workload quite significantly, especially when trying to maintain a fixed *altitude*.
On the practical level, the best practice for maintaining a fixed altitude was to:
- Use the autopilot in its 1st level 'Attitude Mode', where the autopilot trim-wheels provided high precision and responsive pitch correction to maintain any desired attitude, includeing straight and level. Obviously, this method requires continuous monitoring and minor fixes, once the aircraft is stable in power, speed and altitude.
- Use the autopilot speed-hold mode (Mach or KEAS).
Let's talk about the #2 method above:
There is no speed bug for fixing desired Mach / KEAS (!!)
The pilot had to stabilize the aircraft in the desired speed, then to engage the speed-hold modes.
Since the AP tries to maintain speed with pitch attitude, any change in engine power will result with climb or descent. This is tricky.
For example, the aircraft is flying at Mach 0.86, straight and level. The pilot engages the autopilot in the Mach-hold mode:
The 'straight and level' attitude which was set above, was engaged in the given engine power settings. So...when you:
- Throttle up a bit. Extra speed will come-in. The Mach hold will try to maintain Mach 0.86, so it will *Pitch Up* to a new stabilized equation; climbing at Mach 0.86 *for the given, new power*.
- Use the Pitch-up trim-wheel on the AP panel and set a new attitude, without increasing power. The autopilot will "record" the new speed in the new attitude, once stable, as a new Mach-hold number. The aircraft will pitch up or down to a new stabilized equation: For example, climbing at Mach 0.8 (for example), which is a lower Mach number than 0.86.
Sounds easy?...it does...after getting rid of all of the *modern* autopilot habits!
Remember! The SR-71A Autopilot design was 'Mission' oriented, and it works phenomenally during the climb and cruise profiles to 70,000 ft / Mach 3.0 and above!
Depending on your experience, It may take a few flights to realize and understand this method of flying.
Extended usability of the SR-71 autopilot:
On the front panel of the SR-71, just below the VSI indicator, there is small panel titled: Display Mode Selector. In this panel, there is a switch 'ATT REF SELECT - ANS/INS'.
This switch is part of the SR-71 autopilot system (!)
ATT REF SELECT - ANS/INS switch:
- ANS mode: Permits autopilot use of attitude, heading and navigational input from the ANS (Astro-Inertial navigation system) in Auto-Nav mode.
- INS mode: Permits autopilot use of attitude and heading inputs from the INS. Auto-Nav mode is inoperative and cannot be engaged.
The INS mode is similar to a "stupid" NAV mode in old-school autopilot systems; navigating to/from VOR's, ILS etc...
The ANS mode is the *advanced* NAV mode. In the SR-71 it allowed the autopilot to follow a pre-programmed flight-plan (again, with the notes given above regarding the operation of the autopilot..). The VSKYLABS SR-71-TB uses this mode to interact with the FMS flight-plans, allowing the SR-71 to follow a flight plan.
Heading Hold mode (real SR-71):
- The aircraft heading at engagement becomes the reference heading and the autopilot will maintain it.
- The reference heading cannot be changed while heading hold is engaged.
In the real-world SR-71, there wasn't a heading bug which could be managed during autopilot Heading-Hold mode(!)
Real-world SR-71A vs VSKYLABS SR-71-TB autopilot comparison:
Similarities:
- Both have the same 1st level attitude mode: 1st-level autopilot mode which works as you engage the A/P without any secondary modes.
- Both have the same 2nd level modes: Speed hold (KEAS/MACH), Auto NAV, Heading modes.
- Both can follow a pre-programmed flight plans (the VSKYLABS SR-71-TB uses the FMS for this).
- Both required constant monitoring and pilot intervention during engagement of the modes.
- Both have KEAS bleed mode from 450 KEAS /Mach 2.6 to 380 KEAS / Mach 3.2 and above.
Differences:
- VSL SR-71-TB AP can use the heading bug in heading mode for changing the reference heading once it is engaged, while the real SR-71A autopilot could not.
- VSL SR-71-TB AP speed hold mode has a separate engagement switch. Once engaged, the KEAS/MACH toggles the sub-mode.
- VSL SR-71-TB AP has an additional FMS/VNAV push-button. This is due the FMS characteristics for integration.
- VSL SR-71-TB AP CSC button works in toggle mechanism and requires a second press for autopilot reengagement.
More nuances may be noticeable.
SUMMARY: The VSKYLABS SR-71-TB includes the authentic SR-71 autopilot system (update v1.0.4+), which includes a new, authentic panel, and the ability to fly the aircraft as closely as possible to the actual SR-71 autopilot practices!
