13. STABILITY AUGMENTATION SYSTEM (SAS)

Early helicopters all had one serious problem.  They were difficult to control, pilots had little time to look at a chart or attend to other cockpit requirements.  The flight controls required continuous but small control inputs, even to fly straight and level.  This problem persisted thru Army production of the H-21, Army & Navy production of the HUP, and other foreign customers.  It was just an accepted condition of helicopter flying.  As opposed to fixed wing aircraft, which will, for the most part, fly straight and level if controls are released.  A helicopter never wants to perform in the same manner.

Our first attempt to correct this problem was the installation of a Sperry A-12 autopilot from a fixed wing aircraft in a HUP-1 helicopter. The installation contained an electronic Joystick mounted on a bracket convenient to the pilot’s right arm.  Severe problems were encountered.  The autopilot had 100% control authority, so that a malfunction could drive a control to the stop faster than the pilot could regain control.  This is not a problem in fixed wing installations since the gain would be set much lower, resulting in slower control movements.  Also the auto pilot would only be turned on in level flight.  Helicopter installations would be needed in hover also.  So much for the A-12 autopilot.

Our next attempt was the installation of a Roll-Rate-Damper in an H-21.  The intelligence for this device was a Rate Gyro which produced a variable strength signal as a function of the helicopter roll rate.  The faster the roll, the stronger the signal.  The signal was processed and directed to a variable link in the control system.  This link would normally move with the control rod so that normal control was maintained.  The link had limited authority, about 20% of total control.  Therefore a hard over malfunction was easily controlled by the pilot since his hand was already on the control stick.  We tested up to 1 full second delay before pilot recovery was initiated.

This system performed very well, the small roll inputs were taken care of by the rate damper, while full control was maintained by the pilot.  The Roll Rate Damper worked so well, we installed the same basic system in pitch and yaw with some modifications.  We provided coordinated turn in the yaw system.  An intentional roll input produced an opposite direction signal, we reversed the signal so that it was in the same direction and provided the proper amount of directional control to coordinate the turn.  The SAS was born.

Some hardware changes were required to make the system fully operational. It had to be dual.  The H-21 flight system used hydraulic boost actuators made by a company called Waterlift.  We added one of the links to each of those actuators.  The second was an extendible link added to the control.  This provided us with Systems #1 and #2.  Both systems operated full time, each at half gain.  A single would do the same, operating at full gain.

The SAS was installed in all subsequent aircraft including the CH-46 Sea Knight and the CH-47 Chinook.  Later Fly-by-wire systems, which are computer controlled, incorporate the stability augmentation in the program.

The first production aircraft to have the SAS installed were for the German Luftwaff.  They already had 20 H-21 Helicopters, this contract would add 5 more equipped with SAS.  This being the first aircraft with SAS and being delivered so far from home caused some concern with the company.  No one in Europe had ever seen the system.  I was assigned to go to Germany at the time of delivery to perform several tasks:  I was to check out the 5 aircraft SAS systems to be sure they functioned properly, and train mechanics at Dornier Aircraft (our German licensee), the mechanics at Bookeburg Airbase (where the aircraft would be assigned), our field reps from Sweden and France, and the sales people from Bad Godesburg.  Most of the training was done thru interpreters, some had been German prisoners of war held in the United States, who had learned English.

Everything went well, except the aircraft were late being delivered.  We were there seven weeks, much longer than expected.  After three years of operation in Germany not a single SAS problem was encountered.

And added fringe benefit on this assignment wa I could take Nancy along.  We had a great time and traveled between Munich and Minden, about 600 miles, three times.  We had a rented VW.  We always made the trip over a weekend.  Good trip but a little long for both of us to be away from the children.

Charles Kessler is a retired flight test engineer for Boeing’s Vertol helicopter division (formerly Piasecki Helicopter Co.). He joined Piesecki in 1947, in the company’s fourth year, and retired from Boeing in 1983. During his 37-year career he took part in the testing of prototypes and alterations of such models as the CH-47 Chinook and Sea Knight, the H-16, HRP-2, and the V-107. He taught the stability augmentation system to the German Luftwaffe. He has written about his experience in a blog called “Early Helicopter Years,” which can be found at http://helicopterstory.blogspot.com/.