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Aviation History
1956
1956 - 1681.PDF
THE acceptance in recent years of hydraulic and electric powerto operate the primary control surfaces has quickly beenfollowed by the inclusion of more hydraulic and more elec- tronic mechanism in the primary control circuit. To stabilizethe aircraft in regions of its speed range where the pilot is unable to anticipate and correct yawing and rolling oscillations that occur—particularly under conditions of high speed and high altitude where aerodynamic damping is low—one or more autostabilizersmay be required. Alternatively, a sophisticated autopilot may be used which combines in one source both the function of relievingthe pilot in flying a steady course at a level altitude and supplying low-amplitude short-term stabilizing corrections. Operation of aircraft at very high speeds leaves little margin forany kind of error if the aircraft is to function efficiently in the role for which it was designed. An increasing use of automatic controlis inevitable, not only for the supersonic fighter already largely endowed in this way, but for the civil airliner soon to be treadingon the sonic threshold. From a simple flying aid, the autopilot has become a device which—even if it cannot out-think the pilot—is a better sensing servo-mechanism capable of performing any of his functions more efficiently. The present state of the flying control art is perhaps best illus-trated by considering a physical example. The F-104 Starfighter was designed from its conception (reports our contemporaryAviation Week) to incorporate stability augmentation on all three control axes. It is typical of modern fighters in that autostabiliza-tion is not essential—although convenient—over the greater part of the speed range, and only absolutely necessary in order to takeadvantage of speed capabilities (in the case of the F-104) in excess of Mach 1.5. "Essentially," says Aviation Week, "the stability augmentationsystem [of the Starfighter] is the two inner of the four [servo] loops of an automatic flight control system, being the loop for the controlsurface positioning and body stabilization about its own axis. The two outer loops—attitude stabilization (relation of the aircraft'sposition to earth) and flight control (navigational tie-in)—the autostabilizer manufacturers feel could be added to the currentsystem to provide both stability augmentation and automatic flight control (autopilot), without too great a structural and systems re-vision, and less weight than two separate systems." By projecting the use of autostabilization from the start of thedesign, this advanced aircraft bears the first light stamp of the fully integrated control concept. There is no gainsaying that suchsystems are "just around the corner"; they represent the obvious next step in flying control engineering. The logic of the integratedsystem has been well argued by Mr. R. Hadekel, one of the leading British flying control engineers, in his paper Integrated ControlSystems for Aircraft, which was presented at the Society of Auto- motive Engineers Aeronautic Meeting in Los Angeles last October. "Until recently," said Mr. Hadekel, "the three problems—powered actuation, artificial stabilization, and automatic flight control—were dealt with quite independently. Logically, how-ever, it is obvious enough that they are but different aspects of a single overall problem—that of controlling an aeroplane. Theoptimum solution to most engineering problems is almost always the solution which is dictated by the logic of the problem itself,and in the present case this means a single system in which are vested all the control functions. These functions can be per-formed with the maximum of efficiency and the minimum of hardware."The essentials of an integrated system were explained by the author as being automatic flight control and artificial stabilization.Both begin with sensing instruments which detect any departure of the aircraft from the desired path and the desired state ofuniform oscillation-free motion. These instruments are the essen- tial source of information for any automatic system, and they maybe of many kinds, including gyroscopes, aerodynamic sensors, and radio apparatus. The information produced by these instrumentsis fed to a central control device which is perhaps most aptly described as a computor, and which, on the basis of that informa-tion, determines the control surface movements (and possibly movements of other control organs such as engine throttles) re-quired to damp out oscillations and to maintain the desired path. The computor sends out suitable signals to the actuators whichoperate the control surfaces, which are moved by the desired amount. "So far so good," said the author, "but we have not yet abolished FLIGHT, 30 November 1956 843 A near approach to the fully integrated control system—the Lockheed Starfighter. Black Box Flying Controls Integrated Systems Reviewed the pilot, and he too must be given his say, and allowed to fly theaeroplane when he must, or when he wishes to. Hence he too must be allowed to send signals to the actuators, or, putting itthe other way round, the system must be capable of accepting demand signals from the pilot as well as signals from the instru-ments-computor set-up." Mr. Hadekel put forward three main ways in which this mightbe done. One solution was to arrange the actuator to accept at will either signals from the pilot or signals from the computor. Butthis, he said, was an incomplete solution, since it failed to give artificial stabilization while under the pilot's control. This short-coming could be a considerable embarrassment if manual control were used in that part of the flight envelope where stability aug-mentation was an essential need; but the arrangement was con- sidered by the author to serve as a useful emergency system.More satisfactory integrated systems are those where the actuator is arranged to accept either signals from the computoronly, or signals from the pilot and computor together; or the com- putor may be arranged to accept signals from the pilot whenrequired, and re-interpret them before passing them on to the actuators. If, in the former case, the pilot's signals still travel throughthe computor (which superimposes some stabilizing signals), the difference in these two systems becomes one of degree only.Nevertheless, stated the author, it is quite an important difference. Where the pilot's signals are re-interpreted before being trans-mitted to the actuators, the computor usually passes on the pilot's signals—as expressed by motions of the control column and rudderpedals—not as demands for given motions of the control surfaces but as demands for given manoeuvres of the aircraft. "Theresult," said Mr. Hadekel, "is that the aerodynamics of the air- craft can largely be swamped, and that piloting becomes mucheasier and more accurate"—a state of affairs that has been aptly described as "the autopilot doing most of the thinking for you."The design and functioning of the instruments and computor were not discussed. For present purposes, the lecturer said, theymust be regarded as black boxes which receive certain signals from various instruments [the autopilot, autostabilizer and possibly thefeel system] and also from the pilot via the control column. The signals are interpreted, mixed and sent out as signals demandingcorresponding travels of the actuators. Dealing with the construction of the actuators as determinedby the problem of integration, Mr. Hadekel said: "At the present time there appears to be no tendency to departfrom the established hydraulic cylinder and valve type of actuator [as shown in Fig. 1]. In its simplest form this may consist of amoving cylinder with the valve mounted on it [Fig. 1 (a)], but in Fig. 1A. Moving-cylinder actuator with automatic valve follow-up. FLEXIBLEPIPES INPUT LINKAGE C VALVE Fig. IB. Fixed cylinder actuator with follow-up by differential lever. INPUT LINKAGE JiTv. '.:',.% ./ i.•'-' •
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