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Aviation History
1932
1932 - 0859.PDF
August 26, 1932 Supplement to FLIGHT Edited by C. M. POULSEN August 26, 1932 CONTENTS The Determination of Airscrew Design Conditions. Bv Lt.-C'ol. J. D. Blyth, O.B.E.. A.F.K.Ae.S.. M.LAc.E. Strip Manipulation. By W. S. Hollyhock Applications of the Polar Diagram. By E. H. Atkin, B.Sc. (Lond.) Technical Literature— Summaries of A.B.C. Reports and Memoranda Page 58 til 6-1 THE DETERMINATION OF AIRSCREW DESIGN CON DITIONS By Lt.-Col. J. D. BLYTH,* O.B.E., A.F.R.Ae.S.. M.I.Ae.E. A CONDITION generally laid down for the test flights of an aeroplane is that the airscrew shall be so designed that it is impossible to exceed the maximum permissible R.P.M. of the engine in level flight, or the normal R.P.M. at the best climbing speed ; and in estimating the performance of a machine it should be assumed that this condition is fulfilled. Except in rare cases, it is impossible to design an airscrew which will give maximum efficiency and R.P.M. at top speed in level flight, and normal R.P.M. on climb : since the relationship between R.P.M. at top speed and at climb will depend upon two factors, the slope of the power curve against R.P.M., and the pitch/diameter ratio of the airscrew. The latter determines the torque coefficient curve which may in turn be modified by variations in blade width. Until comparatively recently the maximum permissible R.P.M. of an engine exceeded the normal R.P.M. by 10 per cent. With this relationship existing, it was generally safe to assume that the airscrew would be designed to give maximum efficiency at maximum R.P.M., at top speed, as the climbing revs, would not exceed normal in most cases. Lately, however, the amount of over-revving permitted has been increased to 15 per Cent, and in some engines 20 per cent, above normal: and two conditions have to be taken into account in determining the airscrew characteristics. These are the design conditions, and are that the airscrew shall give normal R.P.M. on climb and maximum efficiency at a predetermined value of R.P.M. and speed. The latter will usually be the R.P.M. reached at top speed, and will fix the diameter and pitch of the airscrew, while the climbing condition will determine the plan form. The problem which presents itself at once is the determina tion of the R.P.M. at top speed. This may be found by trial and error ; that is, by assuming a pitch diameter ratio and proceeding to find the R.P.M. at various speeds by the usual methods of performance calculation, and, if the air screw does not fill requirements, modifying it and repeating the process. With practice the preliminary assumptions can be made with fair accuracy ; but if they are incorrect time and labour are wasted. The object of this article is to provide a means of estimating to a close degree of accuracy what the R.P.M. at top speed will be, and so of allowing the airscrew characteristics to be determined after a very short preliminary investigation. The first step is to obtain an approximate value of the maximum speed attainable. To do this the curve of thrust horse-power required for level flight is plotted against speed for the machine under consideration, and an estimate made of the nett airscrew efficiency and maximum R.P.M. reached. The latter gives the thrust horsepower available, and from this and the curve of power required the maximum speed is obtained. In this preliminary estimate great accuracy in guessing the maximum R.P.M. is not necessary. It will be quite near enough to take the maximum R.P.M. as being about 15 per cent, above normal, the slope of the power required curve being so steep that an error of 50 R.P.M. in the estimate will not affect materially the estimate of the top speed. The diameter of the airscrew can be calculated now, and, from the assumed R.P.M.. the value of — and the nett wD airscrew efficiency are obtained. If the latter is so different from that at first assumed that the estimated top speed is * Lt.-Col. Blyth is on the Technical Staff of the Gloster Aircraft Com pany. 802 a D
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