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The Aquila was a 9-cylinder one-row radial aircraft engine produced by the Bristol Engine Co starting in 1934. It saw little use, as its power range was already covered by existing designs. Its primary use was to supply mechanicals to a 14-cylinder version, the Taurus.
The Aquila was developed at the same time as the somewhat larger Perseus, both being sleeve valve designs. The primary difference was in size, the Perseus was based on the 5.75 by 6.5 in (146 by 165 mm) cylinder used in the Mercury engine, while the Aquila used a new and smaller 5 by 5.4 in (127 by 137 mm) sized cylinder. The result was a reduction in displacement from 1520 to 950 cubic inches (24.9 to 15.6 L).
The first Aquila engine delivered a modest 365 horsepower (270 kW), which was hardly spectacular for an engine of this size. It soon developed into more powerful versions as improvements were worked into the line (as well as similar changes to the Perseus), and by 1936 it had improved to 500 hp (370 kW). This made it an excellent replacement for the Jupiter, which ended production at 590 hp (440 kW) three years earlier, but by this time almost all interest was on ever-larger engines. The Aquila saw almost no use.