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Aviation History
1928
1928 - 0106.PDF
FEBRUARY 16, 1928 WINNER OF THE SCHNEIDER RACE First Description of the Supermarine S.5 As the winner of the 1927 race for the Schneider Seaplane Trophy, and as a potential holder of the world's speed record in the near future, the Supermarine S.5 with Napier Lion " racing engine is one of the most interesting of modern British aircraft, and it is with a good deal of satisfaction that we are able to place before our readers this week some par- ticulars and a number of illustrations of its more interesting features. In his paper read recently before the R.Ae.S. & I.Ae.E., Mr. R. J. Mitchell, chief engineer of the Supermarine Aviation Works, gave certain very interesting figures relating to the S.5, but owing to the fact that the results of wind tunnel tests could not be published, much information which would have been extremely interesting had to be withheld. Nor are we, for obvious reasons, in a position to give these here. For instance, the proportion of the total drag represented by the fuselage, the floats, the float struts, and the wings. But in the absence of such information it is permissible to specu- late a little and to attempt to form, from other sources, an idea of the efficiency of a machine like the S.5. Probaby the " Everling Quantities "* afford the simplest available means of doing this, and of the three " Everling Quantities " it is, in this case, particularly the " High-speed Figure " in which we are interested. In his article Professor Everling arrived at a value of the " High-speed Figure " of 40 as a sort of theo- retical maximum or " ideal," and pointed out that actual machines never attained this, but generally reached a value of about half of the " ideal." The Everling formula for " High- speed Figure " is, for ground level flight such as would apply to a racing machine like the S.5 V V'xF C,u 56,000 x N where rj is the propeller efficiency, C,^ the drag coefficient, V the speed in km./hour, F the wing area in square m. and N the brake horse-power of the engine. The brake horse- * See THE AIRCRAFT ENGINEER (Technical Supplement to FLIGHT), November 25, 1926, and February 24, 1927. power of the Napier engine may be assumed to be 875 h.p. The wing area of the S.5 is 115 sq. ft. = 10-68 sq. m., and if we assume a top speed of 300 m.p.h. (484 km./hr), a figure which is probably somewhere very near the actual speed on a straight-line course without previous diving, we obtain a value of the " High-speed Figure " of 484s x 10-68 _ „ 56,000 x 875 " This figure, of course, represents propeller efficiency divided by drag coefficient, and in the absence of accurate information concerning the sort of efficiencies that obtain in the actual machine we are again compelled to make the best guess we can. Probably 80 per cent, would be somewhere near the mark, and if this is assumed as the value of ri, the drag coefficient of the whole machine at top speed is 0-032. As the German coefficients are twice the value of ours, we obtain a drag coefficient, in British " absolute " units, of 0-016. Admittedly we have had to " guess " several of the figures upon which this value is based, but probably it is at least approxi- mately correct. When it is remembered that the machine is a seaplane, and that therefore the float undercarriage must offer considerably greater drag than a land undercarriage, this low value of the drag is very remarkable. In this connection it is not without interest to compare the S.5 with the de Havilland " Tiger Moth " described and illustrated in FLIGHT of September 22, 1927. That machine has a "wing power" of 18-3 h.p./sq. m. (1-7 h.p./sq. ft.) and, at 186-5 m.p.h. (the speed which gained the "Tiger Moth " a world's record in its class) its " High-speed Figure " is 26. If we assume the same propeller efficiency, i.e., 80 per cent., the " absolute " drag coefficient of the " Tiger Moth " at top speed is 0-0154, which is verv slightly lower than that of the S.5. The " Tiger Moth " is, however, a land machine, and has an undercarriage of particularly low drag. The comparison is also interesting in demonstrating how the " Everling Quantities " do serve to show, at least in a general THE SUPERMARINE S.5 : Three views of the all-metal fuselage. The lower photograph shows it in skeleton.In the centre the fuselage is seen undergoing sand loading test, and in the upper picture the Napier " Lion " racing engine has been dropped into place. Note the supports for the cylinder block fairing. 94
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