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Aviation History
1949
1949 - 0053.PDF
JANUARY 13TH, 1949 FLIGHT 37 The Modern Autopilot A Dissertation on the Fundamentals of Modern Automatic Pilot Design IN the paper under the above title which Mr. Meredith, B.A.,F.R.Ae.S , is reading before the Royal Aeronautical Soci-ety this evening, the opening and closing paragraphs arerespectively entitled Retrospect and Prospect, and in the former the author discloses that shortly after production at the Royal Aircraft Establishment of the world's first pilotless aircraft in 1925, this country was also the first to produce a flying bomb. Given the code name "Larynx" (referring to long-range gun and identified with a Lynx engine1), this device was designed to carry a 250-lb bomb at a speed of 200 m.p.h. up to a range of 200 miles. The Larynx controls were almost identical with those used some 15 years later in the German V-i. It was appreciated at the time that the practicability of such a weapon was depend- ent upon the development of a cheap engine designed as an expendable store. It was also realized that London was the ideal target for such a weapon and that, as we as a nation were unlikely to be directing an attack against a target of suitable size at suitable range, it was more important to guard the secrecy of the project than to press the development of a suitable engine. The experience from the recent war in which Germany developed these weapons a few years too late to be decisive, vindicates the wisdom of the answer made by the Air Staff to the very nice question posed by this invention. The general requirements of a successful autopilot to-day are given as (i) capability of rapid, correctly co-ordinated manoeuvre; (ii) ability to maintain a compass course indefi- nitely ; (iii) capability of being coupled to radio devices to seek and follow a radio '' track '' and to follow a glide-path beam for blind approach ; (iv) simple and foolproof operation ; (V) safety in the event of any foreseeable failure; (vi) free- dom from frequent maintenance attention; (vii) capability of rapid replacement by units to avoid dislocation of schedules in the event of failure. In addition, Mr. Meredith states that in the near future, the autopilot must also be capable of controlling aircraft for short periods, of the order of one second, with little or no inherent damping and with unforeseeable lags in the transmission sys- tem, including power-operated controls. After reviewing the development of the hysteresis serva motor, an achievement made in the face of established authori- tative theories as to the impracticability of such work, the lecturer devotes some attention to the amplifier and gives it as his opinion that the magnetic amplifier will, before long, be developed to the stage when it can be employed as the sole amplifying means in such equipment as autopilots. So far, however, it does not provide quite the flexibility of a valve amplifier. In the Smith Electric Pilot (S.E.P.i) "of which the author was the designer, valves are employed for signal ampli- fication, ana a magnetic amplifier for the power stage to drive the servo motors, thus permitting all valves to be conserva- tively rated in the interests of reliability. Nevertheless, the valves have proved to be the least reliable element in the whole system. British Laxity v Mr. Meredith then makes the serious comment: "It is un- fortunate that our manufacturers of valves cannot see their way to producing special valves for electronic equipment, requiring a high order of reliability. There is a large field in industry for electronic control if the required standard of relia- bility could be guaranteed. The American valve manufacturers have seen this and are producing special valves for the pur- pose. Unless something is done about it, either the job will be done without valves or the art of electronic control will be in danger of becoming an American monopoly." Under the head of Safety Requirements, the paper states the general agreement that the most important requirement of an automatic pilot to-day is assurance that it will not endanger the structure of the aircraft by dangerous control movements. The possibility of dangerous control movements must be con- sidered under two separate headings (a) false control move- ments resulting from malfunctioning, and (b) dangerous move- ments resulting during correct functioning from disturbances forced on the aircraft by rough air or manoeuvre. Various suggestions have been made for auxiliary safety devices to deal with both contingencies, but the author con- cludes that only strain-gauges carefully placed on the aircraft B,9 structural members expected to initiate failure can really pro-vide a solution on these lines. All aircraft must be designed with reasonable factors forhandling by the human pilot in gusty weather. An autopilot can control with smaller movements than the human pilot.Moreover, considering the load on the tail structure, for example, due to gusts acting on the tail surfaces, it is evi-dent that, in this case, rapid correcting control movements reduce the structure Idatls, Aileron loads are potentially dan-gerous because of the rapid rolling accelerations which can arise in gusts, and because the aileron may fail independentlyof the main wing structure. This last also illustrates the inade- quacy of accelerometer safeguards, since the rolling accelerationmay well be zero, or even in the opposite direction to that caused by the dangerous aileron loads. However difficult the situation may be in relation to legiti-mate autopilot-induced loads, it is manifestly desirable to limit, as far as possible, the risk of dangerous loads induced by mal-functioning of the autopilot. This means more than the aim of making the system as reliable as possible: it means ensuringthat no foreseeable failure will lead to dangerous control move- ments. The A.R.B. have recognized the possibility of meetingthis condition and have made it obligatory in future auto- pilots. These considerations were mainly responsible for the Smithcompany selecting the rate/rate system whereby servo speeds are related to rates of deviation, instead of controlmovements being related to deviations. Stability and Lag After stating the essential differences between rate and dis-placement type gyroscopes, and appraising the fundamentals of rate /rate control, the paper goes on to survey stability and-the effects of lag, and states that, as aircraft increase in size and speed, it is becoming increasingly difficult, for severalreasons, to control them. In the first place, inherent damping is being steadily reduced, especially in yaw. To make mattersworse, we are now faced with the problem of indirect operation of the rudder with the certainty that this will introduce appre-ciable phase lag in the positioning of the rudder. As a prac- tical rule, inertial lag may be neglected if the natural frequencyof the mass concerned is not less than five times the demand frequency. The subsidence time of the yawing momentresponse of. a Flettner rudder to a step function input is easily calculated if the hinge moments due to rudder angle and rudderrate are known, but the effective lag is somewhat increased by the initial negative response. The conventional spring-tab control clearly contains thislag, and it should be noted that the friction of the rudder hinge enhances the rudder movement damping as the ampli-tude is reduced, so that it is difficult to avoid sufficient lag to destroy all damping of the aircraft motion at some smallamplitude. Here, the "set-up " spring tab appears to possess an important advantage over a simple Flettner control, since,for a small amplitude at which hunting might be expected, the tab may be made inoperative. Power-operated controls require close scrutiny in this respect.Non-linear lags such as the effect of friction are particularly objectionable because they are so unpredictable. The typicalcharacteristics of hydraulic valve mechanisms can easily lead to trouble, since there may well be a "dead spot" before themovement of the valve reverses the movement of the motor. When the amplitude of an oscillation decays till it approachesthe magnitude of the "dead spot," the phase of the motor response will lag by approximately 90 degrees. This effect isreduced by valve leakage at dead centre. By judicious design, sufficient motor spe«d for the small amplitude considered maybe obtained by trie unbalanced leakage while the valve is still working in the "dead spot." In view of the potential difficulties arising not only in the caseof automatic control but also in that of manual control, it is clear that no power-operated control should be accepted forflight test without a preliminary survey of its transmission characteristics over the range of frequencies and amplitudeswith which we are concerned. This transmission characteristic is defined by what is known as the " transfer function " for asimple harmonic motion demand. Mr. Meredith then examines stability and lag in rnathe-
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