As part of our celebration of R&WI's 50th anniversary this year, we are reprinting selected columns by perhaps our most popular writer, rotorcraft aerodynamicist Ray Prouty. At a time when major OEMs and numerous vendors are developing advanced flight control systems for vertical flight, this pre-1993 column presents a vintage summary and perspective on an aspect of them — stability augmentation systems. If you have a favorite that you would like to see again or thoughts to share on Ray's musings, let us know.
Typical series SCAS mechanization
The goal of the helicopter designer is to make the 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 workload and improve pilot performance is well worth it.
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 became 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 swash plate in the right direction to resist the helicopter motion.
If this were done as stated, it would not only resist motions due to costs but would also read cyst 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 a function of stick motion to cancel out the electrical signal coming from the rate 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 something called a series system. The adjacent diagram shows two current ways of mechanizing it: one with an "extendable link" containing a motorized check screw and the other with an 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 the most vivid imagination to predict failures and then figure out ways to keep those failures from being catastrophic. One way is to limit the authority of the SCAS actuator 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-degree-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 redundant 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 it centered position, where it has maximum travel available to compensate for costs or for maneuvering in either direction.
While electronic stability augmentation systems for helicopters were being developed many 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 the nearby diagram shows a typical schematic for such an installation. For maneuvering, some autopilots are turned 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. The decision as to whether it should be a serious system with no cockpit control motion or a parallel system with motion has not yet been settled – since both types are in operation.
Typical parallel autopilot mechanization
In some systems, the onboard instrumentation is combined with radio navigation and landing equipment to operate flight director displays to tell the pilot how to move controls to achieve the desired result. In more advanced concepts, the signals go to 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. These, too, may someday be done by using electronic signals generated by cockpit -controlled motion to replace the push rods, bell cranks and cables used on most helicopters. 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."