Rolls-Royce Kestrel / F Srs
The D-12 was one of the most powerful engines of its era, and continued to swap records with other contemporary high-power engines. No British company could offer anything like it, and when Fairey imported 50 of the type (renaming them as the Fairey Felix) the Air Ministry had enough and ordered Napier & Son and Rolls-Royce to start work on cast-block engines of their own.
Arthur Rowledge, one of Napier's chief engineers and the designer of the Napier Lion engine, became fed up with management and left for Rolls. In this one move any Napier design effort ended while Rolls' got a boost. Applying every known advance since the D-12 was introduced, Rowledge designed the new engine to use supercharging at all altitudes, allowing it to outperform naturally aspirated engines by as much as they were willing to increase the boost pressure.
The year 1925 saw the beginnings of a new engine which differed radically from its forebears in having each of the two banks of six cylinders formed from a single aluminium-alloy block. Of 21.24 litres capacity, it was of compact design and first appeared, under the designation F.10, as a direct drive unit. During June 1927 Flight was able to announce that the F.10 had completed its official 100 hr type test at the first attempt, and had been granted a Certificate of Airworthiness. The maximum b.h.p. was 490 at 2,350 r.p.m. and the fuel consumption, at normal power and speed, 30 gal/hr. Dry weight was 760 lb.
The blocks were set at an angle of 60 deg, the cylinder heads, with the necessary inlet and exhaust passages, being cast integrally with the walls forming the water-jackets. Renewable valve-seating rings were screwed into the heads and the valve-guide bushes were of cast iron. The flanged joint between the upper end of the carbon steel cylinder liner and the head was made gas-tight by a soft aluminium ring, and near the lower end of the liner a sliding watertight joint was formed by means of a rubber ring fitted into a groove in the liner. Just below this joint a flange formed on the liner abutted on the crankcase and the whole assembly was held in position by long bolts. Where these bolts passed through the water space between the liners they were enclosed in aluminium tubes, swaged at the ends to make a watertight joint with the jacket casting. The two inlet and two exhaust valves per cylinder were operated by an overhead camshaft, through a separate rocker for each valve.
The three oil pumps were of the gear type and were driven by spur gearing from the vertical shaft which drove the water pump. Lubrication was on the dry-sump principle.
The engine was fitted with hand-starting gear, comprising a worm wheel incorporating a multi-plate clutch mounted on the sleeve carrying the main bevel wheel for the auxiliary drive; the clutch was set to slip when a predetermined torque was exceeded. In the event of backfire also the clutch would slip, thus saving the gears from damage.
In Kestrel engines of the types A and B, two Rolls-Royce Duplex carburettors were mounted between the cylinder blocks. The two throttles of each carburettor were connected by a pair of toothed quadrants, and one quadrant of each pair was mounted on the end of a longitudinal shaft so that all four throttles were opened and closed simultaneously and to the same extent. The carburettors were fitted with a device by which the flow of petrol from the float chamber to the jet was automatically regulated in accordance with altitude.
The supercharger consisted of a high-speed centrifugal fan mounted co-axially with the crankshaft at the rear of the engine and drawing air through the carburettors. The aluminium-alloy impeller, with radial blades, was driven through a speed-multiplying gear comprising a system of three planets. These were frictibnally driven from a pinion on the crankshaft by means of slippers, held in engagement with the insides of the gear rims by light springs. As the speed of rotation of the crankshaft-and therefore of the slippers-increased, the radial pressure of these springs was augmented by centrifugal force, and consequently the torque which the planets could transmit increased as the square of the speed. As the torque required to drive the impeller increased at a like rate, the gear was capable of driving the impeller with a predetermined margin of torque above that causing slip, which was constant throughout the working range of speed.
The object of this special friction drive was to protect the gearing against damage on sudden acceleration or deceleration of the crankshaft due to the inertia of the impeller. The impeller inertia, however, in combination with the friction drive, served to damp out torsional oscillations of the shaft which drove the camshaft and auxiliaries, including the supercharger itself.
For all the varieties of Kestrel so far mentioned the normal crankshaft speed was 2,250 r.p.m. and at this speed the unsupercharged low-compression engine developed 490 h.p. at sea level. The unblown high-compression types gave 480 h.p. up to 3,000 ft, the medium-supercharged models 525 h.p. at sea level and 500 h.p. at 3,000 ft, and the fully supercharged versions 480 h.p. at 11,400 ft.
The next series of Kestrels bore the series numbers IV, V, VI, VII, VIII, IX, X, XI and XII. Of these the IV, V and VI were fully supercharged, with 0.632, 0.5S3 or 0.477 reduction gear; the VII, VIII and IX were medium supercharged, with the same sequence of gear ratios; and the X, XI and XII were unsupercharged, with a compression ratio of 7A. The fully supercharged engines were rated at 600 h.p. at 11,000ft, and the medium supercharged and unsupercharged versions at 630 h.p. at 3,000ft and 575 h.p. at sea level respectively.
The ultimate in Kestrel development were the Series XIV, XV and XVI, fully supercharged, but more highly rated than the IV, V and VI. The supercharger ran at 9.4 times the crankshaft speed, whereas on the IV, V and VI it ran at 8.8 times that speed. The rating was 690 h.p. at 2,600 r.p.m. (an increase of 100 r.p.m. over the earlier engines) at 11,000 ft, and at 3,000 r.p.m. the output was 745 h.p. at 14,500 Oft. The weight was 975 lb.
The Kestrel XXX was a later development for training aircraft, and had a rated power of 535 h.p. at 12,500 ft.
One key advance in the Kestrel was the use of a pressurised cooling system. Water boils at 100 °C at standard atmospheric pressure, but this temperature decreases with altitude. Since the amount of heat carried out of the engine is a function of coolant temperature and volume, if the coolant has to be kept below boiling point an increasing amount of fluid has to be used, along with an increasingly large radiator to cool it. The solution was to pressurise the entire cooling system, thereby not only preventing the decrease in cooling performance with altitude, but in fact increasing the boiling point even on the ground. The Kestrel was built to maintain enough pressure to maintain the boiling point at about 150 °C.
In May of the following year Flight again reported on the development of the "F" series, introducing the F.12, and desscribing it as "in a general way the geared version of the F.1W' Later, fuller designations and characteristics of the "F" series were made known; thus the FXIA, FXIB, FXIIA and RXIM differed with regard to gear ratio and compression ratio; the FXI engines had a gear ratio of 0.632:1, and the FXII a ratio of 0.552:1. The letters A and B indicated compression ratios of 6:1 and 7:1 respectively. SubsequentIv, the FXIVA and FXIVB were added to the series; the FXIV had a gear ratio of 0.475:1.
During 1930 the "F" type engine was named the Kestrel, and yet another system of designation was introduced. The name was followed by the series number I, II or III, indicating 0.632, 0.552 and 0.475 reduction gear ratios respectively; then followed the letters A, B, MS, or S, indicating 6:1 compression, 7:1 compression, medium supercharger, or full supercharger.
The Kestrel was first produced in 1927 at 450 hp (340 kW), which soon improved in the IB version to 525 hp (390 kW). This variant saw widespread use in the Hawker Hart family that was the mainstay of British air power during the early 1930s. However it was not long before line improvements increased power dramatically; the V model provided 695 hp (520 kW) at 3,000 rpm with no basic change to the design, while the XVI used in the Miles Master delivered 670 hp (500 kW). Messerschmitt also tested its first Messerschmitt Bf 109 V1 prototype, bearing German civilian registration D-IABI, with a Kestrel engine in 1935 as the German designed intended engines were not yet ready. Junkers also used a Kestrel for the first prototype of the Ju 87 "Stuka" dive bomber. The Reich Air Ministry (RLM) acquired four Kestrel VI engines by trading Rolls-Royce a Heinkel He 70 Blitz as an engine test-bed.
The Kestrel was produced in 40 distinct variants which can be divided into three main groups, normally aspirated, medium supercharged and fully supercharged. One variant, the Kestrel VIII was configured as a 'pusher engine' for the Short Singapore flying boat. Apart from supercharging, the variant differences centred around varying compression ratios and propeller reduction gearing.
The total production of Kestrel engines was 4,750. Unit cost in 1934 was £2,051. Further developments of the Kestrel were the Goshawk and the Peregrine (and therefore the Vulture).