That Merlin mystery.
Geoff Leet wrote in about the remains of the Rolls-Royce Merlin engine recovered from a Hurricane crash site (Letters, PE November). I can offer the following to answer his question.
The gear drives in question are provided to drive auxiliary units, such as air compressors, hydraulic pumps, etc, mounted on the rear face of the cylinder heads. The type of unit mounted varies and depends on the particular aircraft installation.
For the Hurricane, the spur gear attached to the end of each camshaft is used to drive two auxiliary units, via other spur gears and lay-shafts, per cylinder block. In the Merlin II installation with a fixed pitch propeller, the starboard cylinder block (A block) would have both drives employed. The outboard drive was for the AVA (pre Haywood) air compressor which was used for the brakes and gun firing mechanism. The inboard drive was for a vacuum pump which was required for the direction indicator, artificial horizon and turn and slip blind flying instruments.
If a Merlin II or III was fitted with either the two position or fully variable pitch propeller, then the vacuum pump drive was relocated to the V drive of the reduction gear casing and as such only the air compressor was driven from the camshaft (in the same position as before). The tacho drive output is through the same gear arrangement as the auxiliary drives but is situated in the rear of B block's rocker cover.
On some later marks of Merlins and all Griffons, the engines were provided with a single dedicated drive to an aircraft-mounted auxiliary gearbox from which all the various auxiliary units were driven. This eased the task when doing an engine change, being able to leave all auxiliary units untouched.
Dave Piggott, Derby