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Aviation History
1934
1934 - 1437.PDF
July 26th, 1934 Supplement to 7660 'FLIGHT ENGINEERING SECTION Edited by CM. POULSEN No. 102 (V°£™e7IX) 9th Year July 26, 1934 THE AUTOMATIC CONTROL OF AIRCRAFT By G. R. M. GARRATT, M.A. (Cantab.)* The Automatic Pilot has been in continuous use in the Royal Air Force for five years, and forms part of the standard equipment of large machines such as night bombers. Recently the instrument has been approved for use in civil aircraft, and there is little doubt that in certain classes of civil work it will be found as useful as it already has proved in military flying. Smith's Aircraft Instruments control the Patent Rights throughout the world. In the article of which the first instalment is published this week, Mr. Garratt, of the Technical Development Staff, Farnborough, describes the principle and details of the two-axes Automatic Pilot, which has been developed by the Scientific Research and Technical Development Staffs of the Air Ministry, notably by Messrs. Meredith, Cooke and Kerr, of the R.A.E. THE general problem of the automatic control orstabilisation of aircraft has received consideration inmany different countries, and although many inven- tions have been recorded, there is little data available of the results which have been achieved. The instrumental problems and the aerodynamic requirements are not easily satisfied, and the credit for the practical development of the automatic pilot in this country belongs very largely to Mr. F. W. Meredith, Mr. P. A. Cookc and Mr. P. S. Kerr. The principal objects of automatically-controlled flight may be briefly summarised as follows : — 1. Reduced fatigue for the pilot, who is able, when necessary, to leave his seat or to attend to other duties, such as navigation or the operation of his radio equip- ment. 2. Greater accuracy of control and course-keeping. 3. Reduced risk of loss of control in bad weather, or during night-flying. 4. Improved stability of the aircraft when accuracy of flying is of importance, e.g., air survey and photography. There are a large number of possible forms of automatic control, and while some of them might be practical, others would be impossible of satisfactory achievement. For ex- * The author desires to acknowledge the courtesy of Messrs. Smith's Aircraft Instruments in providing the facilities for the production of the illustrations in this paper. ample, one might consider an azimuth control depending fundamentally upon a magnetic compass or upon some detector of the sun's rays. One might equally consider a pitch control depending on air speed, altitude, acceleration, the rate of change of altitude or on almost any function which varies with the pitch attitude of the aircraft. The main disadvantage of such controls, however, would be the fact that they would be useless unless they were applied to a completely stable aircraft on account of the phase and time lag which would be involved. An aircraft in steady flight frequently receives distur- bances, and it depends on the stability characteristics of the aircraft what form of motion will result from the initial disturbance. Assuming a stable aircraft, a complex oscilla- tion will generally be set up, anr! when the oscillation is eventually damped out the aircraft will usually be fly- ing on a new course. The first function of an automatic control should, therefore, be to counteract the effect of the initial disturbance and to ensure that the aircraft returns to .its original course. Some datum is necessary from which it is possible to detect any deviation and to which the course may be reset. In preference to such unreliable data as the earth's magnetic field or the sun's rays, the gyroscope is supreme. A carefully balanced spinning gyroscope has the property of keeping the direction of its axis fixed in space unless acted upon by some external force, and this characteristic stability is used in a few of the largest liners to reduce th'
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