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Aviation History
1927
1927 - 0813.PDF
OCTOBER 20, 1927 FAIREY METAL AIRSCREWS IN SCHNEIDER RACE Some Notes on their Design and Manufacture SINCE Mr. S. A. Reed evolved the original solid blade metalairscrew, its adoption has resulted in an unbroken series of successes in all speed events. Commencing with its initial victory in 1923, every winner of the Schneider Trophyand, incidentally every winner of the Pulitzer Trophy as well—has been fitted with these airscrews. Their superiorefficiency arises from the use of a blade of thin section, which is stabilised wholly or partly by the centrifugal forces actingthereon. The following notes are intended to outline the policy-adopted for the design and final selection of the winning airscrews in this year's Schneider Trophy Race, and to give abrief description of the conditions under which they were manufactured and tested. Selection of Airscrew Types for Maximum Efficiency It was realized at the outset that although different types should be tried in order to ensure the greatest possible chance of success, the number of effective tests would have to be strictly limited. The engine and other components of the aircraft would justly claim a large share of the competitors' increased up to a point which would just lead to a theoretically inefficient value of the tipspeed, but would correspondingly reduce slipstream resistance on the aircraft. In another, the blade widths were distributed in a form not properly consistent with the most efficient distribution of work, but more conducive to the avoidance of boss interference. As a result of this scheme, it was found possible to deliver for trial a systematic series of airscrews in which success or failure could be most readily traced to any particular feature, the basic, or ideal form serving merely as a starting point for subsequent variations. It was, however, of the utmost importance that the four types should form a continuous series, for had the characteris- tics been chosen at random, there would have been no possi- bility of connecting the results or drawing conclusions from a limited number of tests. Continuity was, in fact, equivalent to testing not four but a complete family of airscrews. Blade Stability and Strength The above considerations refer exclusively to aerodynamic performance, but it can be stated that much greater anxiety FAIREY METAL AIRSCREWS : Various designs experimented with for the Schneider Trophy Seaplanes, propeller in the lower right-hand corner is shown complete with spinner. The attention, whilst weather conditions and the problem of transport to Venice would still further restrict the time avail- able for trial flights. With these limitations in view it was decided to evolve four different distinct types of airscrew for each of the two engines (ungeared and geared). The first of each series was designed to conform strictly with the ideal theoretical airscrew, giving maximum efficiency for the speed and horse power proposed ; the term " ideal " must here be understood to apply to an airscrew isolated from the aircraft and engine and working in undisturbed air. The solution for such an airscrew is determinate in every respect, and the conclusions of theory in these special circumstances are known to be well supported by laboratory tests. It was not, however, expected for a moment that this type could be final when applied to the full scale aircraft and airscrews combination, and, in order to cover as completely as possible the non-calculable full scale effects, the following line of action was adopted. In each of the three remaining types of the series at least one of the essential features of design was made to depart from the ideal form, progressively, and in the directions which were considered most appropriate to overcome suspected full scale difficulties. For instance, on one type the diameter was 735 was felt in regard to stability and strength in view of the unprecedented conditions under which the blades would be working. Blade flutter and wholesale hunting of airscrew and shaft in resonance with certain engine or aircraft compo- nents are two of the most formidable enemies of airscrew efficiency. With regard to blade stability, reliance had to be placed in the accumulated experience which the Fairey Aviation Co. has been able to acquire in the course of several years of practical research on this subject, and it is gratifying to note that none of the types submitted developed blade flutter. Resonance with the engine or aircraft was encountered in one instance, and, although it was found possible to mitigate it considerably by subsequent alterations to the distribution of masses in the blades, the type of airscrew had to be discarded owing to lack of time for further trials. In the absence of flutter, static strength afforded no essential difficulty and the stresses occurring were undoubtedly quite moderate. Manufacture In order to give full effect to the provisions made in design, the greatest skill and efficiency was required from the works department concerned with the manufacture of these air-
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