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Aviation History
1936
1936 - 0952.PDF
r v APRII 16. 1936. FLIGHT. bomber. This machine was fitted with a pair of A B.C. radials in nacelles between the wings. The engine cowling merged into the tail of the nacelle to form a well-shaped body almost equal in diameter at its maximum cross-section to the engine itself. Nine pockets—one for each cylinder—were recessed into the cowling, the cylinder heads being open to the airstream. The arrangement never passed into general use. A later development was the fitting of "tails" or fairings behind each cylinder of a radial, but their efficacy was small. The present Service-type Bulldog IIA fighter with Jupiter engine has its power plant cowled after this fashion, the crank-case cover being formed by the exhaust collector ring. Small inter- cylinder fairing panels are fitted between the exhaust collector ring and the main fuselage fairing. Some years ago a number of firms attempted to encase the projecting cylinders in helmets, each with a very small aperture in front to admit the cooling air. On certain machines built purely for speed (including the Short A combination of " helmets " with modern long-chord cowlings is a feature of the Avro Anson (Cheetah IX's). Crusader racing seaplane and the French Gourdou Leseurre onoplane) this arrangement gave reasonably good results j-certamly better than those obtained with any other con- ^ mporary arrangement—but for everyday use it was ruled because of inadequate cooling and inaccessibility. of th Speeds constantly on the increase the high drag otilv 6 >iadlal became m°re and more apparent. It is dJtiiTr1 tfae Iast eiSht or nine years that the intro- ings h 0t the Townend rin§ and tne N.A.C.A.-type cojwl- same tVe °ffered a verY S^ea± reduction in drag, at the Engin "?u retaining acceptable cooling characteristics, the ne mselves- of course, have had to be adapted to the intr rf° -g techniclue- This fact. in conjunction with in deenp1 *10n of gearing and supercharging, has resulted Perhan \k m°re closely spaced fins on the cylinders. toward th l8?63-*^* contribution made by Great Britain • efficient cowling of radials is the Townend ring, A close-up of the nose of the Blackburn Shark showing how the 760 h.p. 14-cylinder Tiger is fitted with the long- chord Siddeley cowling. (Flight photograph.) introduced by Mr. H. C. H. Townend, B.Sc, A.F.R.Ae.S. Model tests show that a nine-cylinder radial engine could increase the drag of a streamline body to as much as five times that of the body alone. Even a five-cylinder engine could increase it to three times the original figure. These results spurred the National Physical Laboratory, where Mr. Townend was working, to develop the ring. It was found during experiments that the cylinders of a radial normally deflected the air outward, forming a broad turbulent wake. An annular aerofoil placed round the cylinders exerted an opposing, inwardly directed deflec tion on the airstream, preventing this breaking-away and maintaining a greater degree of smoothness in the flow. The reaction on the ring may be regarded as a radial '' lifting '' force tending to burst it outwards and having an axial component in a forward direction tending to push the ring toward the nose of the machine. Some outstandingly good results were obtained soon after the introduction of the ring. A Vickers single-seater fighter with a Bristol Jupiter XF geared engine had its maximum speed increased by 12 m.p.h., bringing it to 191 m.p.h. at 12,000ft. A Boulton and Paul Sidestrand twin-engined day bomber increased its speed from 154 m.p.h. to 167 m.p.h. at 12,000ft. The ring also proved itself very effective on two-row engines, and added 7 or 8 m.p.h. to the speed of the Armstrong Whitworth Siskin fighter with fourteen-cylinder Jaguar engine. Experience showed that for rings of the usual single- surface or plate type a camber of about 10 per cent, of the cord length was desirable. The licence for the Townend ring was acquired by Boulton and Paul, Ltd., who proceeded with its develop ment. They embodied in the nose of certain rings annular exhaust collectors (the Sidestrand mentioned was fitted with this type) and made others of polygonal form. Tests with a streamline nacelle showed that the drag with a polygonal ring was slightly less than with a circular type. To be concluded next week.)
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