POWER FOR SPEED

Otago Daily Times, Issue 23332, 26 October 1937, Page 11

POWER FOR SPEED

NEW BRITISH AERO ENGINE REDUCING COOLING DRAG (From Oub Own Correspondent) (By Air Mail) LONDON, Oct. 2. High power for exceptionally low drag is achieved in the Rolls-Royce Merlin aero engine, details of which the Air Ministry has recently released for publication. With a maximum power rating of 1030 h.p., the drag is kept down by the compactness of the V-Twelve design and by the use of ethylene glycol as the cooling liquid in conjunction with a ducted radiator. The engine is designed to operate under high temperature liquid cooling conditions, with a 95degC. outlet temperature for cruising and 120degC. for climbing and level flight. The big temperature difference between the cooling liquid and the surrounding air enables the radiator area to be Kept small. Further assistance in reducing drag is given by the low velocity ducted cooling, in which the radiator is enclosed in a pressure cowl with the flow of air controlled by an adjustable flap at the rear. Searching flight tests by the RollsRoyce company in conjunction with the Air Ministry have shown that this form of ducted radiator reduces cooling drag to a very small proportion of the engine output. The high speeds obtained by the latest R.A.F. aeroplanes which are equipped with the Merlin engine are accounted for to some extent by this low velocity ducted cooling. Another feature which contributes to good aircraft performance besides the excellent power-drag ratio, is the excellent power-weight ratio. The net dry weight of the Merlin is 13351 b, a figure of slightly less than 1.31 b per horse-power. For the takeoff the maximum permissible revolutions are 2850. EXTRA RESPONSIBILITIES Modern aero-engines are expected to undertake a great many additional responsibilities compared with earlier types, and the Merlin incorporates in its design provision for a complete range of accessory drives, which enable all the multifarious operations which form part of the technique of flying to be done for the pilot when he simply trips a switch or turns a cock. These accessory drives deal with such things as high and low pressure, air compressors, shaft-driven electrical generator, hydraulic pump for retractile undercarriage, hydraulic pump for gun turret operation, and gun synchronising gear. The arrangement of the drives is such that they do not increase the frontal area of the engine. The Merlin has a bore and stroke of 5,4 in and 6in, and the total cylinder displacement is 27 litres, which is 27 per cent, more than that of the RollsRoyce Kestrel, which for long has been in extensive use in the Royal Air Force. Each cylinder has four valves, two inlet and two exhaust, the exhaust valves being sodium cooled with the head and seat surfaces coated with a nickel chromium alloy to resist corrosion. The pistons are of the full

skirted type, machined from aluminium alloy forgings and having three pressure rings and two scraper rings. Heart of the engine, the crankshaft provides in itself alone sufficient material for a prolonged and interesting study. It is a one-piece forging in hardened steel, dynamically balanced and carried in seven main bearings. The method of securing the main bearing caps in the crankcase by long transverse bolts has been proved by long experience in the Kestrel and is used in the Merlin. Forked marine type connecting rods are used with the plain rod working on the outside of the forked rod big end bearing. The divided steel bearing shell is lined internally and externally with a special lead-bronze mixture, the inner surface bearing direct on the crankpin. STARTER AND SUPERCHARGER A 12-volt electric starting motor is mounted on the engine, driving through a gear ratio of 393 to 1, and provision is also made for hand-start-ing through a gear ratio of 14 to 1. The supercharger is at the rear of the engine and is of the centrifugal type, the impeller having radial blades and being built up of two components, an aluminium alloy member with straight blades and a steel member with curved blades, the two parts being so assembled as to form continuous blades. When the engine is turning at its normal speed of 2600 revolutions per minute, the impeller is turning at 22,330 revolutions per minute. The design of the supercharger is based on previous Rolls-Royce practice, but its capacity has been increased to deal with the larger volume of air required by the Merlin working ai maximum output. Boost pressure is controlled by an automatic servo mechanism operated from the pressure side ol the supercharger. This works through a differential linkage and limits the throttle opening to that giving the maximum power permitted at any altitude. Thus the engine Is protected from the damage that might result from overboosting. The Merlin has been successfully type tested for use with high octane leaded fuel, and has completed supplementary tests for running at economical cruising consumptions and for dive bombing at speeds in excess of the maximum permissible revolutions per minute. CONSTRUCTIONAL PROCESSES The highest traditions of British workmanship are maintained throughout the Derby factory where the RollsRoyce Merlin is built. The factory is to a large extent self-contained, possessing its own foundries for both ferrous and non-ferrous materials, having a well-equipped stamping shop, producing its own mouldings, making much of its own electrical equipment and even producing Its own oxygen, which is conveyed by pipelines to the various sections where it is needed for welding operations. A special work planning system is adopted whereby the planning of operations and the design of tool equipment are sectionalised. Groups of tool designers arc responsible for each class of work. One sec-

tion of the jig and tool department, for instance, is concerned solely with milling operations, another with lathe work, another with drilling and boring operations. This method provides an intimacy of knowledge in the detail work which is of great advantage in production.