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Aviation History
1926
1926 - 0603.PDF
August 26, 1926 Supplement to FLIGHT ENGINEERINGSECTION Edited by C. M. POULSEN August 26, 1926 CONTENTS Aircraft Performance. By J. D. North, F.R.Ae.Soc Spindled and Hollow Spars. By Lieut.-Col. J. D. Blyth, O B EM.I.Ae.E., late R.A.F. ... ". " PAGE 73 77 OUR CONTRIBUTORS Holidays have been responsible for a somewhat '-thin" section of THE AIRCRAFT ENGINEER this month, a number of contributions which had been promised for the present issue having failed to turn up in time to be included. While the Editor naturally deplores this fact, he has been unable to remedy it. but it is hoped that the next issue will make up by being a fairly large one. Among the "missing" articles is Dr. Aitchison's instalment in the series on Duralumin—Dr. Aitchison having be?n among those who took a](doubtless well-earned) holiday during August. We trust he will feel all the better for his "heat treatment," and that when he resumes his series of articles he will show no signs of " fatigue.' Mr. J. D. North turns his attention from aerodynamic to structural design in the present issue and commences a dis- cussion on "Structural Policy in Design." The subject of load factors and factors of safety is dealt with at some length, and some very interesting information is contained in the tables of data from two Boulton and Paul machines, the information being here published for the first time. As the subject is one of very great importance at the present moment we feel that thanks are due to Mr. North's firm for having given permission for the material to be published for the benefit of other designers. The subject of stress?s in a terminal velocity nose dive, and when pulling out of such a dive, is considerably to the fore just now, and any light that may ba thrown on such conditions as still remain obscure is to be welcomed on all sides. In the United States load factors considerably greater than those demanded in this country are now used, and in his article Mr. North calls attention to some interesting experimental results that have b?en obtained in America relating to the accelerations that may be expected. Thus in a vertical bank the highest loads-, appear to have been reached, amounting to no less than 5'7 times g, while in pulling out of a dive at a speed of 1H2-5 m.p.h., the accelera- tion was found to reach the very high figure of 7.8 g. When it is realised that modern single-seater fighters probably have terminal velocities of close upon 300 m.p.h., it will be realised that some quite extraordinary loads may be expected, and, apart from structural considerations, the physical effect on pilots is likely to be a very serious factor. AIRCRAFT PERFORMANCE. Structural Policy in Design. By J. D. NORTH, F.R.AE.S. (Continued from]). 71.) A number of complex considerations go to determine the structural policy adopted in aeroplane design. The condi- tions peculiar to flying, which make it impossible to produce an aeroplane capable of withstanding any possible flight loads, have given rise to a policy where a compromise between utility and safety is adopted. Attempts have been made to arrive at a solution to the problem analytically, but to a large extent the structural design depends on experience. It is quite obvious that where experience of flying large numbers of aeroplanes designed to meet certain conditions has shown that these machines are immune from structural failure, it is not advisable to increase the weight to meet modified conditions suggested by analysis without very careful investigation. A rough indication of the factors governing structural policy in aeroplane design is given in Table I, which is self-explanatory. On the analytical side attention has been principally directed to the study of loads arising from motions due to the pilot exercising his fullest physical capacity ; in some cases this conception has been simplified by assuming that the physical strength of the pilot is suffici- ently great to move any or all of the control members to their fullest extent under any conditions. It seems not unreason- able that the real motions of an aeroplane will have a psycholo- gical basis, that is to say, that they depend on the voluntary action of a pilot. As the loads found by studying the motion on a physiological basis demand structures outside the range permitted by considerations of utility, it is found necessary to adopt fractional factors of safety. This seems to imply the somewhat unjustifiable assumption that the actions of a reasoning individual can be expressed as a numerical factor of the actions of a physical being without intelligence. It is important not to undervalue the traditional influence of pre-war design and flying. From 1910 to 1914 successful flying was sufficiently common to enable considerable experi- ence to be obtained structurally to enable a factor (at this time of the order of 6) to be given which, if applied in the rather crude manner then possible, produced a more or less consistent structure of about the same weight and strength as the successful machines of the period. It must be remem- bered that looping and upside-down flying were considered as aerobatic feats for which the aeroplane required to be specially designed or strengthened, that pulling out of a terminal nose dive was considered to be (and in all cases 532a E2
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