| T40 | |
|---|---|
| T40-A-10 at the National Museum of the United States Air Force | |
| Type | Turboshaft |
| National origin | United States |
| Manufacturer | Allison Engine Company |
| Major applications | A2D Skyshark Convair XFY XF-84H Thunderscreech |
| Developed from | Allison T38 |
The Allison T40 (Allison Model 500)[1] was an early turboprop engine, composed of two Allison T38 power sections driving a common gearbox.
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The T40 was an attempt to produce a high power turboprop engine by combining two T38 engines side-by-side with a joint gearbox to combine power and reduce shaft speed to that necessary for a propeller. This combination was used with a contra-rotating propellers system.
In the late 1940s and early 1950s this engine was favored by the US Navy and was used in several aircraft, the XA2D, XA2J, P5Y, R3Y, and the several VTOL developments including the Hiller X-18, the XFV-1, and XFY-1. The USAF's XF-84H Thunderscreech also tried this engine with a single 3-bladed propellor.
As the individual power plants were clutched into the gearbox, it was intended in most instances that the aircraft could cruise on one half of the engine and only engage the second power-section when there was need. In practice the system did not work so well. Failure to recognize that one of the T38s had failed, and its compressor was devouring power produced by the other section, led to the loss of the first prototype XA2D-1 and its pilot Lt. Cmdr. Hugh Wood on 14 December 1950.
The major problems of the T40 were its fragile gearbox, and the prop-control system which used 25 vacuum tubes, and was far from reliable.
The T40 never produced the 5,500 horsepower (4,100 kW) it was supposed to deliver, and was plagued by gearbox failures, runaway props and prop control failures.
The failure of the T40 was a blow to Navy aviation in the early 1950s almost as severe as the even bigger J40 fiasco which affected diverse but important programs such as the F4D, A3D, and F3H.
The only aircraft using the T40 to actually enter service was the Convair R3Y Tradewind. These large four-engined flying boats served primarily between Alameda and Hawaii during the mid-1950s (replacing the Martin Mars flying boats).
Numerous problems with the T40s, ending in a near disaster where an R3Y managed to land with a runaway engine, resulting in a collision with a seawall in 1958, caused the Navy to ground the R3Y.
The company proposed the Allison Model 503 (military designation T44), with three power sections. Not built and the project was cancelled.[2]
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