High Quality Content by WIKIPEDIA articles! The Hydra (also known as the Double Octagon) was an experimental 16-cylinder, twin row radial aircraft engine built by Bristol Aero Engines. It is a relatively rare example of a radial with an even number of cylinders – it is often claimed that radial engines require an odd number of cylinders, but this is simply easier, not physically required. Only two Hydras were built, the type never entered production. It appears the Hydra was built as a "backup" design in case the newer sleeve valve engines being designed by Roy Fedden at the same time proved unworkable. Design of the Hydra was led by Frank Owner, who built an experimental V-4 design to test a new twin overhead cam design, a first for Bristol engines which normally used pushrods and rockers. When the V-4 ran successfully, it seems they used four such engines connected to a common crankcase to produce the Hydra.

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