AUTOPILOT - EXPLAINED! (Part 1/3)
Update v1.0.4 includes a major update to the SAS/Autopilot system.
The following explanations and information are based on the official SR-71A-1 manual.
The general operational principles, modes and way of operation are being simulated in the VSKYLABS SR-71-TB (v1.0.4+), along with a very few nuances, which are explained down in this thread.
Autopilot Operation (REAL SR-71A and VSKYLABS SR-71-TB commentary):
* The VSKYLABS SR-71-TB autopilot mimics the real SR-71A autopilot. A few nuances may be noticeable during operation. These includes separate 'FMS/VNAV' and 'SPEED HOLD' buttons on the RH side of the autopilot panel. The reason is to allow easier operation with the FMS.
** When loading/creating an FMS flight-plan, make sure to enter cruise altitude for the initial leg, before engaging the FMS/VNAV mode.
For more detailed information/guidance, 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.
SR-71A Autopilot - Here we go...
The autopilot (AP) system includes separate pitch and roll autopilots, which together provide control of aircraft attitude about the pitch and roll axes.
The pitch autopilot operates in a pitch attitude-hold mode or in one of two selectable speed control modes, KEAS hold or Mach hold, while also providing automatic pitch trim.
The roll autopilot operates in a roll attitude-hold mode or in a selectable heading-hold mode or an auto-steering (AUTO NAV) mode. Autopilot output signal strength is scheduled as a function of altitude and airspeed by inputs from the ADS.
IMPORTANT: Pitch and roll autopilots are enabled by operation of PITCH and ROLL engage switches on the AFCS control panel. Both autopilots affect operation of only the elevon control surfaces.
Engagement of an autopilot causes the related autopilot command signal (pitch or roll) to be summed with the related pitch SAS or roll SAS command signal. Combined SAS and autopilot commands, applied to the inboard elevon servos, cause the elevons to move and be positioned so as to counter undesired aircraft motion about the pitch and roll axes and to respond to pitch autopilot and roll autopilot commands.
When the pitch autopilot is engaged, the aircraft is retained in the pitch attitude established at the time of engagement.
With MACH HOLD selected, pitch hold is released and the autopilot controls pitch attitude to maintain the Mach number established at the time of Mach-hold engagement.
With KEAS HOLD selected, pitch hold is released and the pitch autopilot maintains the existing airspeed until a KEAS bleed line is intercepted.
KEAS bleed: The autopilot follows a schedule that bleeds off KEAS, from 450 to 380, as Mach increases from 2.6 to 3 plus. In a normal climbing profile, when performed with the autopilot engaged on the KEAS mode at 450 KEAS, once intercepting the Mach 2.6 line, the autopilot will start bleeding KEAS in the following rate: For each Mach +0.1, decrease of 10 knots KEAS.
When the roll autopilot is engaged, the aircraft is retained in the roll attitude established at the time of engagement.
With HEADING HOLD selected, roll hold is released and the autopilot controls roll to maintain the aircraft heading established at time of heading-hold engagement. Heading hold can be engaged with the aircraft in any bank angle up to 50 degrees and, when engaged, causes the aircraft to roll out and capture the engaged heading.
With AUTO NAV selected, roll hold is released and the autopilot controls roll to maintain aircraft flight along a great circle course in response to bank angle steering commands from the ANS (in the VSKYLABS SR-71-TB this mode allows GPSS navigation along a pre-planned FMS flight-plan).
IMPORTANT: The pilot's ATT REF SELECT switch is used to select either the ANS or backup INS as the source of pitch, roll, and heading reference inputs for the autopilot. Only the ANS, when selected, provides a steering command signal for AUTO NAV operation.
Manually operated PITCH and TURN (roll) wheels, on the AFCS control panel, permit changes in pitch and roll attitude during autopilot engagement. Control stick inputs cause only momentary departure from engaged pitch or roll attitude unless the CSC/NWS (control stick command/nose wheel steering) switch is pressed on the control stick grip.
Pressing the CSC/NWS switch, temporarily disengages the pitch and roll autopilots, allowing the pilot to make attitude changes with the control stick without having to disengage then reengage the autopilots.
REAL-SR-71A: Releasing the switch reengages the autopilots at the new attitude; however, engaged submodes (KEAS HOLD, MACH HOLD, HEADING HOLD, or AUTO NAV) must be reengaged.
VSKYLABS SR-71-TB: The switch works in a toggle mechanism. NWS indication light can be used as a reminder. Pushing the switch again reengages the autopilots at the new attitude; engaged submodes (KEAS HOLD, MACH HOLD, HEADING HOLD, or AUTO NAV) will continue to be reengaged.
Both aircraft (Real one, VSKYLABS): On the ground, operation of the switch allows the rudder pedals to be used for nose wheel steering.
