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Aviation History
1921
1921 - 0192.PDF
MARCH 17, 1921 CAMBRIDGE UNIVERSITY AERONAUTICAL SOCIETY (OFFICIAL ORGAN "FLIGHT") ON February 23, 1921, a most interesting and instructivepaper was read by Professor L. Bairstow, F.R.S., C.B.E., F.R.Ae.S., entitled " The Control and Stability of Aeroplanes." Professor Bairstow in his introduction pointed out that so long as an aeroplane is within its " normal range " of speed, there are few difficulties, but that, as it is just when the machine gets outside this "normal range" that trouble commences, we ought to make a study of non-normal flight. After mentioning such troubles as engine failure, resulting in forced landings, which has nothing to do with control and stability, the lecturer referred to the danger of spinning into the ground as a result of turning without the engine, and to the difficulty of flying in fog and mist. The latter is facilitated by fitting turn indicators ; a compass course can then be held and the horizontal balance of the aeroplane maintained by (1) Keeping the turn indicator at zero. (2) Keeping the bubble of the cross level at zero, and (3) Flying to compass. Most aeroplanes tend to deviate widely from the straight if uncontrolled. The causes are complex; lack of symmetry of rigging ; slipstream effects on the fin and rudder; and lateral instability. Both the latter are common features, and the study of them has been greatly neglected, owing to lack of appreciation of the importance of the problem by the authorities. There is no difficulty in principle in producing an aeroplane which will not, with controls locked, tend automatically to bank to an angle of 50 or 60 degrees. The difficulties of application are thosj of detail. On the effect of size, the lecturer said that as the aircraft gets bigger, the forces on the control column and rudder increase, and the pilot becomes relatively less and less powerful. Balancing the control surfaces is not an ade- quate remedy, and we should study the natural motions of aircraft by the methods of stability, and so find out what mechanisms are suitable. That this can be done is clear from papers written at the N.P.L. by Nayler and myself on flight in a natural wind. It is shown that the movements of the elevator to correct for gusts are not wholly arbitrary, and can be produced to a high degree of approximation by suitable fluid damping. Professor Bairstow stated that, after the reliable engine, the most important element in the progress of aviation is the reliably stable aero- plane. Slides were then shown of diagrams representing the well- known instability of cambered wings. A curve was drawn, having as absissa the speed in m.p.h. and as ordinate the moment coefficient about the e.g. This was the wing curve. Another curve was shown for the tail, whose object is, of course, to produce an opposing couple to that from the wings. The difference is the moment acting on the aircraft. With an unstable aeroplane, the lecturer said, it is only necessary to hold the stick steady to find out, that one of two things occurs. Either the machine goes straight into a steep dive, or it stalls. If it stalls and is controlled laterally a steep dive rapidly follows. If stalling occurs, and lateral control is not exercised, or is ineffective, the result is a spin. The remedy for this defect is simple in principle but troublesome in application. Movement of the tail plane alone is useless. The real corrective is to move the e.g. forward relative to the wings, which is usually attained by shifting the top wingfurther back. Professor Bairstow then referred to some observations made on a complete model of an S.E. 5, corresponding to the following positions of the e.g. from the leading edge : 0.2, 0.3, 0.4 and 0.5. It was found that the aeroplane would be very stable with the e.g. forward at 0.2 of the chord. The amount of movement of the control to change the speed from 100 m.p.h. to 60 m.p.h. was very large (about 20 degrees) and would impose limits on the manoeuvrability. With the e.g. at 0.3 of the chord the aeroplane was very sensitive to elevator position over a large speed range (120 m.p.h. to 80 m.p.h.) but became very stable at landing speeds of 55 to 60 m.p.h. With the e.g. position at 0.4 of the chord the aeroplane would be unstable at high speeds, but more controllable at landing speed. The last instance (0.5 chord showed the aeroplane to be violently-unstable, and also heavy on controls. The lecturer then showed some records obtained on stable . aeroplanes, showing how the oscillations were damped and tended to die down altogether. He then said that it is to be hoped that the stability of the aeroplane will now receive the attention which has in the past been devoted to performance. It is easy to make an unstable aeroplane. It is not difficult to make a very stable aeroplane. On the other hand, it needs considerable care to give an aeroplane a desired degree of stability. It is to be hoped that we have for ever ceased to regard stability with indifference or as an undesirable quality which tends to diminish the skill required of the pilot. With regard to lateral stability Professor Bairstow said that the application of the theory of stability to the lateral motions of an aircraft is far less advanced than the applica- tion to longitudinal motions. Pilots and others have not yet distinguished clearly between lack of trim and instability. He then referred to instability at large angles of incidence and dealt at some length with auto-rotation experiments, showing slides of models mounted in the wind channel. Such models had been found to rotate about an axis in the wind direction with a definite speed for each angle of incidence, the speed of rotation also depending upon the wind speed. In conclusion Professor Bairstow quoted the final para- graph from his lecture to the Royal Aeronautical Society about two years ago, saying : " Once it is conceded in the necessary official quarters that the study of stability is import- ant, there will be no difficulty in beginning the progressive collection of data, but if this work is to proceed satisfactorily it is necessary that an appreciable part of the time of the staff of the N.P.L. and the R.A.E. shall be continuously devoted to it. Incomparably more laborious than that relating to performace. It is work which can only be broken into at frequent intervals at a cost of serious loss of time and accuracy, and in the days to come, when the number of aeronautical engineers with mathematical knowledge is sufficient for the needs of the industry, this data will be the fundamental stock-in-trade. The immediate needs of the industry have much support, but a policy of looking two or three years ahead has fewer advocates in spite of the fact that such provision is necessary if Britain is to keep her lead in the air." Aircraft and the Navy Estimates \ r •- IN a long memorandum, which accompanies Lord Lees' statement upon the Navy Estimates, reviewing the history of the past year, the following reference is made to naval air development, which rather discounts the impression that the Admiralty have been blind to the extension of sea-power through the development of aircraft :— During the year the Naval Staff has been in close co-operation with the Air Staff both in matters of development and the tactical use of aircraft for Naval purposes. The information gained from exercises and experiments has been fully inter- changed, and specifications for improved designs of aircraft have been laid down. It is hoped that during this financial year the air equipment of our fleets with heavier-than-air craft will make a big stride. With regard to the suspension of the provision of airships for the Navy, the memorandum says :—" The Air Ministry informed the Admiralty that the economies imposed upon them in the preparation of the Air Estimates would compel them to reduce considerably the provision of heavier-than-air craft unless the Airship Service was suspended. The Admiralty considered that any such reduction in the vital Naval requirements in heavier-than-air craft, which are essential to Naval efficiency, and to which Naval votes are heavily committed in the provision of aircraft carriers, could not be accepted. While the Admiralty regret the decision of the Air Ministry to suspend the Airship Service, they realise that in view of the stringent financial restrictions, no other decision could have been arrived at. The limited funds now available to meet Naval air requirements will, therefore, be expended on heavier-than-air craft instead of on airships. In view of the importance of training R.A.F. personnel in Naval work, arrangements have been made to put an aircraft carrier at the disposal of the Air Ministry for certain periods for this purpose." In a comparative table of the world's navies. Great Britain is the only country shown as having aircraft carriers, specially designed as such. The number is given as three, and includes Furious, A rgus and Eagle, which, although not actually carriers, were very extensively altered, and it is claimed may be- considered to embody lessons of the War. 192
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