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Rolls-Royce Gem 42 Turboshaft

The Rolls Royce Gem 42 is a helicopter turboshaft engine in the 1,000 horsepower class with a few notable features. The Gem is sort of a british version of a Lycoming T53, similar in size and power output. One major distinction between the Gem and the T53, however, is that the Gem employ's Rolls Royce's three spool philosophy which features a split compressor on separate spools to optimize engine efficiency, particularly at off-design settings, while also improving engine responsiveness. The Gem can be found in the British Westland Lynx helicopter, which is particularly noteworthy for the fact that a Lynx holds the world speed record for a helicopter of approximately 250 miles per hour.

The Rolls Royce Gem is a compact three spool free shaft turbine shaft engine with a takeoff rating of 1,000 shaft horsepower at an output speed of 6,164 revolutions per minute, though the engine is capable of 1120 shp for 2.5 minutes in an emergency. At the very front of the engine is a compact output reduction gearbox and torquemeter which features a 4.38:1 reduction to slow down the rated power turbine speed of 27,000 rpm to the nominal output speed. Load is taken off a flange at the very front of the engine.

Engine intake air must flow around this gearbox and past fixed inlet guide vanes through an inducer type inlet before reaching the low pressure compressor. The low pressure compressor is a 4 stage axial compressor with fixed stator vanes. The low pressure compressor has a maximum speed of 37,000 rpm, and speed automatically changes depending upon inlet conditions to optimize compressor performance. True engine power output is almost directly proportionate to low pressure compressor speed. The discharge air from the axial low pressure compressor is fed past guide vanes and an interstage air bleed manifold to the single stage centrifugal compressor, which rotates at a maximum speed of 43,000 rpm. The high pressure compressor accelerates the airflow outwards into the diffuser, where airflow is slowed down and pressure increases to 180 psi. The twin spool compressor allows for good surge control without the need for variable inlet guide vanes and variable stator vanes like the T53.

High Pressure air from the diffuser surrounds the annular reverse flow combustor, forming an insulating blanket of air. Air is fed through holes in the inner combustion liner into the burner, where 17 fuel vaporizers mix jet fuel with the air. The mixture is initially ignited by a torch igniter but once the engine reaches self sustaining speed fuel flow to the igniters is cut off. The combustion gas is turned 180 degrees before it escapes through the high pressure turbine nozzle. The nozzle vanes accelerate and direct the gas onto the single stage axial high pressure turbine, which drives the high pressure compressor and the accessory gearbox. The fuel control schedules fuel flow primarily as a function of N2 (high pressure spool) speed, compressor discharge pressure, and turbine inlet temperature. After passing the high pressure turbine, the gas is expanded through the low pressure axial flow turbine which drives the low pressure compressor. The LP spool is ungoverned and is essentially "along for the ride," but for a given N2 speed, N1 (low pressure spool) will vary based upon inlet air pressure and temperature, as well as power turbine speed and compressor air bleed function. The HP and LP spools form the gas producer, which generates high energy gas to drive the two stage axial flow free power turbine.

The free power turbine, or N3, rotates at a rated speed of 27,000 rpm, and drives the front mounted output reduction gearbox through a shaft that runs coaxially through the center of both the low pressure and high pressure spools. A neat feature of the engine is the way three shafts run coaxially with one another, the high pressure shaft being on the outside while the power turbine shaft is on the inside. After driving the power turbine, the gas exits the engine through an axial flow exhaust diffuser, where the exhaust gas is expanded to atmospheric pressure to escape the engine without producing excessive residual thrust.

The accessory gearbox is located on the top side of the engine above the centrifugal compressor. The accessory gear train is driven off of a bevel gear in front of the centrifugal compressor. Accessories include the usual fuel pump, fuel control, oil scavenge pumps and pressure pump, and an electric starter/generator. The engine has an integral oil cooler and engine driven fan, and an integrated engine mounted oil tank. Fuel control is by a hydromechanical system with an electronic fuel trimming system for more precise engine control during acceleration and governing.


Rolls Royce Gem 42 Free Shaft Turbine Engine

  • Type: Three spool, free shaft turbine
  • Inlet: Axial, around output reduction gearbox
  • Compressor: Split compressor; 4 stage axial LP compressor, 1 stage centrifugal HP compressor
  • Burner: Annular reverse flow combustor
  • Turbine: Three spool; sing stage axial HP turbine, single stage axial LP turbine, dual stage axial power turbine
  • Exhaust: Rear facing, single exit axial flow diffuser
  • Power Rating: 1,000 shaft horsepower at 6,164 rpm (Takeoff rating)
  • Rated Torque Output at Full Power: 852 lb/ft @ 6,164 rpm
  • Theoretical Peak Torque Output: est. 1,600 lb/ft @ stall
  • Weight: 404 lbs.
  • Power/weight: 2.48:1 hp/lb
  • Air mass flow: 7.5 lbs/sec
  • Pressure Ratio: 12.1:1
  • Specific Fuel Consumption: .60 lb/shp/hr

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