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Aviation History
1946
1946 - 0469.PDF
MARCH 7TH, 1946 FLIGHT Stress Without Strain A Simplified Method of Airscrew Blade Stressing, with Special Reference to Wood or "Improved Wood" Blades By ANTHONY A. FLETCHER, M.A. (Oxon.) FIRST of all, I wish to acknowledge my vast debtto that brilliant epitome of experimental research,the de Havilland Report No. 83. At one sweep this invaluable treatise has cleared away the difficulties which, to all but the "experts," have hitherto encumbered air- screw design, and has made it possible for any or all to produce airscrews which, from the aero- dynamic standpoint, will not only work, but work very well indeed. But, alas, however beautiful and efficient an airscrew may be, it is of no use as a practical work- aday article unless it is strong enough to withstand, without fly- ing to pieces, the appalling num- ber of "horses" that is now pumped into it as a matter of course. The whole pith of D.H. Report No. 83 consists in carrying all questions of aerodynamic design back to a certain predetermined '' Standard'' airscrew, upon which the changes are rung, so to speak, by a succession of experi- mentally determined constants and coefficients, until the even- tual outcome marries-up with the particular set of requirements. It occurred to the writer, in one of those blinding flashes of the obvious to which even a mediocre intelligence is liable from time to time that perhaps a similar line of treatment might be usefully applied to eluci- dating and simplifying the Great Stress Problem. A start was, therefore, made by selecting a plan form of blade which was known from practical experience to be very satisfactory and sweet-running. This plan form is shown in Fig. 1, which gives the chord at any given section in terms of the greatest chord width. This occurs at 0.45 radius, and is known as section C-C. Note that Fig. 1 represents proportional chord widths only; in actual prac- tice, of course, the blade is symmetrical in plan, or, if not quite symmetrical, disposed about a straight radial line passing through the centre of gravity of each section (the so-called " Locus of Centroids "). The same remark applies to the blade as viewed in side elevation. The next point to be settled is the distribution of the load along the blade, because the actual stresses incurred in the airscrew depend upon how the load is distributed. Concerning this, there are many schools of thought, but for "TONY" Fletcher is one of the old-timers in British aviation. He started with the Martinsyde firm at Brooklands in the early days, and his job, previous to his well-earned retire- ment, was that of chief designer to Hordern- Richmond Aircraft. In submitting the article to the Editor, his covering letter took the following form :— A CCE.PJ, I beg, dear C.M.P.. this unpretentious jeu d'esprit; a bid to simplify the mess that seems to cumber airscrew stress. It's not for highbrows and their pals, who sport with double integrals and solve, without the slightest fuss, terrific sums in calculus. Rather, 'tis hoped that it may be of comfort to the P.B.D., enabling him to do his stuff and make his windsticks strong enough. So, should you feel inclined to smile and crab its literary (?) style, just glance again at stanza wo— it is not meant for such as you ! the purposes of this method it is assumed that the total air load varies along the blade as the integral of the radius squared times the width plotted against the radius. This method of allowing for the effects of area is quite as accurate as any, and, it is believed, was first adopted by Hamilton Standard and then by de Havillands. The justifica- tion for its adoption lies in the fact that under static conditions— which nearly alwa57s impose the greatest loads upon an airscrew blade—the inner portions of the blade run at a coarser angle of attack, relative to the outer por- tions, than they do at high speed ; of course, the aerofoil sections of all airscrews, whether V.P. or F.P., become thicker towards the root. By neglecting these factors we tend to calculate higher loads on the outer portions of the blade than actually occur; in other words, by adopting the system of load distribution (Fig. 2) we err, very decidedly, upon the safe side. The next assumption to be made (heigh-ho, these assump- tions!) is that the distribution of the load does not extend farther inboard than 0.2 of the airscrew radius. This seems entirely logical, as nowadays this inboard portion is nearly always covered up by a spinner or some similar "gubbins." Here we are, then, with our load-distribution curve ready to hand; what is to be done with it? The answer is very simple. By means of a planimeter we integrate it graphic- ally, and the result of so doing is seen in Fig. 3, which is the Shear Force Curve, expressed in proportional intensity of shear. To those who are not familiar with graphical integration, all it means is that we construct a new curve, the ordinates of which represent the total area, and the proportions of the total area, of the original curve. For example, in Fig. 3 the ordinate at the 0.2 section, which bears the value unity, represents the total area of Fig. 2; the ordinate of the 0.3 section in Fig. 3 represents the area of the curve in Fig. 2 from the 0.3 section to the tip, as a proportion of the whole, and so on, and so forth. We now take our Shear Force Curve and, doing another spot of graphical integration on precisely similar lines, pro- duce another curve (Fig. 4), which is the Curve of Bend- •20 30' Fig. 1. C : -if 60 75 -90 -95 it i RAWS. ax plotted against radius. AICSCR; Fig. 2. Load-dretribution diagram. -20 30 -45 -60 . AIRSCREW RADIUS •90 95 1* Fig. 3. Shear-force intensity versusradiu£. v/
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