Stability Augmentation System (SAS)

While there is no X-15-type stability augmentation system available in Microsoft® Flight Simulator X or Lockheed Martin® Prepar3D®, we thought it would be a good idea to provide our users with the basic autopilot modes offered in the simulator instead. The X-15A-2 SE addon contains a modified SAS panel [48, fig. 5-1] with the basic autopilot controls for altitude hold, wing leveling, attitude hold and yaw damping modes. These controls are described in the "Stability Augmentation System Controls and Indicators" section.

Known Issue

We noticed that some of the simulator's basic autopilot functions such as "wing leveler" and "altitude hold", when engaged, induce unwanted oscillations at very high speed and altitude (instead of helping the pilot). Unfortunately, we have no other solution at this time but to recommend disengaging these functions when such behavior is encountered until a fix is found. Note that this is no longer an issue in Prepar3D® v4.

SAS Description

On a real mission, at a Mach number of about 3.0 and an angle-of-attack greater than 10°, the X-15 was uncontrollable without damping when normal piloting techniques were used. The stability augmentation system (SAS) [48, fig. 5-1], installed in the real aircraft, provides damping inputs to the aerodynamic flight control system about the pitch, roll, and yaw axes. Major components of the system are a three-axis gyro, an electronic case assembly, two pitch-roll servo cylinders, one yaw servo cylinder, and a gain selector switch assembly. In-flight testing of the SAS channels can be done by use of an in-flight test unit controlled from the cockpit [7, fig. 5-2].

The SAS has fundamentally three semi-independent channels, each comprising a working circuit, a monitor circuit, and a malfunction detector. Each working circuit received its commands from the gyro assembly in its particular axis and in turn commands associated servo cylinder displacement. The monitor circuits receive commands from the gyro assembly identical to those of the control circuits. The malfunction detectors compare the commands passing through the monitor circuits with the associated servo cylinder displacement. If a predetermined error between the working and monitor circuits is exceeded, the malfunction detector locks out the particular channel in which the error occurred. The working circuit command to the associated servo cylinders is an electrical signal which drives an electro-hydraulic transfer valve on the servo cylinder. The transfer valve controls hydraulic pressure on each side of the servo cylinder piston.

The pitch-roll servo cylinders, powered by the No. 2 hydraulic system, are mechanically linked to the horizontal stabilizer [1, fig. 3-1] control linkage by mixer bell cranks and move surfaces by simultaneous movement of the bell cranks. Roll is achieved by differential movement of the bell cranks. In addition, loss of No. 2 hydraulic system pressure will automatically engage the SAS emergency hydraulic system. Should an abrupt loss of No. 2 system pressure occur, the pitch and roll damping channels may trip. If this occurs, they must be reset.

The yaw servo cylinder, powered by the No. 1 hydraulic system, is mechanically linked to the vertical stabilizer [4, fig. 3-1] control linkage through a mixer bell crank and moves surfaces by movement of the bell crank.

An interaction of the yaw and toll damping working circuits is provided whereby signals from the yaw axis of the gyro are fed into the roll circuit to augment roll damping. This is referred to as the "yar" function.

The SAS (and the addon's basic autopilot) is powered from the No. 2 primary AC bus and the primary DC bus.

Important

The avionics must be turned on for the autopilot to engage. To turn on the avionics, turn the radio on by moving the radio function selector switch [1, fig. 5-8] to the right. If the radio function selector switch is set to OFF, the avionics is turned off and the autopilot is disengaged. Refer to "Radio Communication and ADF Controls" for more details.

The SAS circuit breakers [4, 7 and 8, fig. 5-9] must be closed (pushed in) before the airplane is fueled, and must be kept closed until the end of the flight, so that the SAS gyro heaters will be energized to prevent freezing damage to the gyro.

In this section:

Stability Augmentation System Controls and Indicators