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Aviation History
1942
1942 - 1685.PDF
AUGUST I3TH, 1942 FLIGHT V Side view of a com plete unit taken from a Dornier 217 bomber. One of the blower intakes is on the lower left. The internal air sooop is attached to the cowl ing. A panel re moved from the nose gives access to the magneto. THE B.M.W. 801A Details of Germany's Latest Twin-row Radial Power Plant By F. C. SHEFFIELD IT would be somewhat misleading to refer to the power unit employed on the Focke-Wulf 190 and Dornier 217 machines merely as the B.M.W. engine. In the fullest sense it is a complete power plant, conceived and constructed as a compact whole. Here is no question of an engine fitted with a number of auxiliaries and enclosed •• a cowling whilst coolers and air bops are added externally. The idea maj' not be novel in conception, but the practice is new in production and of outstanding interest. Using the fourtetn-cylinder air- cooled twin-row radial B.M.W. engine as a basis, the aim was to obtain the maximum power output whilst reducing overall dimensions and frontal area to a minimum and presenting an aerodynamically smooth exterior. Every thing is subordinated to this end. Many ingenious constructional features are employed, and the practical result must be regarded as a justification of the design. Within a cowling of 52m. diameter and baving-an overall length of 58111. is packed a remarkable power output. The rating under various conditions is as follows: — Boost r.p.m. b.h.p. Altitude Takeoff 1.32 atm. 2,700 1,580 Emergency maximum 1.3 ,, 2,550 1,585 15,750ft. International rating .. 1.27 ,, 2,400 1,460 16,300ft. Continuous maximum 1.15 ,, 2,300 1,280 18,500ft. Economical cruising .. 1.1 ,, 2,100 1,125 19,500ft. To achieve the low overall diameter the cowling closely encircled the 5oin. diameter engine and is supported by front and rear rings and an inter-cylinder baffle ring, all attached to the valve rocker s by rubber-bushed bolts. Special measures ere taken both to introduce the requisite amount of air and to ensure its distribution throughout the in terior. Cooling' Fan At the front end the cowl ing is faired down to shroud a twelve-bladed magnesium alloy fan of 32m. diameter. The fan is driven from the air screw- reduction gear at 1.72 times crankshaft speed, which is equivalent to 3.17 times the airscrew speed. An adequate air flow is thus assured when the engine is running on the ground or in flight at low speeds. Of course, considerable power is required to drive a fan of this size, and it is esti mated that 100 h.p. is needed B.M.W. 801A Type ..... 14-cylinder, twin row, radial. Bore and Stroke - 6.15in. X 6.15in. Swept capacity 42 litres. Overall dimensions ... 52in. dia. X 58in. long. Rated Output - 1.460 b.h.p. at 16,300ft. Take-ofl power - 1,580 b.h.p. for 3 min. Front view showing the fan which is driven at 1.72 times crankshaft speed. It supplies all air for the engine, cylinder cooling, oil cooling, and cabin and wing heating. for this purpose at the take-off. When the aircraft has a forward speed of 170 miles per hour, however, the fan is motored round by the air stream, and no power is absorbed directly. The loss of 100 h.p. of the critical take-off output must be reckoiied a disadvantage, but this is offset by the advantages in flight conferred by a positive cool ing air flow and by the small diameter low-drag cowling. It is an indication of how the unit is considered as a whole. Exit of air from the interior of the cowling is by way of two circumferential slots controlled by sliding gill rings, as indicated in the diagram of the airflow system. Each of these gill rings is actuated by an electric jack and a push-pull cable. A series of bell crank levers spaced around the diameter of the support member and coupled to the gill ring and an equaliser ring ensure axial move ment, giving a uniform annular slot. The rear ring, con trolling the airflow past the cylinders, has two positions only, with an axial movement of 3.12m. Maximum and minimum discharge areas, after deducting the cross-sec tional area of the exhaust pipes which project through this slot, are 3.7 sq. ft. and 1.2 sq. ft. respectively. The forward ring, sealed to the shrouding of the cooling fan by a flexible rubber joint, is variable throughout its permissible axial movement of i.i2in. to control the reverse flow of air from the interior of the cowl ing through the oil coolers. This slot has a maximum area of 0.73 sq. ft. Internal Air Scoops The air intake for the super charger is also completely in side the cowling, and fed from the air delivered by the fan. Two shallow segmental scoops are fitted inside detachable sections of the cowling on each side of the engine. It is, of course, not practical to make a normal bolted connection, and actually the junction is effected by the flange of the D
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