The Rolls-Royce Gem is a turboshaft engine developed specifically for the Westland Lynx helicopter in the 1970s. The design started off at de Havilland Engine division (hence the name starting with “G”) and passed to Bristol Siddeley as the BS.360. Rolls-Royce bought out Bristol Siddeley in 1966 and after it dropped the Bristol Siddeley identity the engine became the RS.360.
The Gem’s three-shaft engine configuration is rather unusual for turboshaft/turboprop engines. Basic arrangement is a four-stage axial LP (Low Pressure) compressor, driven by a single stage LP turbine, supercharging a centrifugal HP (High Pressure) compressor, driven by a single stage HP turbine. Power is delivered to the load via a third shaft, connected to a two-stage free (power) turbine. A reverse flow combustor is featured.
The thermal efficiency is expressed mathematically as:
where:
- is the thermal efficiency,
- is the thrust produced by the engine,
- is the jet velocity,
- ṁf is the mass flow rate of fuel,
- is the specific enthalpy of the fuel.