Representative of the name of Rolls Royce today are the Kestrel and Merlin types of engines, both of which are extensively used in high-performance service aircraft.
MANY MILITARY TYPES OF AIRCRAFT are fitted with Rolls-Royce Merlin engines. These engines are high powered units, fully supercharged and cooled with ethylene glycol. The twelve cylinders give a total displacement capacity of 27 litres. These engines are used where the maximum performance of aircraft is required at altitudes of the order of 15,000 feet.
The Series XVI Kestrel is a fully supercharged engine developed for use with modern fuels. It has an international rating of 690 bhp at 11,000 feet, and its maximum power rating for level flight is 745 bhp at 14,500 feet. It is a twelve-cylinder engine, with cylinders arranged in two banks of six inclined to each other at an angle of 60°. The cylinders have a bore of 5 in. and a stroke of 5½ in.
The Kestrel design has been improved from time to time without its basic features being altered. Higher degrees of supercharging and improved forms of supercharger, for instance, have been used, and many minor refinements have been introduced. The engine uses “composite cooling”; the cooling system may be operated right up to its boiling point under a slight degree of pressure, which makes possible the use of a smaller radiator than would otherwise be necessary. A slight amount of boiling when the aircraft is climbing is allowed for, as the radiator is not big enough to dissipate all the heat generated during a climb.
In level flight the engine is entirely water cooled. The radiator is located in a cowl which has an adjustable flap at its exit to control the degree of cooling. This system reduces the drag at high speeds.
The Series XVI Kestrel engine has a compression ratio of 6:1 and is fitted with a “boost control” of the automatic servo type. Another fitting is an accelerator pump with delayed action, which allows rapid and positive acceleration to be obtained. There is also a device for enriching the mixture when taking off. The Series XVI Kestrel is available in three types with different reduction ratios for the airscrew drive - 1:632, 1:553 and 1:477. Its performance figures with a variable-pitch airscrew differ slightly from those already quoted. With a variable-pitch airscrew it has an international rating of 715 bhp at 12,250 feet. The dry weight of the Kestrel engine is 955 lb. Hand starting gear is fitted to the engine, but provision is made for other methods of starting, to be fitted when preferred. Among the accessories that can be fitted to the engine are pumps for a retractable undercarriage and for a hydraulically operated gun turret.
The Merlin engine is a more recent production of the Rolls-Royce Company. It is of the 12-cylinder fully-super-charged V type. Its 54 in bore and 6 in stroke are larger than those of the Kestrel. These figures give the Merlin a total cylinder displacement capacity of 27 litres.
This engine is fitted to many types of military aircraft, including such widely different types as high speed fighters and heavy bombers. It is used where the maximum performance of the aircraft is required at altitudes of the order of 15,000 feet.
In its main constructional features the Merlin follows well-tried Rolls-Royce practice, but incorporates innovations of recent years. It is designed for cooling by ethylene glycol. This method of cooling operates at a relatively high temperature, and the high temperature difference between the cooling liquid and the surrounding air permits the use of a small radiator - an enormously important feature in modern streamlined aircraft of high performance. The radiator is enclosed in a pressure cowl and the flow of cooling air is controlled by an adjustable flap at the rear of this cowl. Drag is reduced to the absolute minimum by this means.
A centrifugal pump of high capacity is used for the cooling fluid. The supercharger, although its general design is based on previous practice, is arranged to deal with the large volume of air required by the Merlin engine working at maximum output.
The carburettor is of special design and embodies many interesting features. It is of the updraught type and the richness of the mixture is automatically controlled by aneroids operated by atmospheric and supercharger pressures.
Special Carburettor Design
Should the aneroid system fail the mixture returns automatically to fully rich, thus preventing damage to the engine through its running on a too weak mixture. Also, over-boosting of the engine with the cruising control in the weak position automatically opens the power jet to the full rich position.
Any possible tendency towards freezing of the throttles at high altitudes is prevented by circulating the scavenge return oil through the hollow throttle valves. A by-pass is incorporated in the system to prevent damage should the pressure become excessive.
The twin fuel pumps are driven by independent shafts, so that if one fails, the working of the other is not interfered with. Each pump, working alone, is of more than sufficient capacity to meet the maximum demands of the engine.
Dual ignition is provided, two twelve-cylinder type magnetos being used. The complete ignition system is screened for use in radio-equipped aircraft. The standard accessories include high and low-pressure air compressors, a hydraulic servo pump for operating a retractable undercarriage, a shaft-driven electrical generator, a hydraulic pump for gun turret operation, a revolution counter, gun synchronizing gear, and electric and hand starting gear. The electric starting gear is a twelve-volt electric motor mounted in a vertical position on the starboard side of the engine.
The Merlin engine, as will be seen from the illustration, has a small end section. Its lines are clean, and the various auxiliary drives are arranged in neat fashion. Provision is made in the lubricating system to provide the oil pressure for the operation of a variable-pitch airscrew.
The international rating of the Merlin is 990 bhp at an altitude of about 12,000 feet. Under these conditions the engine is turning at a speed of 2,600 revolutions a minute. The maximum power is obtained at about 16,000 feet and 3,000 revolutions a minute, and is approximately 1,030 bhp. Without oil the engine weighs 1,325 lb.
COMPOSITE COOLING is a feature of the Rolls-Royce Kestrel engines. In level flight the engines are entirely water cooled. When climbing a slight amount of boiling is allowed for, thus permitting a smaller radiator to be used than would otherwise be permissible. Reduction gearing for driving the propeller is provided, and a choice of three gear ratios is available.