Rolls-Royce Kestrel

Origin Earlier in-line engine designs were generally built on top of a cast aluminium crankcase, with individually-machined steel cylinders, bolted on top. Given the forces involved, the connection of the cylinders to the crankcase had to be robust, adding weight, and as a whole the engine was reliant on the structure of the crankcase to hold it together. In 1923, the Curtiss CR won the 1923 Schneider Trophy. The CR was powered by the recently introduced Curtiss D-12 engine, which replaced the individual cylinders with a cast aluminium block. This gave the engine much greater strength, allowing the rest of the engine to be greatly simplified, making it much lighter overall, as well as easier to assemble as the two parts simply bolted together in a single operation. It was also easy to convert existing assembly lines to the new system as the cast blocks were already being produced for the crankcase, and all that was required was new machines to mill the blocks to the accuracy needed for the pistons. The Curtiss D-12 was one of the most powerful engines of its era, and continued to exchange records with other contemporary high-power engines such as the Napier Lion. At the time, none of the British aero engine manufacturers could offer an engine which offered a similar power which was also as light and compact as the D-12. The D-12 was licensed by Fairey and introduced to as the Felix. Arthur Rowledge, Chief Designer at Napier and the designer of the Napier Lion engine, joined Rolls-Royce in 1921 to take up the role as "Chief Assistant to Mr F. H. Royce". Rowledge built a team to introduce a new engine using the cast block, but set the goal to surpass the D-12. This would be accomplished using supercharging at all altitudes, allowing it to outperform naturally aspirated engines. Previously, supercharging (and turbocharging) was primarily used for high-altitude designs to offset the loss of ambient air pressure as the aircraft climbed, and thereby maintain power. But with the new construction technique, the engine would be so strong that it could be supercharged at all altitudes without overstressing the cylinders, allowing a smaller engine to operate as if it was larger, and thus improving its power-to-weight ratio. The prototype Kestrel engine was first run in 1926, and one first flew in 1927, with a power rating of 450hp (335kW). The engine was normally aspirated in its initial form. Improvements . The driving dog for a Hucks starter can be seen The engine was first produced in 1927 at , which was soon improved in the I-B version to by increasing the compression ratio to 7:1. The I-B variant saw widespread use in the Hawker Hart family of aircraft, a mainstay of British air power during the early 1930s. Development continued and the V model introduced the centrifugal supercharger, increasing power to . 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. The mark-XVI engine used in the Miles Master M.9 prototype delivered , and the XXX variant of 1940 saw service at . Cooling system 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 pressure decreases as altitude increases, and therefore the boiling point of water decreases with altitude. The amount of heat rejected by an air-to-air cooling system is a function of the maximum coolant temperature and volume, so the resulting decrease in cooling capacity became a limiting factor for engine power in this period, as the coolant has to be kept below boiling point. The solution was to pressurise the engine's entire cooling system, thereby raising the temperature at which the coolant would boil: not only does this help mitigate the decrease in cooling performance with altitude, but allows a smaller cooling system to be used in the aircraft for the same heat load. The Kestrel was built to maintain coolant pressure to keep the boiling point at about 150°C. ==Variants==