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Aviation History
1955
1955 - 0220.PDF
220 FLIGHT, 18 February 1955 AMERICAN FIGHTER AUTOPILOT Lear Announce the L.10 Installation for Supersonic Aircraft : IT is well known that for some years many Americanfighters have been equipped with autopilots for useeither during long transit and operational flights or in connection with automatic fire-control systems. All F-84Gs, for example, have the Lear F-5 for long-distance flights, while sucn aircraft as the F-86D Sabre and F-94C Starfire carry out the final phase of the collision-course attack entirely under autopilot control. The rockets which constitute the offensive armament of the two last-named aircraft are launched by the fire-control system. The fighter-type aircraft so equipped have hitherto presentedtheir autopilots with comparatively simple aerodynamic problems. In supersonic aircraft, however, a number of additional factorsarise, such as the supersonic fighter's great sensitivity, in terms of fore-and-aft stability, to speed changes. It may even becomenecessary, in order to obtain sufficient accuracy of flight manoeuvre, to fly the aircraft at second hand, so to speak, through an autopilotrather than under direct human guidance. One method of achiev- ing this is by making the control for the autopilot an integral partof the stick. The pilot then selects an autopilot-controlled manoeuvre through the stick rather than by manipulating theknobs on the standard controller. The Lear L-10 is a new autopilot, just out of the test stage,which has been designed specifically to meet conditions such as those outlined above. One of its most valuable features, incident-ally, is the fact that it is almost directly developed from its predecessor, the F-5; the proven reliability of many of the F-5'scomponents, and the existing production facilities, are certain to be of great assistance in getting the new equipment into service.The L-10 has so far been tested in an F-84E and is shortly to be installed in an F-86D Sabre and an F-84F Thunderstreak forevaluation by the U.S.A.F. The standard F-84F autopilot is at present the Lear MB-2, which the company turned out on a pro-duction basis only three weeks after the initial contract was placed. The MB-2 has been referred to as an "L-7^" (the developmentmid-point between F-5 and L-10) and it is to be further tested in a B-57 Canberra and a reconnaissance version of the F-84F. As the accompanying diagram shows, the L-10 includes severalnew features, such as an automatic Mach number control or "Mach hold," an automatic g-limiter, all-attitude manoeuvrability,automatic heading selection, and control through either a conven- Schematic layout of the L-l(ys components, showing the distribution of its controlling, executive, reference and guidance sections. Either the knob controller or stick top director, or both, can be installed, and the navigation, fire-control or bombing systems would be incorporated as required for particular applications. PILOT REMOTE VERTICAL AND ALL- ATTITUDE REFERENCE SYSTEM REMOTE DIRECTIONAL GYRO AND COMPASS SYSTEM RATE GYROS (ROLL,PITCH AND YAW) | SIDE-SLIP SENSOR DYNAMIC PRESSURE TRANSDUCER THREE-AXIS AMPLIFIER CONTROL (CENTRAL COMPONENT) VORAND ILS RADIO EQUIPMENT HYDRAULIC SERVOS (SERIES PARALLEU ROTARY SERVOS AND FOLLOW-UPS EXTENSIBLE-LINK SERVOS (WITH INTEGRAL FOLLOW-UPS) TRIM ACTUATORS (MANUAL AND AUTOMATIC) AIRCRAFT tional panel or by articulated stick-top. Continuous automatictrimming for all surfaces is provided, together with a sideslip eliminator. The latter involves a new principle of autopilotoperation which is described below. The central component or "brain" of the L-10 is the mainamplifier which, as in the F-5, is assembled on the "building- block" principle with plug-in amplifier units. One of its featuresis that it provides more gain than before (about ten times as much as the F-5), resulting in increased accuracy of control-surfacepositioning. Several other design-points lead to faster control- response, and speed of reaction in such a system is a valuableadjunct in the control of supersonic aircraft. Continuous stability- augmentation by the extensible-link servos in the L-10, and auto-matic damping of lateral and longitudinal control modes, even when the autopilot is disengaged, result in a stability in speed andattitude which is most useful to the pilot. Moreover, he cannot feel the damping action on the controls and no separate electronicdamping systems are required. Another feature of the L-10 is the fact that it can operate froma variety of types of remote vertical gyro and close-tolerance gyro direction-indicators or from three-gyro stable platforms—a sig-nificant step towards the integration of systems which make use of remote attitude-gyros. This is a real problem in many Americanaircraft, particularly in intercepters, where there may be as many as five separate vertical gyros serving such systems as panel instru-ments, flight directors, fire-control computors, autopilot and radar aerial stabilizer. If any one of these vertical gyros should fail, avital link in a chain of systems would become inoperative. The L-10 is designed to be able to take its attitude and directionalsignals from the panel instruments supplying the relevant infor- mation. Moreover, Lear has developed a series of stable platformsdesigned to act as master references for all types of fire-control and bombing systems, as well as for instrument-panel indications.The stable platform, which has no manoeuvre limitations (it is fully free in all axes), could be conveniently duplicated to providea stand-by; it has even been mooted as a suitable master reference for such exacting applications as V.T.O. instrumentation systems.The master-gyro concept has, of course, wide implications in the field of aircraft instrumentation and automatic control in general.One way in which the L-10 differs from a great number of the present autopilots is in its method of achieving directionalcontrol. The L-10's rudder control is relegated largely to the role of turn co-ordination and side-slip correction—the latterespecially in case of asymmetric power or external load conditions. For course control the L-10 feeds azimuth error, detected by thecompass, to the roll servos rather than to the yaw servos (under certain conditions a proportion of the course signal is fed to therudder as well). The principal course control is, however, achieved by banking the aircraft. This conforms both to theflight characteristics of jet aircraft in general and to those of very-high-speed machines. Full co-ordina-' .. • - tion of the turn is assured by a rate gyro orientated in yaw and by a side-slip detector, both controllingrudder positioning. This type of control is particu- larly important when the autopilot is being directedby automatic guidance systems either for interception or bombing. Consistent response for both attitude and tradecontrol is accomplished by automatic variation of the system gains with varying flight conditions. It isalso possible to adjust gain-variation to suit the characteristics of various types of aircraft. The L-10 is designed to accommodate a newline of electronically-controlled Lear hydraulic servos consisting of two different types—firstly,actuators set to apply output in parallel with manual effort, and secondly, actuators whose output is intro-duced differentially into the control system to give stability augmentation. An aircraft can be flown with this autopilot throughalmost any type of manoeuvre and for this purpose there are, as already briefly mentioned, two alterna-tive methods of directing the autopilot. One is by the conventional type of control panel, and the otheris through a force-control lever which is substituted for the handgrip of a conventional control column.If, using this latter type of controller, the pilot ' wishes to turn, he applies the normal pressure on thestick and the aircraft will begin to roll. When he has FIRE-CONTROL, BOMBING AND VECTORING SYSTEMS ALTITUDE CONTROLLER MACH HOLD (SERVO AND AMPLIFIER)
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