What is the purpose of the differential gear at the front of a Bristol Hercules aero engine?

Answer 1

It was not strictly a differential gear, it was a reduction gear to reduce the relatively high engine revs by a fixed amount so that at full throttle at maximum speed in fine pitch the propeller tips would not go supersonic and break up through unstable shock wave generation. All high power propeller aero-engines had one, both piston types and turbo-props, especially turbo-props of which the turbine speed was very very much higher than the permissible propeller speed.

Variable pitch controlled the speed of the propeller by varying the angle at which the blades met the airflow, the reduction gear was a fixed ratio which in turn governed the engine revs. Fine pitch = high revs, Coarse pitch = low revs.

Engines like Hercules,Centaurus, Proteus (turbo-prop), Merlin, Griffon, etc. and their American counterparts all had one.