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Aviation History
1927
1927 - 0453.PDF
June 23, 1927 Supplement to FLIGHT ENGINEERINGSECTION Edited by C. M. POULSEN June 23, 1927 CONTENTS Aircraft Performance. By J. D. North, F.R.Ae.Soc The Guggenheim Competition. Some Speculations by " Marco Polo " PAGE 41 43 EDITORIAL VIEWS Owing to the holding of the Royal Air Force Displav at Hendon next week, it has been necessary to include the June number of THE AIRCRAFT ENGINEER in the present issue of FLIGHT. AS a result, the A.E. is somewhat small this week, but we hope to make up for this by increasing the size of the July issue. As usual, readers will hnd much to interest them in Mr. North's article, which this week deals with aero engines and gearing. This is a subject which seems to be just beginning to attract the attention that it deserves, and was dealt with to some extent by Mr. C. C. Walker in our issue of January 27, 1927. Mr. Walker showed how power is wasted by using high-speed engines in relatively slow machines. Mr. North goes even further than that and expresses the view that even on racing aircraft the geared propeller would result in a considerable increase in performance. As Mr. North points out, l>efore one can really form any opinion of the efficiency of an engine, it is necessary to take into consideration propeller efficiency and fuel consumption at the speed and height at which the machine is to operate. His reference to modern aero engines as " light and efficient prime movers for operating dynamometers." is rather apt, although doubtless engine designers will not altogether agree. Mr. North's statement that, for almost any practicable aeroplane specification, the reduction in fuel weight, fuel tank weight and space, obtainable by the use of a geared engine, will more than compensate for the extra weight of gearing, propeller and engine mounting, should be taken to heart and carefully studied. The Guggenheim Safe-Aircraft Competition is a subject well to the fore at the moment. The prize is such as to make it well worth while entering for this competition, and in the present issue. " Marco Polo '' discusses some of the problems attending the design of a machine with a speed range of from 35 to 110 m.p.h. Doubtless, many aircraft designers will disagree with him, and it is hoped that they will send along their views. We shall be very pleasedto publish these in the next issue of THE AIRCRAFT ENUINEEE. AIRCRAFT PERFORMANCE By J. D. NORTH, F.R.Ae.S. (Continued from p. 38) In view of the advantages to be obtained by the use of low R.P.M. values of „. _ it would seem surprising that many modern engines have such unfavourable values of K. The reasons for this are complex. In the first instance, scale has an important effect on engines themselves. In geometrically similar engines, stresses in moving parts are the same at the same piston speed : hence, with constant B.M.E.P., from- stress considerations, the horse-power from a given volume decreases with increase of scale. For example, in two geometrically similar engines, one twice the scale of the other, the relative capacities would be as 8 to 1. and the speeds as 1 to 2 : hence, horse-power would be as 4 to 1 (i.e., horse-power varies as L2). Cooling conditions similarly permit higher speed with smaller engines, since the cooling surface of a cylinder obviously varies as Ls. Primarily, if engines could be made geometrically similar, the weight per horse-power would increase with increase of scale and directly as L, since the weight would increase as L3 and the horse-power only as IA Although conditions of stress, cooling and lubrica- tion* suggest that small engines could be run up to very high speeds, there are other factors which limit the extent to which this can be practically utilised, though induction boosting opens up special possibilities. Similarly, the weight cannot vary as L3 because it is not practicable to make engines geometrically similar, the minimum scantlings practicable being reached at moderate powers and sizes. At the same time, accessories such as the ignition system, carburettors, &e., vary in weight only comparatively slowly as scale changes. Here we have conditions governing optimum size closely analogous to those governing optimum size for the aeroplane. Data hardly exist to enable an estimate to be made of most economical size, since engines vary so much in design. The reduction of the number of cylinders in the lower powers, for example, tends to keep revolutions per minute more nearly uniform in large and small engines. It is obvious that, could revolutions per minute vary inversely as L, the K value for direct drive engines would be independent of horse-power : but actually, it is the modern engine of 500 to 1,000 h.p. which stands to gain most from gearing. There is, of course, with a geared engine, considerable * i.e. p.v. 414a
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