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Aviation History
1924
1924 - 0092.PDF
AEROPLANE PERFORMANCE ESTIMATES [THE paper under above title, read by Mr. R. Chadwick, Chief Designer to A. V. Roe and Co., before the Institution of Aeronautical Engineers on February 8, 1924, was of more than ordinary interest, forming as it did a veritable text-book on the subject of aeroplane performance estimates. While it is tme that Mr. Chadwick did not introduce any new methods, his explanation of the various procedures followed in estimating the performance of an aeroplane was so clear as to be easily understood by almost anyone, and perhaps the greatest merit lay in the compilation of average figures for weights, air resistances, etc., of a number of items. The lecturer was careful to point out that, like all averages, these figures did not always hold true, especially if radical departures were made from orthodox design. Nevertheless such figures are of the very greatest assistance in preliminary estimates, and it is for this reason that we have chosen, out of the 35 illustrations, six sets of curves which seem to us to be of such everyday utility as to deserve to be widely known. We should have liked to publish Mr. Chadwick's paper in full, but to do so would have required several issues of FLIGHT. We have, therefore, had to be content with publishing these six graphs, and the brief risume of the paper which follows. It is to be hoped that the.Institution of Aeronautical Engineers will be able to publish Mr. Chadwick's paper in full in their Minutes, complete with all the illustrations, since the paper—as we have already mentioned—would form an excellent text-book on the subject, especially suitable for those who have not the opportunity of doing performance calculations in the ordinary course of their duties, but who are interested in the subject and wish to be able to carry out such calculations should the necessity arise.—ED.] The Three Methods THE lecturer pointed out that there are three methods in general use for estimating the probable performance of a new aeroplane design. The first consisted in predicting the performance from formulae or graphs obtained by averaging the performance test results of a large number of aero planes. The second method consisted in estimating the performance by calculating the wing lift and drag from model tests, and the drag of the remainder of the aeroplane from tests on component parts. The propeller efficiency also had to be calculated, and the performance obtained from the resulting values for h.p. required and h.p. available at all speeds within the flying range. The third method was to calculate the performance from the results of tests in the wind tunnel on a complete model of the proposed aeroplane. The lecturer pointed out that the first method was generally employed for making a preliminary estimate, and was particularly useful when considering the possibility of meeting a prospective purchaser's requirements. By the first method a close approximation could be made to the probable performance without the necessity of preparing any drawings, while the graphs used were also useful in considering the effect on performance of an existing aeroplane of varying the load carried. The second method was more laborious, and was usually employed to make a detailed estimate when the design had been more or less settled. The third method was not frequently resorted to unless the proposed design was a considerable departure from the average type. This method was also expensive, and it took a considerable time before the model test results were available. Weight Estimates Mr. Chadwick then proceeded to illustrate the two first methods by working out a numerical example, assuming that certain figures relating to useful load, range, maximum and minimum speed, etc., were furnished, to which the designer had to work First of all it was necessary to be able to estimate the total loaded weight of the machine, and in order to enable this to be done the lecturer showed slides giving weight of honeycomb radiators, tanks, struts, ' propellers, oleo undercarriages, etc. Unfortunately, we have not the space to reproduce these curves, valuable as they are. The weight of aeroplane components as percentage of the gross weight was given in another slide, and this is summarised as follows, it being clearly understood that the figures given are averages : Wings, complete with bracing, 15 per cent. ; tail, elevators, rudder and fin, 2 per cent. ; undercarriage and tail skid, 4 per cent. ; body, with engine mounting, seating, etc., 11 -25 per cent. ; machine and engine controls, 0 -75 per cent. ; total structure weight, 33 per cent. Engine, 18 -5 per cent. ; radiator, shutters and cowling, 2 -75 per cent. ; cooling water, 2 -75 per cent. ; fuel and oil tanks, 3 per cent. ; piping, cocks, pumps, etc., 0 -5 per cent. ; propeller, 2 -5 per cent. ; starting gear for engines, 0 -5 per cent. Total power plant weight, 30 per cent. Total bare weight as percentage of gross weight, 63 per cent. The lecturer gave the following average figures of weights in lbs./h.p. : Water-cooled engines, 2 -5 ; air-cooled engines, 2 -33 ; water-cooled engine accessories, 1 -0 ; twin water- cooled engine accessories, 1 -25 ; air-cooled engine accessories, 0 -383 ; water-cooled engines and accessories, 3 -5 ; twin water-cooled engines and accessories, 3 -75 ; air-cooled engines and accessories, 2 •! ; the engine accessories include radiator and shutters, cooling water, piping, cocks for fuel, oil and water, exhaust pipes, pumps, starting gear, propeller. Fuel and oil tanks are not included, as these vary with the duration of flight. The weight as percentage of engine weight of accessories was given as follows : Water-cooled engine accessories, 43 per cent. ; twin water-cooled engine accessories, 47 per cent. ; air-cooled engine accessories, 16 per cent. Performance Estimates Using the figures in the first five slides as a basis, Mr. Chad wick then arrived at an estimate of the total loaded weight of the machine taken as an example, and then proceeded to the actual performance estimates. In the six following slides (which are reproduced herewith) were shown data collected from a large number of performance tests on different aeroplanes. Fig. 1 (we have re-numbered the six illustra tions) gives curves of wing loading—ranging from 5 to 11 lbs. per sq. ft.—with speed in m.p.h. on a base of power loading in lbs./h.p. As stated on the graph, the upper curve of each band refers to single-seat, single-bay machines, while the lower curve refers to two-seat, two-bay aeroplanes. The curves refer, of course, to machines of average proportions, and are affected by radical changes in wing section used, in wing bracing, and in general " fineness " of the machine. For biplanes of more or less normal proportions they should, however, give fairly accurate results. In Fig. 2 are curves of absolute ceiling, " service " ceiling, i.e., height at which the rate of climb is 100 ft./min., and rate of climb at sea level, are plotted on a somewhat unusual base ; " combined loading," i.e., wing loading in lbs./sq. ft. multiplied by power loading in lbs. /h.p. Thus the '' combined loading " of a machine having a wing loading of 7 lbs./sq. ft. and a power loading of 20 lbs./h.p. will be 140. The curves would probably not hold good for abnormal loadings, such as, for instance, if the figure 140 for combined loading were obtained from reversing the power and wing loadings. For normal loadings, however, they are probably extremely close to actual figures. The lecturer pointed out that the curves in Fig. 2 (and also in Fig. 4, to which reference will be made later) refer to R.A.F. 15 wing section, and that if a different aerofoil section was used an '' equivalent wing loading " should be found for use with the curves. This " equivalent wing .. . . , . Atmax. of R.A.F. 15 loading was : Actual wing loading x , -. - ;. «L max, of section used It is also assumed that if a different wing section is used it will have round about the same efficiency as R.A.F. 15, otherwise the curves will not apply. It should be pointed out that for the speed curve, Fig. 1, tire actual wing loading should be taken. Fig. 3 is a rate of climb chart for normal machines, in which a series of straight-line curves of " combined loadings " ranging from 53 -2 to 188 are plotted on a base of rate of climb in ft./min. From these curves the probable rate of climb at any altitude can be found if the " combined loading " is known, and the rate of climb at ground level has first been ascertained from the curve in Fig. 2. In Fig. 4, as already stated, it has been assumed that the wing section used is R.A.F. 15, and for other sections the " equivalent wing loading " should be found as explained above. The curves give the time to altitude for various " combined loadings " and sea-level rates of climb. Fig. 5 gives a speed variation curve at various altitudes below the absolute ceiling of the machine. The use of this curve is explained by the note on the actual graph. Finally, in Fig. 6 we have a series of curves for the rapid determination of landing speeds, or more correctly speaking stalling speeds. The lecturer pointed out that in designing for any given landing speed a slightly lower stalling speed should be allowed than specified maximum landing speed. The curves in Fig. 6 were, of course, obtained from the formula Weight W = lift = A^pAV2, from which landing speed V — A. / — x / Wing loading 1 V £, max. x 0-0051 x A V kj, max. x 00051 In 92
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