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THIS paper seeks to draw from current research work on flutter and related problems results of general design significance ; and, avoiding mathematics, endeavours to set these results out in relation to past and present problems.

IN so far as it is possible under wartime restrictions, the purpose of this paper is to present a comprehensive picture of the more important problems in aircraft structural design and research in the interest of advancing the knowledge of those engaged in industries which formerly had but slight connexion with aeronautical engineering, but today are deeply involved in various phases of aircraft work. A similar objective was stated by Dr. A. G. Pugsley as follows:

IN the 1940 Wilbur Wright memorial lecture on the future of British Civil Aviation Dr. Roxbec Cox has stated that the study of passenger convenience leads to consideration of over weather flying and that the study of improvements in safety leads to consideration of flutter prevention. He has stated also that all aviation authorities recognize the danger of flutter and warns that the chief danger to be guarded against in the future is a sense of security. It has, therefore, been thought worth while to examine the effect on wing flutter speed of flying at great heights. The effects have already been briefly considered by Williams, Pugsley and Duncan in this country and by Kassner and Fingado in Germany.

Under this heading are published regularly abstracts of all Reports and Memoranda of the Aeronautical Research Committee, Reports and Technical Notes of the U.S. National Advisory Committee for Aeronautics, and publications of other similar research bodies as issued

The conference was opened by Professor A. G. Pugsley, who welcomed the members to Wills Hall and commented on the importance and appropriateness, at the present time, of the subject of the meeting. In doing so, he drew attention to some general considerations relevant to the detailed subjects they were to discuss.

IN aeronautical work the general engineering outlook in regard to the specification of strength factors has been supplemented by an appreciation of the unusually variable nature of the loads coming upon the aeroplane structure. The two questions have generally been treated as separate ones, and it is only recently that efforts have been made to bring them together by relating both to accident rates. In doing so we have on the one hand to allow for the frequency of occurrence of loads of different magnitudes and on the other hand to allow for the variation in strength among aeroplanes produced to a given design. The purpose of this report is to try, in a preliminary and elementary way, to weld these various aspects of the design of aeroplane structures into a logical and consistent whole—a philosophy of strength factors —and, by so doing, to reduce accident rates, to bring into better perspective some of our past problems, to point to ways of further development, and to prepare for making the fullest use of the speed and acceleration loading statistics now accumulating.

STRUCTURAL engineers have always been accustomed to think of the strength of a structure in terms of gradually applied, or “static”, loading. It was natural, therefore, that when strength tests were first made on aeroplane structures, the primary object was to find the load that, when gradually applied to the structure, just broke it. This “ultimate” load was easy to determine, because when it was reached the structure collapsed and refused to carry any more load. At a later stage attempts were made to define a “proof” load at which the structure, because of its deformation or other damage, just ceased to be regarded as airworthy. This proof load was a much vaguer load to determine experimentally; how long should it be left on the structure, should it be applied more than once, who should be regarded as competent to pronounce on the airworthiness of the deformed structure, and should the airworthiness be judged when the structure is actually underload or after the load is removed, were all difficult questions to answer. As a result, in spite of an increasing realization of the relatively greater importance of the proof load, or at least of some comparable concept, in practice reliance has continued to be placed on measurements of ultimate load.

THOUGH resonance tests are now becoming familiar to most aeronautical engineers, it is perhaps desirable at the outset to indicate their general nature. They are vibration tests carried out on an aeroplane with the immediate object of determining the natural frequencies and modes of its parts, and in particular, of its wings. The ultimate aim of resonance tests on a given aeroplane is to provide data to assist in the estimation of its critical flutter speeds, so that its liability to flutter troubles may be assessed.

AERONAUTICAL research has until recently been conducted by men trained in other and varied branches of science and engineering. Mathematicians and physicists, civil and mechanical engineers, have all taken part from time to time, and their work has naturally borne the imprint of their several outlooks. Structural research, which has hitherto been largely overshadowed by aerodynamic and engine work, has suffered particularly from a continual changing of personalities and a confusion of interest. As a result it has in many respects failed either to adopt the traditions of the general body of structural engineers or to build up a fully ordered tradition of its own. And so we may sometimes see on the one hand, for example, the anomaly of an approach to the local buckling problems of monocoque construction as though Stephenson had never built and experimented upon his Britannia Tubular Bridge; and, on the other hand, a growth of stressing methods inadequately linked by generally accepted basic principles.

CERTAIN types of strut, when tested in compression, fail by twisting. In such cases the twisting occurs suddenly at some critical compressive load corresponding to the incidence of torsional instability. This kind of failure is found in tests of “thin” section struts, of angle or channel section, for example, and sometimes in tests of “open” section struts with lattice or other light bracing, and is ascribed to the low torsional stiffness of such struts. The failure is often accompanied by a distortion of the sectional shape of the strut due to some secondary instability of section, but attention is here directed to the primary torsional instability involved.