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Aviation History
1958
1958-1- - 0568.PDF
572 FLIGH1 Automatic Stabilization for Helicopters THE major part of a lecture given before the HelicopterAssociation on October 3 consisted of an exceptionallyinformative description of the A.S.E. (automatic stabiliza- tion equipment) used in the Sikorsky HSS-1N helicopter. EntitledAll-weather Helicopter Systems, the paper was by Walter Gersten- berger, chief of dynamics, Sikorsky Division of United Aircraft.After discussing basic considerations of bad-weather flying for military and civil helicopters, Mr. Gerstenberger turned to theHSS-1 and HSS-1N (U.S. Naval designations for the two versions of the S-58). Two independent sets of powered controls were provided, he said,one operated by an engine-driven hydraulic pump, the other by a rotor- driven pump. D.C. power was furnished by engine-driven and rotor-driven generators, and A.C. was supplied by three inverters, so arranged to take full advantage of the redundancy to provide safe flight duringemergencies. Automatic stabilization had been in operation for several years on theHSS-1 and had proved a good solution to the problem of providing the pilot with a vehicle he could handle easily and without too much depend-ence upon external visual cues. It had a reliability of about one malfunc- tion in 800 flight hours, and it was probable that this reliability couldbe at least doubled. There was little doubt that during the 801st hour the pilot could fly this helicopter on his instruments, without the A.S.E.equipment and at a speed above 40 knots, to a safe destination. A.S.E. was developed by Sikorsky exclusively for helicopter use fromfundamentals first conceived ten years ago. Describing its operation, Mr. Gerstenberger said that whilst the pilot must be the prime authorityin the direction of his vehicle he needed help, in order that his mental and visual capabilities should not be saturated. A.S.E., which introducedstabilization signals without taxing the pilot with the details, was an example of this concept. It permitted certain functions which could bedelegated to automatic control. By limiting the authority of the control to the point where it could still perform its task, yet leave the pilotinherently in full authority at all times (especially during emergencies) a fail-safe condition existed, in which 100 per cent reliability was notrequired for the aid. This might be contrasted to a control which could not be duplicated by the pilot, e.g., the fuel control of a turbine, wherefuel management was so difficult and so closely related to the structural integrity of the turbine that it appeared more feasible to increase thereliability of the automatic control, or rely on several engines, than it is to allow the pilot to control the turbine manually. Incorporation of additional features for improved limited-visibilityflying in the HSS-1N preserved this concept of delegation of limited authority used in the HSS-1, but extended it to the rotor r.p.m. controlin the form of a throttle governor and to the Doppler hover mode with the facility of automatic approach to the hover. The HSS-1N was ananti-submarine helicopter, and the additional facilities provided for round-the-clock operation. Here was an interesting example of a solu-tion to the difficult problem of the approach-to-hover over water with sufficient accuracy to permit the sonar transducer to be lowered into thewater without damage, all this being accomplished with no dependence on outside visual references. The results were from the combined effortsof the U.S. Navy's Bureau of Aeronautics, Naval Air Development Center. Naval Air Test Center, numerous electronic component manu-facturers and Sikorsky Aircraft. Apart from the automatic control equipment, the instrument panel[illustrated below—Ed] has been redesigned to provide additional infor- mation and integrated to provide for a more convenient scan pattern.To the left of the pilot's panel were all instruments for engine monitoring. When everything was normal, all needle positions were vertical. Under-neath these were the switches for selection of alternative electrical power sources. In front of the pilot and co-pilot were the basic flight instruments,and in the centre of this group were the essential instruments needed for going into hover from forward flight, all of them entirely new: thedual-purpose hover indicator, which showed Doppler ground speed or cable angle; the improved radar altimeter, with an expanded scale toshow height above the surface to within two feet; and the remote attitude-indicator, with both roll and pitch-trim adjustments. Instru-ments for power monitoring were to the pilot's right. The motor box was still at the back, between the pilots, but was nowequipped with individual channel-disengagement switches so that any malfunctioning channel could be disconnected without cutting outA.S.E. completely. The cyclic stick had been shortened to give a better view of theinstrument panel, and a new, "beeper" trim-button now allowed the pilot to make small stick-feel corrections without losing his establishedreference. The redesigned grip contained all the familiar switches except the rum-button, which has been removed. The collective-pitch stick had also been changed. The hoist switchwas now on the collective, and did double duty as an emergency inter- phone control. The engine starter button served as a disengage-switchfor the A.S.E. coupler once the engine was started. As before, the A.S.E. panel provided engagement and disengagementfor the A.S.E. pitch, roll and yaw; barometric altitude; automatic hovering using the sonar transducer cable as a reference; and hydrostaticaltitude. It also provided for small heading-trims and corrections for fore-and-aft e.g. trim, and gave a null indication for the servomotorposition. Controls were also provided for two new functions; automatic engine speed, and the seek-and-retain ground-referenced hover coupler.Adjacent was a switch to select between Doppler reference and cable reference and another switch to select between hydrostatic altitude orradar altitude. The lecturer next recapitulated A.S.E. procedure in the earlier HSS-1."When you engaged A.S.E.," he said, "you had pitch, roll and yaw stability. BAR ALT gave you altitude retention. After you lowered thesonar transducer you switched to ASW cable and ASW ALT for auto- matic hover over the cable. The conventional cyclic stick, withoutA.S.E., normally fed through mechanical linkage and a hydraulic servo to the rotor blades. A.S.E. input was introduced by an electric servoworking through a differential link, to make small, limited-authority corrections through the hydraulic servo, to the blades. Electric servoinput was derived from a comparison of signals from the vertical gyro and the stick-position senser. The resulting signal was then amplifiedand fed to the electric servo. Thus, in normal operation, the attitude of the helicopter was stabilized at an attitude determined by the pilot'sstick position. The stick-centering spring was used in conjunction with a magnetic clutch. When the clutch was engaged, the spring providedstick-feel, and a referenced position, if the pilot removed his hand from the stick. Disengaging the clutch allowed the pilot to shift his reference. "Another feature of the HSS-1 was the sonar coupler, which operatedfrom a cable-angle senser. The senser induced a signal intp the coupler which sent a command signal that was introduced into the A.S.E. tomaintain the helicopter in position over the sonar transducer." In the new HSS-1N, said Mr. Gerstenberger, important features hadbeen added in the pitch-and-roll channel. A Doppler ground-speed system had been added and could be selected as the primary sensingfor the coupler. The automatic control circuits of the coupler would bring the helicopter to a hover from any lateral or longitudinal flightcondition. However, in order to make it possible for this control to work through the whole flight regime, it had been necessary to extendthe automatic control authority beyond the limits provided by the servomotor input. The magnetic clutch had therefore been replacedby another actuator so that, when the coupler called for a signal in excess of what could be provided by the differential input, a signal wassent to this trim control to move the stick slowly in the proper direction; this maintained the automatic control within its limited authority. Whenthe coupler was not engaged, the trim control could be used by the pilot to make small adjustments to the stick trim. The altitude channel was basically the same as the pitch channelexcept that the pilot's control was the collective. Altitude errors were detected by a barometric senser, and the coupler was fed either by thehydrostatic depth-senser or by the expanded-scale radar altimeter. In order to accomplish the large power-changes necessary between forwardflight and hover, a collective open-loop-and-balance spring was used across the hydraulic servo, in conjunction with a collective-stick position-senser,to adjust automatically the position of the pilot's control. One of the most important additions to the HSS-1N was the Dopplerground-speed sensing equipment, actually a radar transmitter and receiver. As the Doppler beams struck the surface, the impulses werereflected back to the receiver-antenna on the helicopter. Any motion ; which existed between the helicopter and the reflecting surface, normal4to the radar beam, would cause the received signal to undergo a frequency change. Frequency shifts were compared and longitudinal, lateral andvertical velocities computed. This information was fed to the coupler and the pilot's hovering indicators. In the coupler, they were combinedwith signals from the radar altimeter and other information to bring the helicopter from forward flight to hover at preselected altitude. (Concluded ax foot of opposite page) The HSS-1 N's instrument panel. A, co-pilot's flight instruments; B, navigation; C, powerplant and services; D, pilot's flight instruments.
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