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Bristol Hydra
by Brian Perkins
"The Hydra is a 16-cylinder, twin row, twin overhead camshaft, poppet valve
unit with the two rows of cylinders in line to allow for the camshafts. There
are only two valves per cylinder, which are operated by the overhead cams
through bucket tappets. The whole design of the engine is so completely
different to anything else that Sir Roy Fedden had been concerned with that I
cannot help feeling that the designer, Frank Owner, was given a much freer hand
than normal, probably due to the concentration by Fedden on the sleeve valve
concept.
An immediate question asked by anyone who knows about radial engines is, 'why
an even number of cylinders per row?' The generally accepted reason is that the
prototype test engine was built as a V4 and when this showed promise they just
multiplied by four.
Only two Hydras were ever built and one of these was test flown in the Hawker
Harrier, which Bristol used for a variety of test engines. It apparently
suffered from severe vibration at critical speeds and although I am sure that
this could have been cured by an intensive development programme, the engine was
dropped in favour of the new sleeve valve engines, which had started to work
well enough to be put into production."
Hydra Displays
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Original Hydra
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Trial Assembly
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Trial Assembly
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Trial Assembly
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Crankcase and Crankshaft
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Crankshaft Installed
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Cylinder Heads
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Cam Boxes
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Crankcase
"The blank originally weighed 22 lb and ended up weighing only 1.65 lb. For a
one-off it is still the best way."
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Mini-Prototype
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Blank
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Finished Crankcase
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Finished Crankcase
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Finished Crankcase
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Crankshaft
Cylinder Heads
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Blanks
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Blanks
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Complete Cylinder
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Complete Cylinder
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Machining Fixture
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Machining Fixture
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Cam Boxes
Sitting on top of the cylinder heads are the 32 cam boxes which again caused
a considerable amount of head scratching as to the best way of producing them. I
could see no way to machine them from the solid, tried producing a rubber mould
for casting but eventually decided that to achieve the finning and other
features the best way was to fabricate them. It was a lot of work but produced a
pleasing end result. These 32 cam boxes had to be made into 16 front (easy), 4
rear RH with gearbox, 4 rear LH with gearbox, 4 rear RH with gearbox and drive
and 4 rear LH with gearbox and drive.
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Fin Bending Jigs
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Fin Bending Jigs
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Cam, Accessory, Magneto and Supercharger Drive Gears
Magneto Drive
Rear Crankcase Cover
Supercharger Diffuser and Rear Cover
Speed Reducer Cover
Fins on the speed reducer housing were machined with the aid of two rotary
tables.
Drawings
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Longitudinal GA
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Tranverse GA
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Crankshaft
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Connecting Rods
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Gears
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Firing Order
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Magneto
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Cam Profile
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