| Kestrel | |
|---|---|
| Rolls-Royce Kestrel XVI at the Royal Air Force Museum Cosford | |
| Type | Piston V-12 aero engine |
| Manufacturer | Rolls-Royce Limited |
| First run | Late 1926 |
| Major applications | Hawker Hart Miles Master |
| Number built | 4,750 |
| Unit cost | £2,051 (1934)[1] |
| Developed into | Rolls-Royce Goshawk Rolls-Royce Peregrine |
The Kestrel or type F is a 700 horsepower (520 kW) class V-12 aircraft engine from Rolls-Royce, their first cast-block engine and the pattern for most of their future piston-engine designs.[2] Used during the interwar period it remains somewhat obscure, although it provided excellent service on a number of British fighters of the era. The engine also sold to international air forces, and it was even used to power prototypes of German military aircraft types that were later used during the Battle of Britain. Several Kestrel engines remain airworthy in 2010.
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The Kestrel came about as a result of the excellent Curtiss D-12, one of the first truly successful cast-block engines. Earlier designs had used individually machined steel cylinders that were screwed onto a crankcase, whereas the cast-block design used a single block of aluminium that was machined to form cylinders. The result was both simpler to build as well as lighter and much stronger, requiring only an investment in new machining equipment.[3]
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.[4] 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.
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.[5]
The engine 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.[6]
Increased availability of higher octane aviation fuels in the late 1930s allowed the engine to be boosted to higher power levels without suffering from detonation, and the Kestrel eventually attained a power output of 720 hp (537 kW) in the XXX variant of 1940.[7]
Further developments of the Kestrel were the Goshawk and the Peregrine (and therefore the Vulture). In practice, development of the Peregrine and Vulture was troubled and they were both cancelled with comparatively few built.
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.[8]
Note:[9]
Several Rolls-Royce Kestrel engines remain in service powering restored Hawker biplane types:
Preserved examples of the Rolls-Royce Kestrel engine are on public display at the:
Data from Lumsden[14]
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