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Aviation History
1957
1957 - 0485.PDF
FLIGHT, 12 April 1957 Autopilot to Flight-control System . . . 487 elevator control only, since the requirement was largely for steeringand height-holding. For this purpose a single inclined-axis dis- placement gyro was coupled to two rate-gyros, one for pitch axisand one for yaw. An electrical precession control on the displace- ment gyro was applied from a compass pick-off, and a mechanicallinkage joined the barometric altitude control to the displacement- gyro frame. Thus the autopilot could be set to put the weaponon a fixed compass heading and to make it climb to and hold a preset height. Until the advent of the more sophisticated, precision aircraft-control systems of the later war years and the post-war period, and the advent of jet propulsion, military autopilots had in effectcombined both short-term stabilization about one or more axes and long-term steering and pitch control as dictated by aero-dynamic characteristics and the need to relieve pilot fatigue. Prac- tically all manual flying controls had up till then been purelymechanical with no servo- or power-assistance. Jet propulsion brought much higher speeds and a substantialincrease in short-term instability. These factors, allied with the much higher stick-forces required to move the control surfaces,radically altered the requirements and introduced the need both for power assistance in the manual control channels and for auto-stabilization in yaw, in pitch and (later) in roll. Requirements expanded until present Service high-speed aircraft invariably havea hydraulically-powered control system for all three axes, with servos providing all power for surface movement, while artificial"feel" is applied to the control column. Separate auto-stabilizer detectors and servos provide short-term stabilization. The engin-eering of these manual powered-control and auto-stabilizer systems has been handled by aircraft constructors themselves, and by suchfirms as Automotive Products ((Lockheed Servodyne), Messier, Hobson and Elliott Brothers. Fairey and Boulton Paul, as con-structors, have also offered their systems to others. In fighter development up to the present day, it has becomeincreasingly apparent that the pilots not only need powered- control assistance and short-term auto-stabilization, but also someform of completely automatic control, especially for weapon- system handling; and here a direct comparison may usefully bedrawn with the guided missile. It has frequently been stressed that the human pilot's greatest asset is the power of judgment for whichno economic and efficient machine substitute has yet been devised. In the missile the human command is applied from outside, eitherby presetting or programming of the automatic control or by continuous radio (or other) control. The German V.2 is an earlyexample of both these methods. In the first models, remote radio control was used to cut off propellant supply for ranging, whilelater models carried an integrating gyroscopic accelerometer to provide a programmed or pre-set range-control. In the manned fighter the human command is, of course, appliedby the pilot, but he requires as much mechanical aid as possible to give him the necessary time to decide his course of action. Thusthe "weapon concept" of today, applied to a manned fighter, demands the greatest possible degree of integration and auto-maticity, particularly in the aircraft/powerplant and weapon- system combination, with the human pilot as the overridingmonitor. The pilot needs information to enable him to carry out the monitoring function, and integration of instruments thereforeassumes vital importance. But before elaborating on this theme of complete integration itwould be worth while to continue the story of the control func- tion. The pace of development in the last few years has been sofast, and the specialized problems presented to the systems de- signers so increasingly complex, that history has tended to repeatitself once again. Each new requirement or aid has been developed on its own with ever-narrowing specialization in each aspect orfunction. The aircraft designer has had to build bigger and better aircraft. The powerplant designer has had to achieve ever-higherthrusts to carry more and more equipment required to operate the bigger and better aircraft. For example, in the control functionalone, manual powered-control required its own specialized hydraulic valves, artificial feel units and actuators; the auto-stabilizer needed its own gyroscopic and accelerometer sensors, amplifiers and actuators; the autopilot needed its own gyro andother references, amplifiers and actuators or servos; and all had their own adequate duplication to ensure reliability and safety.One particular aircraft carried no fewer than seventeen separate actuators or servos to operate its control surfaces. Somebody at this stage must have recalled the advice of the lateMr. William Stout, the distinguished American designer, and decided that it was about time to "simplicate and add more light-ness." It has become apparent that a much greater degree of integration of all control functions will in future be demanded inorder to reduce unnecessary duplication. The result has been the establishment of co-ordinated teams such as the Elliott-Hobsonconsortium, Smiths and Messier collaboration, and Sperry's in- tegrated centrol-system project; and there is much closer liaison Autopilots assisted in the flying of the Empire air routes with the Empire flying-boats. The photograph shows "Cavalier" on the water at Bermuda. between aircraft constructors' own systems-design teams and thespecialist gyroscopic, hydraulic and electronic designers and manufacturers. Displacement first- and second-derivative sensers with elec-trical and mechanical signalling and hydraulic actuation form the commonest combination for such joint projects. Integrated controlsystems are being designed in such a way that they can (and, indeed, must) be capable of still further integration with thepowerplant control, radio, radar, automatic navigation, weapon systems and pilot's instrument display. In America similar lines of development are being followed. TheLockheed F-104, for example, is an advanced illustration of this trend, while, a year or two earlier, an F-86D Sabre in Korea usedan advanced weapon-control system with a substantial degree of integration and achieved considerable success. With a U.S. Armymajor as sole occupant, it shot down a Mig-15. Of the F-104 it has been said that, if a failure occurs in one of the three axes of theautomatic control system, the pilot might just be able to get the aircraft down on the ground in one piece. If two axes fail hebales out. On the subject of present high-performance aircraft controls, itis of interest to quote the American Mr. William Lear, who is well known for his part in the development of such systems. Hesaid that in the early days "the pilot placed [the autopilot] in operation cautiously, without confidence that it would work satis-factorily. A few years hence some particularly daring pilot may even more timidly turn off his automatic flight control systemjust in order to see what will happen." For military aircraft of the V-bomber, B-52 and B-58 type, diereis a similarity of basic requirements, as far as the control function is concerned. But to give a clear picture it is advisable to tracedevelopment from the end of World War 2 because, for obvious reasons, certain characteristics and influences differ substantiallyfrom those of fighters. There has, of course, always been a very close parallel in development between military and civil heavyaircraft, if only because many of the successful airliners of today owe their existence to the financial resources made available to theaircraft industries on both sides of the Atlantic by governments with predominant interest in defence. While the bomber requiresa control system to make it a stable platform in the target area, passenger comfort in a civil airliner requires the most stableflight-path possible all the time. Both transport and bomber need as much relief for the pilot as possible on long-range flights andalso the precise cruise control which, over long periods, can be more adequately provided by automatic control. Immediately after the war most civil airliners were adaptedmilitary aircraft; and the autopilots of "civilized" C-47s, Lancas- trians, Yorks, Sunderlands, and Liberators remained in the air-craft. These autopilots were mostly of the Sperry A.3 three-axis, pneumatic/hydraulic displacement type. In addition, the firstspecifically civil designs of the 1945-1948 years were similarly equipped, including the Viking, Bristol Freighter and Wayfarerand the early DC-4s and Constellations. But the possibilities of electrical operation had been increasingly appreciated in militarycircles and the Sperry A-12, Pioneer-Bendix PB-10, the com- pletely new Smiths electric autopilots, the Mk 9 (S.E.P.l) andMk 10 (S.E.P.2), all of them originally designed to military require- ments, were widely adopted. These were much more sophisticated to provide precise controlboth in transit and during autopilot-coupled automatic approaches. The Sperry and Pioneer-Bendix equipments combined displace-ment and rate, the latter information by means of electronic differentiation of the displacement-gyro signals which, after ampli-fication, drove electrical servos coupled to all three control channels. Refinements such as automatic trim-tab control andheight-locks were added. The Smiths team, under the direction of Mr. F. W. Meredith, •"...-.;.,_• f•* -,-!', ,T:V..- 4 ;
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