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Aviation History
1953
1953 - 1275.PDF
FLIGHT, 25 September 1953 THE WILBUR WRIGHT LECTURE A Digest of Professor Hoff's Paper on Buckling: R.Ae.S. Awards Presented THE steep-tiered lecture room of the Royal Institution was crowded on September 14th, as members of the Royal Aeronautical Society and the Institute of the Aeronautical Sciences met together to hear the 1953 Wilbur Wright Memorial Lecture and to see the presentation of the Society's foremost awards. The lecture, given annually and alternately by American and British speakers, was on this occasion entitled Buckling and Stability, and was presented by Professor N. J. Hoff, who is head of the Department of Aeronautical Engineering and Applied Mechanics at the Polytechnic Institute of Brooklyn, New York. Before die lecture, as we briefly recorded last week, die awards were presented by the President, Sir William Farren. Honorary Fellowships were awarded to Sir Geoffrey de Havilland and Sir Ardiur Gouge, and mention was made of die same honour—to be conferred at a later date—to Lord Hives. Sir William then paid a tribute to the late Professor L. Prandd, one of the few men to have the triple honour of being a Gold Medallist of the Society, an Honorary Fellow and a Wilbur Wright lecturer. The Society's Gold Medal, die highest honour it can confer (awarded first in 1909 to the Wright brothers), was next presented to Mr. E. F. Relf, C.B.E., F.R.S., F.R.Ae.S., Principal of the College of Aeronautics from 1946 to 1951, for his outstanding contribution to aeronautical science over a period of many years Mr. H. Grinsted, C.B.E., B.Sc, F.R.Ae.S., received die Society's Silver Medal for his outstanding work in aeronautical engineer ing; and the Society's Bronze Medal was awarded, for his work on the development of Naval aircraft, to Mr. L. Boddington, Director of Military Aircraft Research and Development, Royal Navy. The British Gold Medal for Aeronautics went to Mr. R. E. Bishop, C.B.E., F.R.Ae.S. (de Havilland Aircraft), for his out standing contribution to aircraft design, and Mr. J. E. Gordon, B.Sc. (R.A.E., Farnborough), received the British Silver Medal "for his excellent work in the sphere of aircraft structural plastics." For his paper, The Fatigue of Aircraft Materials with Special Reference to Micro-Structures, Major P. L. Teed, F.R.Ae.S., received the Simms Gold Medal of die Institution of Aeronautical Engineers, whose Wakefield Gold Medal was presented to Mr. F. W. Meredith, B.A., F.R.Ae.S., for his work in the design of automatic pilots and aircraft instruments. G/C. E. A. Whiteley, D.F.C., B.A., A.F.R.Ae.S., who qualified for the award of the George Taylor Gold Medal for his paper The Spacing of Aircraft under High-Density Conditions, was unable to receive his award diat evening, for his work lies overseas; he is Director of Plans, Far East Air Force. Column-theory Developments The Wilbur Wright Lecture was then given by Professor Hoff. He began by tracing the historical development of the theory of columns, stating that, although columns built almost 5,000 years ago still stood in Egypt, the first theoretical calculation of die load- carrying capacity of columns was published by Euler only 209 years ago. This classical theory, applying to perfectly elastic long columns, he continued, was widely known at die time when Wilbur and Orville Wright designed and constructed their first gliders and powered aircraft; and, although such a rational theory of die inelastic buckling of short columns was not generally avail able, practical engineers had enough empirical information to permit them to design safe columns without undue weight. An interesting extract from Orville Wright's diary referring to strength tests made on the completed powered aircraft before its first flight was quoted by the lecturer. The establishment of a rational theory of short columns was first made possible by die results of investigations by von Karman published in 1910, Professor Hoff continued. On the assumption of small deformations, von Karman derived a simple formula for the buckling load of a short column, and was the first to give simple closed-form expressions for the reduced modulus of the solid rectangular and die idealized I-sections. The major features of the experimental work performed to verify the theory were the careful determination of the compressive stress-strain diagram of the metal of the column, and the accurate centering of the speci mens under load by means of adjustable knife-edge end-fittings. Von Karman also made rational corrections in the evaluation of die test results to allow for die increased rigidity of the column in the region of die end-fittings. A new approach differing considerably from the classical analysis was put forward by Shanley in 1946. While the results of ordinary tests on slender columns gave good agreement widi the classical theory, the performance of short columns, thin-walled cylindrical shells and flat plates provided large discrepancies, die buckling load in general proving considerably smaller man mat predicted by die Eulerian theory. Shanley's thesis was that the tangent-modulus load, suggested earlier by Professor Engesser, should be accepted as the correct buckling load. Assumptions on the two dieories were different (according to the earlier theory, die column end-points were subjected to a fixed axial compressive load, while on the Shanley system die end-load changed continuously during the buckling process), but die acceptance of the newer concept did imply die rejection of the classical view of stability. The lecturer went on to give an analysis of the dynamics of die motion of a column during the buckling process in a normal column-test, beginning widi the equations of motion of a per fectly elastic column. From these, he went on to deal in mm widi the elastic buckling process and a consideration of die static solution widi vibrations superimposed. His description of the experimental test apparatus and results was followed by the development in detail of a dynamic theory for short columns. Phase plane diagrams were then shown by Professor Hoff, who went on to describe the continued experiments performed in order to check the theoretical conclusions. Static and Dynamic Criteria The penultimate section of the paper was concerned with the determination of die buckling load in the cases of slender columns, short columns, flat plates and thin cylindrical shells respectively. An account of die Duberg-Wilder definition of critical load, and the Pearson-Prager criterion of buckling was followed by a con sideration of experimental proof of the correctness of the tangent- modulus load, and of die relation between the static and dynamic criteria of stability. Summarized conclusions were then expressed by the lecturer. A short column compressed in the testing machine widi the dis tance between its two ends fixed could be stable when the dis turbance was small, and unstable when die disturbance was large; consequendy it might buckle before its stability limit was reached in the sense of the classical small-deflection theory. A column whose material was subject to creep was unstable under a com pressive load of any magnitude, but it might not buckle during the lifetime of the structure of which it was a part. Buckling under a static load following a disturbance had many aspects differing significantly from buckling during a loading process, particularly when the latter was rapid, and there were material differences between the various kinds of loading processes. For these reasons it was desirable to distinguish between the stability of a fixed system in the classical sense and the stability of a loading process. There was no reason to believe that an arbitrarily chosen loading test would furnish values that could prove or disprove the classical theory of static stability. There was even a great deal of an arbitrary nature connected with the definition of die buckling load in a loading process. It seemed advisable to reserve the terms "critical load" and "characteristic value of the load" for the designation of tiieoretical loads at which the state of equilibrium was of a special character. Thus die load at which the limit of stability was reached in accord ance with the classical small deflection tiieory, or even according to a large deflection dieory, was properly called a critical load. Common sense required diat the load at which a column or a thin-wailed cylinder buckled suddenly in a loading process should be designated as the buckling load. The audience's thanks to the lecturer were voiced in a forth right and amusing manner by Mr. George Edwards, chief engineer of Vickers-Armstrongs aircraft division and a vice-president of the Society, who pointed out first that "this business of struts and tilings has been plaguing people in the structures game for many years." After commenting that the assumptions of some present- day engineers made the prognostications of the Greeks (the pro portions of whose columns were based on those of die human body) seem most scientific by comparison—yet another reason for "the creeps"—Mr. Edwards referred to "putting a bit extra on everything" after theoretical calculation: this, of course, was what every stressman did. In serious vein, Mr. Edwards congratulated Professor Hoff on the unparalleled presentation of the lecture—a highly specialized one, in contrast to the reviews of wide fields covered by his pre decessors—and welcomed him and his fellow delegates to die joint aeronautical conference.
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