Gem
Rolls-Royce Gem at The Helicopter Museum (Weston)
Type	Turboshaft
National origin	United Kingdom
Manufacturer	Rolls-Royce Limited
Major applications	Westland Lynx

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.^[1]

Design and development

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.^[2]

The Gem 42 develops 1,000 shp (750 kW) at Take-off, Sea Level Static, ISA, but the Maximum Contingency Rating (MCR) is 1,120 shp (840 kW).

Until recently all versions of the Lynx have been Gem powered. However, now that Rolls-Royce own Allison, they have been marketing the more modern LHTEC T800, developed jointly with Honeywell.

Applications

• Agusta A129 Mangusta
• Westland Lynx

Engines on display

• East Midlands Aeropark
• Midland Air Museum
• The Helicopter Museum (Weston)
• 111 (Sunderland) Squadron ATC
• South Yorkshire Aircraft Museum

Specifications (Gem 42)

Data from Rolls-Royce^[3]

General characteristics

• Type: Triple-shaft two-spool turboshaft
• Length: 43.4 in (1090 mm)
• Diameter: 23.5 in (590 mm)
• Dry weight: 414 lb (187 kg)

Components

• Compressor: 4-stage axial LP, single-stage centrifugal HP
• Turbine: 2-stage power turbine, single-stage LP, single-stage HP

Performance

• Maximum power output: 1,000 shp (746 kW)
• Overall pressure ratio: 12:1
• Air mass flow: 7 lb/sec