Aircraft Auxiliary Power Unit  

The Solar T41-M9A is an aircraft auxiliary power unit designed to provide power in the form of electricity as well as cabin heating and ventilation. The APU was designed in the late 40's/early 50's to be used in large aircraft; to provide auxiliary power when the main engines are not running. The T41 engine was produced by the Solar Turbine company, a division of International Harvester. Later Solar was purchased by Caterpillar, who own it today. The T41 was used in other applications as well, including water pumps, and ground power units. A hand crank start version of the T41 with a magneto type igniter was produced by the engine Manufacturer Perkins, and was rebadged the Mars.

The Solar T41 gas turbine engine is about as basic a gas turbine engine design as can be, and its less than stellar performance shows the engine's age. The T41 can be classified as a single spool, fixed shaft turboshaft engine, with a power output of 50 shaft horsepower at a rated speed of 6,000 rpm. Peak rotor speed is 40,000 rpm, which is actually low for such a small rotating group.

The engine features a single stage centrifugal compressor, with a pressure ratio of only 2.6:1, and an air mass flow of 2.5 lbs of air per second. The compressor draws air in through a screened annular inlet in between the turbomachinery and the reduction gearbox, and feeds a diffuser and a single can burner. Fuel is sprayed through a single fuel nozzle, and is ignited initially by a high energy spark plug. Peak turbine inlet temperature is 1250 degrees F. The combustion gas is expanded through a single stage radial inflow turbine. The exhaust then exits from the center of the turbine towards the rear of the engine. The turbine and the compressor sit very close to one another, on the same shaft, in a back to back configuration, making for a very compact engine. The engine reduction gearbox sits at the front of the engine and is driven off of a bevel gear on a shaft that protrudes from the front of the compressor. The bevel gear changes the rotation through 90 degrees, and all of the accessories and the engine load are driven at right angles to the main engine rotor's plane of rotation. The accessories that are mounted to the gearbox include an oil pump, a high pressure fuel pump, a starter generator, and a fuel controller. The oil system is a wet sump type, using the reduction gear housing as the sump from which the oil pump draws lube oil to feed to the bearings. Oil drains back into the sump via gravity. Fuel control is completely through a hydromechanical and pneumatic governor and acceleration limiter unit. Load is taken off from the engine's reduction gearbox at an output speed of 6,000 rpm.


Solar T41 Gas Turbine Engine

  • Type: Single spool, simple cycle turboshaft
  • Inlet: Radial
  • Compressor: Single stage centrifugal
  • Burner: Single can combustor
  • Turbine: Single stage radial inflow turbine
  • Exhaust: Rearward facing, straight through single exhaust outlet
  • Power Rating: 50 shaft horsepower at 6,000 rpm
  • Peak Torque Output: 44 lb/ft at 6,000 rpm
  • Weight: 95 lbs.
  • Power/weight: .53:1
  • Air mass flow: 2.5 lbs/sec
  • Compression Ratio: 2.63:1 at 40,000 rpm
  • Maximum TIT: 1250 degrees F
  • Specific Fuel Consumption: 2.3 lb/shp/hr

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