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Aviation History
1940
1940 - 0386.PDF
FEBRUARY 8, 1940 The frontal views shown on the right are more or less to scale. The Merlin represents the latest practice in the construction of vee-twelve liquid-cooled units and makes an interesting comparison with the inverted Daimler-Benz. The big Allison is virtually a combina- tion of two vee-twelve units. The first successful twenty-four cylinder engine of H layout is the Napier Dagger which is of approximately the same power as the Bristol Taurus radial shown next to it. Although the Dagger is of smaller frontal area it is longer than the Taurus and has a larger "wetted area." AIRCRAFT POWER 1. Rolls-Royce Merlin. 2. Allison XB-342 frontal area or lengthy and of small cross-section. It is unwise to take frontal area as the deciding aero- dynamic factor ; wetted area is the safer criterion. The method of arranging cylinders is largely determined by the cross-section and is influenced by the need for good balance, reasonable bearing loads, even distribution, satisfactory cooling and even torque. Although radial combinations of 30 cylinders are quite feasible, the most powerful radial now running— the Wright Duplex Cyclone—has only 16. Small-dia- meter radials and vees in cowlings of circular section make good " airscrew supports" due to their shape, whereas engines of H, X or W layout may be happier when submerged completely within a wing or fuselage. A particularly interesting development which has lately appeared is the American Pratt and Whitney Double Wasp 14- cylinder two-row radial with extension shaft. This combination results in a cowled radial installation comparable in efficiency with contemporary liquid-cooled practice. Cooling air is taken in through an adjustable scoop below the cowling and just behind the spinner, and is exhausted through con- trollable gills round the trailing edge of the long-chord cowling. The vee-type engine will probably be re- tained for powers of 1,500-2,000 h.p. In this country we are continuing to develop the upright vee (e.g., Rolls-Royce Merlin and Peregrine), and the Americans and French are working on similar lines (Allison, Hispano-Suiza and Lor- raine), but the Germans are pro- ceeding with the inverted-vee arrangement (Daimler-Benz and The top view of the three on the left shows one of the most modern installa- tions of an air-cooled radial engine. This is on the Double Wasp-powered Curtiss P.42. Below it is one nacelle of the Bell twin-engined pusher fighter showing how one completely cowled Allison engine drives its pusher pro- peller. The exhaust-driven super- charger and the gun position ahead of the engine are points of interest. TTM bottom view shows the exquisite cowling lines of the Boulton Paul Defiant, which has a Rolls-Royce Merlin engine.
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