Allison T40

The Allison T40, company designation Allison Model 500, was an early American turboprop engine composed of two Allison T38 power sections driving a contra-rotating propeller via a common gearbox.

Design and development

The T40 concept originated at Allison in 1944, where design studies were carried out on a 4,000 shp (2,983 kW) turboprop engine. Taking advantage of this effort, the US Navy awarded Allison a contract for the design and development of a 4,100 shp (3,057 kW) turboprop engine. The resulting T40 combined two Allison T38-A-1 power sections side-by-side with a common reduction gearbox powering a contra-rotating propeller. Similar in layout to the Armstrong Siddeley Double Mamba, the T40 differed in that each engine drove both the forward and the rear propellers, unlike the Double Mamba, where each Mamba power section drove either the front or the rear propeller separately.

The T38-A-1 power sections each contained 17-stage compressors giving a pressure ratio of 6.3:1, eight combustion chambers, and four-stage turbines. A combining reduction gearbox was driven via extension shafts from the power sections. Each transmission shaft incorporated a clutch to allow the power sections to be run independently.