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Aviation History
1932
1932 - 1322.PDF
SUPPLEMENT TO FLIGHT THE AIRCRAFT ENGINEER DECEMBER 29, 1932 26 d. I 2 2-4 < ID 22 IC 00 GAL ^ L5. PER m OR FIG.17 FART Z. 2000 3000 NORMAL R.P.M. (CRUISING) 43 42 41 40 39 58 4000 Fuel Economy. slightly at the expense of fuel economy. With fuel at Is. 7|d. per gallon it is estimated that the extra fuel economy of 7 m.p.g. would more than compensate for the small increase in engine price due to running at 1,750 instead of 3,200 r.p.m. II. & M. 1267 Reference 6.'—Reduction of drag Of radial engines by the attachment of rings of aerofoil section, including interference experiments of an allied nature, with some further applications.—H. C. H. Townend, B.Sc. (To be concluded.) CEILING CAPACITY AS A MEASURE OF PERFORMANCE By CLIFFORD W. TINSON, F.R.Ae.S. IN dealing with aeroplanes fitted with supercharged engines, sea level loses its former significance as a datum level to which to refer performance, and because different engines have various rated altitudes, the rated altitude is not a suitable datum level for this purpose. Speed-range, rate of climb, and ceiling all depend on the excess power available over that required, so that an improvement of performance, whether attained simply by an increase of engine power, or by reducing air resistance by better streamlining, or by improving the thrust horse-power available by gearing the air screw shaft or otherwise, may be related to the height of the absolute ceiling. The ceiling capacity of an aeroplane, therefore, em braces certain aerodynamic qualities in addition to its relation to the face values of power loading and wing surface loading. The increase of speed from ceiling down, provided that the airscrew be of what may be termed average form, is substantially similar for a wide range of wing and power loadings, whilst the slope of the rate of climb curve, as it leaves absolute ceiling (taking this point as origin instead of sea level, as formerly), is intimately connected with ceiling capacity. By making absolute ceiling the datum level of refer ence, and drawing curves to show the appreciation of performance from ceiling downwards, using such portion of them as lies between ceiling and rated altitude, it is possible rapidly to forecast the performance of an aero plane at the altitudes which matter, irrespective of the relation between rated altitude and sea level. Further, the method should improve the accuracy of obtaining climb times between heights nearing the ceil ing, since the errors are convergent towards the ceiling instead of divergent from the ground. With the usual method, the accuracy is greater where the order of climb time figures is small, and less where the climb times are double figures, so that a comparatively small percentage error may make a difference of perhaps a minute to climb to 20,000 ft., whilst the same percentage error makes no appreciable difference low down. By working the opposite way, the error will be rela tively greater in the times between heights furthest from ceiling, at altitudes at which the performance is generally relatively unimportant. When it is inconvenient to make complete calculations for the estimation of aeroplane performance, an approximation may be made from formulae connecting wing surface loading and power loading. The basic formulae for maximum speed, maximum rate of climb, and absolute ceiling are of the following form: — (1) V max = K (2) Vc max Wing loading \« Power loading Power loading (3) H max = 40,000 log10 K, v Wing loading K5 Power loading X -x/Wing loading From the rate of climb, times to heights are obtained from (4) T = 2-303 logio Aj Ai where r = the slope of the rate of climb curve, and Ai and A2 are the rates of climb at heights h1 and 7ia respectively. These formula? are discussed in a number of works, among them being " Airplane Design," by Warner, the " Handbook of Aeronautics," and the current issue of the " Air Annual of the British Empire." The constants K, K„ K2 and K3 are based, naturally, upon results of actual performance, and the published values are representative of average design. The values vary, however, not only with type of aeroplane, but with different authorities. For example, in the speed formula, the value origin ally given by Warner to K as representative generally of the (then) average design is 124. Liptrot's average would be 125, as he gives the value 120 at one end for boat seaplanes, rising to 130 for single-seater fighters, these being the average figures in their respective classes. Another authority gives 126 as the average. Gassner, however, in an article published in " Aviation " in June, 1932, gives the value of K for a single-seater fighter as 137, rising to 150 for what he calls a " refined " aeroplane of similar type. (For a number of years FLIGHT has been in the habit of using the Everling " High-speed Figure " t;/2fcD, which is in effect identical with DiehVs formula quoted by Warner, but which is directly comparable with top speed, etc., in the metric system as used by Conti nental journals. For example, an Everling high-speed figure of 26, which is approximately the highest achieved by a normal aircraft, i.e., one not fitted with retract able undercarriage, corresponds to a value of K = 156 y> in DiehVs formula Fmax — K\/ *- ; #=138 corresponds to rifikj, of 18.—ED.) It is clear that the determination of a suitable value in any particular case demands some experience, and in addition a mental picture of the degree of stream lining or cleanness which the above values represent. Further, if K has an original value of 137 in a par ticular case, and by better streamlining the value can rise to 150—roughly 15 miles per hour faster—the power and wing loading being unchanged, then the values of Ku K2 and K3 in the climb and ceiling formulae must change in sympathy, for less power is required, render ing more available for climb, and a higher ceiling will be attainable for the same face values of power and wing loadings. In the ceiling formula, the average value of K3 given by Warner is 88, but here again the values must vary 1232/
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