ATTITUDE VS. RATE STABILIZATION FOR HELICOPTERS
(IS ATTITUDE STABILIZATION REALLY A SAS?)
Depending on one’s background, there are varying opinions of what constitutes a stability augmentation system or SAS for helicopters. Early rotorcraft SAS systems consisted of feedback of pitch, roll, and yaw angular rates to limited authority series servo-actuators. The series servo is essentially an extensible link that is inserted in series with the flight control system. It is typically limited to 10% or less authority so as to prevent excessive excursions in the event of a hardover failure.
This mechanization has been so historically common, that there is a misconception that it defines a helicopter SAS. The more modern definition of a SAS is any stability augmentation that allows the pilot to fly through the system. For example, the widely accepted Military Flying Qualities Specification for Military Helicopters (ADS-33E) requires an attitude-command-attitude-hold (ACAH) SAS for flight in degraded visual environments (DVE).
The requirement for ACAH SAS stems from a large body of simulator and in-flight variable stability research conducted by Hoh Aeronautics, Inc. under contract to the U.S. Army Aeroflightdynamics Directorate at the NASA Ames Research Center. That research showed that the old fashioned Rate-based SAS did not reduce pilot workload or enhance safety in degraded visual environments. In contrast, the ACAH SAS provided significant reduction in pilot workload and improved safety in conditions of poor visibility. For the interested reader, these results are summarized in detail in a technical paper given to the American Helicopter Society in 1998. (Hoh Roger, H., “ACAH Augmentation as a Means to Alleviate Spatial Disorientation for Low Speed and Hover in Helicopters” AHS International Meeting on Advanced Rotorcraft and Disaster Relief, 12 – 23 April 1998, Gifu, Japan). The reader is also referred to the Military Handling Qualities Specification for Military Helicopters, ADS-33E-PRF, where ACAH is required in the DVE.
As an example, a spatially disoriented pilot gets no help from a Rate SAS because it is just as happy in a 90 deg bank as in normal flight. With an ACAH SAS the pilot only needs to relax the controls and the helicopter will return to and maintain level attitude flight. Similarly the ACAH allows significantly more divided attention as it will maintain a level attitude while the pilot diverts his or her attention away from aircraft control. The Rate SAS does not provide this benefit as it will continue to pitch or roll with any pressure on the cyclic and will not recover from turbulence upsets.
Another finding from the Army-sponsored research was that it is not necessary to use a series servo to achieve a SAS. Conventional thinking was that a parallel servo causes the cyclic to move and that this would confuse the pilot. However, flight testing showed that if properly harmonized, the cyclic motion due to the SAS does what the pilot expects and is therefore unnoticeable. In order to provide fly-through capability, the parallel servo must be back-drivable. These important findings were used by Hoh Aeronautics, Inc to develop a low-cost lightweight ACAH SAS for the civil market (HeliSASTM). The concept used to create HeliSAS is protected by US Patent 7,108,232 B2.
Because a SAS that is implemented with a series servo must limit the authority to 10% or less, attitude feedback was found to be unsafe. Specifically, attempts to implement an ACAH SAS with a limited authority series servo resulted in the servo hitting the limits, and a sudden loss of stability when maneuvering. This was found to be unacceptable by the test pilot evaluators.
To summarize the definition of a SAS is not limited to a series servo, or to the use of angular rate feedback. The old fashioned Rate-based SAS does not reduce pilot workload and is therefore of little value. On the other hand, the parallel mechanization with attitude feedback provides significantly reduced pilot workload and much improved safety margins in poor visibility at an affordable cost.
The implication of these results is that a SAS that is intended to reduce rotorcraft accidents for operations in degraded visual environments (e.g., EMS accidents) must be ACAH. A Rate-based SAS will not provide the desired safety enhancement.