Some helicopters incorporate a stability augmentation system (SAS) to help stabilize the helicopter in flight and in a hover. The original purpose and design allowed decreased pilot work load and lessened fatigue. It allowed pilots to place an aircraft at a set attitude to accomplish other tasks or simply stabilize the aircraft for long cross-country flights.
Force trim was a passive system that simply held the cyclic in a position that gave a control force to transitioning airplane pilots who had become accustomed to such control forces. The system uses a magnetic clutch and springs to hold the cyclic control in the position where it was released. The system does not use sensor-based data to make corrections, but rather is used by the pilot to “hold” the cyclic in a desired position. The most basic versions only applies to the cyclic requiring the pilot to continue power and tail rotor inputs. With the force trim on or in use, the pilot can override the system by disengaging the system through the use of a force trim release button or, with greater resistance, can physically manipulate the controls. Some recent basic systems are referred to as attitude retention systems.
Active Augmentation Systems
Actual systems use electric actuators that provide input to the hydraulic servos. These servos receive control commands from a computer that senses external environmental inputs, such as wind and turbulence. SAS complexity varies by manufacturer, but can be as sophisticated as providing three axis stability. That is, computer based inputs adjust attitude, power and aircraft trim for a more stabilized flight.
Once engaged by the pilot, these systems use a multitude of sensors from stabilized gyros to electro-mechanical actuators that provide instantaneous inputs to all flight controls without pilot assistance. As with any other SAS, it may be overridden or disconnected by the pilot at any time. Helicopters with complex Automatic Flight Control Systems (AFCS) and autopilots normally have a trim switch referred to as “beeper trim” or a “coolie hat.” This switch is used when minor changes to the trim setting are desired.
Stability augmentation systems reduce pilot workload by improving basic aircraft control harmony and decreasing disturbances. These systems are very useful when the pilot is required to perform other duties, such as sling loading and search-and-rescue operations. Other inputs such as heading, speed, altitude, and navigation information may be supplied to the computer to form a complete autopilot system.