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Aviation History
1924
1924 - 0748.PDF
NOVEMBER 27, 1924 "SOME FURTHER PRACTICAL POINTS IN THE STRUCTURAL DESIGN OF AIRCRAFT" THE paper read by Dr. A. P. Thurston before the Institution of Aeronautical Engineers on November 21 was not nearly as well attended as it ought to have been. Dr. Thurston's paper was of a somewhat different character from what was to be expected from the title given in the Institution's fixture list, where the paper was announced to be headed " Graphical Methods of Aircraft Structural Design." In point of fact, Dr. Thurston did not give any new methods of graphical structural design, but confined himself to showing lantern slides of curves of aeroplane percentage weights, stability formulae, average curves for fuselage strengths for machines of various sizes, crippling loads on struts and longerons and such-like matters. The curves of average item weights were, the lecturer admitted, based on pre-War machines, and in the discussion following the lecture Dr. Thurston was criticised for attempting to compare modern machines with those indicated in the curves. Basing his average figures on the curves for pre-War machines, the lecturer arrived at the following percentage weights : Body 17 per cent., under-carriage 4, wing structure 20, engine and installation 26, and useful load 33 per cent, of total loaded weight. Turning to the question of longitudinal stability of aero planes, Dr. Thurston showed a slide in which tail plane and elevator moments were plotted against the product of wing area and chord. Drawn through the points representing a number of German machines was a straight-line curve repre senting Barnwell's formula, al = 0-4 AC, in which a is the area of tail plane and elevators, / the distance of elevator hinges from centre of gravity of machine, A the equivalent area of main planes, and C the average chord. The lecturer stated that the constant might be somewhat less than 0-4 in certain cases. Most English machines, he said, fell either on the stable side of the curve or near the line of Barnwell's formula, while many War-time German machines fell far over on the unstable side. He then gave values of al/AC for a number of German machines, ranging from 0-0815 for the Brandenburg mono plane to 0 -572 for the Albatros. The remainder of the paper was devoted to sections dealing with the strength of fuselage with vertical loading, the strengh of fuselage with lateral loading, strength of longerons, strength of body struts'and inter-plane struts, and properties of wood. Slides were shown illustrating how timber strength decreases with absorption of moisture. The lecturer also stated that the strength of woods varied as the specific gravity. An exception appeared to be oak, which appeared to have a smaller strength in proportion to its weight than other woods. A lively discussion followed the reading of the paper. Mr. Oswald thought such average curves as those shown should be treated with respect, and personally he preferred to use fundamental equations, as there were such great variations in detail design and in the materials used. The curves of percentage weights were, he thought, unfair to light 'planes, which according to the curves would show up badly, but which in reality were very efficient. He also thought he recognised some of the curves as having been previously published in " Aircraft Engineering." Capt. W. H. Savers said that such average curves as those shown by the lecturer did a lot of harm and slowed up progress. Such curves of average item percentages when used by the Air Ministry had the effect of hampering design because if a designer came along with a design in which the structure weight, for instance, was much smaller than the average percentage, the Air Ministry said that the machine could not be safe with such a low weight, and the design would then probably be turned down. With reference to the <•> <•> Barnwell formula and the figure of 0-0815 for the Branden burg monoplane, he did not know exactly which Brandenburg the lecturer was referring to, but if it was the Brandenburg seaplane of which examples had been obtained by this country at the end of the War, it would seem that the formula given broke down completely, because although the figure for the Brandenburg was only about one-fifth of the average value of 0-4, the Brandenburg had proved in practice to be extremely pleasant to fly and very manoeuvrable. So much depended upon the wing section used that one could not use such average figures. For instance, with most of the German " tadpole " sections of the Schoukowsky type a constant of 0 -4 would result in a totally uncontrollable machine. Mr. Evans thought the formula wide of the mark, and considered it fundamentally wrong, because it was based upon chord length, and it was c.p. travel and not chord length that mattered. He asked what would, for instance, be the result of trying to apply the formula to a machine with reflex section which, being stable, required no tail. He was not even sure that the formula was right for biplanes using R.A.F. 15 section. With regard to the German machines falling on the unstable side of the curve, he thought that there again comparisons might be unfair, because the curve was based upon the use of R.A.F. 15 sections, while, as a matter of fact, the Germans used quite different sections during the War, and the tail planes might be, and probably were, quite satisfactory for the wing sections employed. Lieut. Olechnovitch would like to ask the lecturer if he had any information concerning the use of certain fillers for wood to prevent moisture absorption and strengthen the timber. He himself had carried out some experiments in which the timber was placed in a vacuum so as to draw out all juices. The test specimen was then soaked in dope, and on test afterwards was proved to have nearly double the strength of the natural timber. The Chairman, Mr. W. O. Manning, said he was not sure the lecturer was right in stating that the curves representing item weights referred to machines ten years' old, as there "was one weighing 21,500 lbs. (Here the lecturer interposed the explanation that this machine was the Sikorsk.) He did not think that the structure weight of modern machines could be compared with that of pre-War types. He shuddered, he said, when he saw the curves showing decrease of strength with moisture absorption, but regained some of his composure when he recalled that machines had been flying for many years and shown no signs of becoming dangerously weak, so, pre sumably, they did not absorb as much moisture as might be thought. He would like to see experiments carried out on how absorption of oil affected the strength of timber. He thought it might be found to weaken it considerably. In replying to his various critics, Dr. Thurston said the curves were meant to indicate a method, and were not intended as curves of reference. They were useful for checking one's own calculations. Barnwell's formula was on the safe side, and the machines designed to it were among the most successful of their period. He did not agree that average curves slowed down progress. On the contrary, he thought they should be an incentive to designers to improve upon the average results. During the War the curves had proved useful in showing that structure weight did not increase with the size at the rate certain experts claimed, and, consequently, it had been possible to start work on large machines which might other wise not have been built. The reason why large machines did not increase in the theoretical ratio was that they were not mere geometrical copies on a larger scale, but employed different detail structural members. Lord Thomson's Eastern Tour ON November 21 last Lord Thomson, Air Minister in the late Government, gave a most interesting lecture before the Central Asian Society at the Royal United Service In stitution. Viscount Peel presided, and Lord Thomson's subject was " My Impressions of a Tour in Iraq." Lord Thomson began with a brief outline of the aerial tour he made last September, and most of these opening remarks—and indeed much of what followed—were, naturally, a repetition of what has already appeared in FLIGHT dealing with Lord Thomson's Eastern tour (FLIGHT, October 16, 1924), so that it will be unnecessary to repeat it here. Lord Thomson, however, dealt at greater length on the political situation in <$> <•> Iraq, and gave his impressions on this aspect of the case obtained during his short but comprehensive tour. Lack of space will not allow us to quote these impressions—which are not directly connected with aviation—in full, but, briefly, they may be condensed into the following salient points : The bombing expeditions of the R.A.F., though regrettable were absolutely necessary for maintaining law and order, and, in fact, actually were in the end the means of lessening the, amount of suffering ; Great Britain should not yet withdraw from Iraq, which required above all things, security ; this security was being largely maintained by the R.A.F\, which, as Lord Thomson put it, was the cement that kept the bricks together. 748
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