From the Ray Prouty Archives: Stability Augmentation Systems

By Ray Prouty | January 30, 2018
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The goal of the helicopter designer is to make his aircraft have such good inherent flying qualities that the pilot requires no extra help. The fact that so many helicopters are flying today with just a pilot in control demonstrates that the goal can be reached — if expectations are modest. For many applications, however, such as flying in reduced visibility or while the pilot has other critical chores, the cost of providing the extra help to reduce pilot workload and improve his performance is well worth it.

Stability augmentation

Early stability augmentation systems (SAS) were primarily mechanical — such as the Bell stabilizer bar, the Hiller servo rotor and the Lockheed gyro. As electronic/hydraulic/mechanical systems become smaller and more reliable, they took over the job — primarily because of their weight advantage and the ease with which other tasks, such as navigation, could be added to their capabilities.


The primary use of the SAS equipment is still to improve the flying qualities by damping the pitch and roll motions caused by gusts — just as the earlier systems did. This is done by installing small gyros to generate electrical signals proportional to pitch and roll rates. These signals are then used to control hydraulic or electrical actuators that tilt the swashplate in the right direction to resist the helicopter motion.

If this were done as stated, it would not only resist motions due to gusts but would also resist motions due to deliberate pilot-control motions. This would result in a helicopter with sluggish control response. To prevent this, the system must be designed to distinguish between the two types of inputs.

In many modern systems, this is done with a command augmentation system (CAS or, as a combined system, SCAS). The CAS uses an electrical signal, which is as function of stick motion to cancel out the electrical signal coming from the gyros. Thus, the response to control inputs can be made as snappy as you want — while still providing damping to inadvertent gust disturbances. Of course, a similar system can be installed in the tail-rotor control system to improve directional stability and control characteristics.

If the system is used primarily for stabilization, it is usually set up as doing its thing without moving the cockpit controls. This is sometimes called a "series system," and Figure 60-1 shows two current ways of mechanizing it: one with an “extendable link” containing a motorized jack screw and the other with a SCAS electro-hydraulic valve on the hydraulic power actuator.

Since no system that relies on electrical signals can be considered wholly foolproof, the designer must use his most vivid imagination to predict failures and then figure out way s to keep those failures from being catastrophic. One way is to limit the authority of the SCAS actuator to 10% or 20% of full control throw That way, if a gyro unit suddenly puts out a hard over signal corresponding to a 1,000 deg per second rate, the resulting control motion is limited and can be overridden by the pilot.

Another scheme is to use dual or even triply independent systems so that the failure of one can be at least partially compensated for by the continuing operation of the other(s).

There are conditions that, if permitted, might use up all of the limited actuator authority. These include pitch rates over a long time, as in a steady turn, and semi-permanent changes in control positions reflecting changed flight conditions To avoid actuator saturation due to these effects, both SAS and CAS signals are “washed out” over a period of 2 to 20 seconds to permit the actuator to drift back to its centered position where it has maid-mum travel available to compensate for gusts or for maneuvering in either direction.

Expanding the system

While electronic stability augmentation systems for helicopters were being developed about 20 years ago, the airplane people were refining their autopilots, which had originally been developed for a somewhat different purpose: to hold course, altitude and speed while flying from one point to another by using signals from attitude gyros, altimeters and airspeed systems. Most of these autopilots moved the cockpit controls as well as the aerodynamic control surfaces, so that they were most effective when the pilot flew hands-off, or at least did not resist the control motion. This is called a “parallel system,” and Figure 60-2 is a typical schematic for such an installation. For maneuvering, some autopilots are tuned off — either manually or with pressure-sensitive switches.

Once it was accepted that electric signals from rate gyros could be used [for] helicopter stability augmentation, it was not much of a step to include the autopilot functions and to develop an automatic flight control system (AFCS). The decision as to whether it should be a series system with no cockpit control motion or a parallel system with motion has not yet been settled — since both types are in operation today.

In some systems, the onboard instrumentation is combined with radio navigation and landing equipment to operate flight director displays to tell the pilot ho to move his controls to achieve the desired result. In more advanced concepts, the signals go into a computer and then directly to the controls. The pilot’s duty becomes primarily one of monitoring that other “pilot”.

The use of a computer makes it possible to combine the signals from the various sensors to provide the best possible flying qualities in various flight conditions. This makes it a “fly-by-wire” system for all functions, except for full-authority pilot inputs. And these, too, may some day be done by using electronic signals generated by cockpit-control motion to replace the push rods, bell cranks and cables used on most helicopters today. This scheme promises some weight saving but will have to be 101% reliable. I heard one pilot say, “I am willing to accept fly-by-wire as long as the wires go up the center of sturdy control rods.”

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