The T56 turboprop, evolved from Allison's previous
T38 series, Propeller upgrades to eight-bladed NP2000 propellers from
UTC Aerospace Systems have been applied to the E-2 Hawkeye, C-2 Greyhound, and older-model C-130 Hercules aircraft, and will be adopted on the P-3 Orion. Production of the T56 engine is expected to continue to at least 2026, with the U.S.
Naval Air Systems Command (NAVAIR) order in 2019 of 24 additional
E-2D Advanced Hawkeyes (AHEs) powered by the T56-A-427A engine variant.
Experimental and non-turboprop uses The T56/Model 501 engine has been used in a number of experimental efforts, and as something other than a turboprop powerplant. In early 1960, two Allison YT56-A-6 experimental turbine engines without propellers were added next to existing propulsion engines on
flight tests of a Lockheed NC-130B 58-0712 aircraft. The YT56-A-6 produced pressurized air for blowing over control surfaces to demonstrate
boundary layer control (BLC), which helped to enable
short takeoff and landing (STOL) performance. In 1963, Lockheed and Allison designed another STOL demonstrator, this time for a
U.S. Army requirement. Lockheed internal designation GL298-7 involved a C-130E Hercules that was re-engined with 501-M7B turboprops. The 501-M7B produced more power than the normally installed, T56-A-7 engines by about 20% (though the 501-M7B was limited to to avoid additional structural changes), because the introduction of air cooling in the turbine's first-stage blade and the first and second-stage vanes allowed for an increase in the turbine inlet temperature. In 1963, an
aeroderivative line of industrial gas turbines based on the T56 was introduced in under the 501-K name. The 501-K is offered as a single-shaft version for constant speed applications and as a two-shaft version for variable-speed, high-torque applications. Series II standard turbines included the
natural gas-fueled 501-K5 and the liquid-fueled 501-K14. The air-cooled Series III turbines included the natural gas-fueled 501-K13 and the liquid-fueled 501-K15.
Marinized turboshaft versions of the 501-K were used to generate electrical power onboard many United States Navy cruisers and destroyers, and for propulsion on 4 Canadian Navy destroyers. During the late 1960s, the U.S. Navy funded the development of the T56-A-18 engine, which introduced a new gearbox compared with the early gearbox on the T56-A-7. The 50-hour preliminary flight rating test (PFRT) was completed for the T56-A-18 in 1968. In the early 1970s,
Boeing Vertol selected Allison (at that time known as the Detroit Diesel Allison Division (DDAD) of
General Motors) to power a dynamic-system test rig (DSTR) supporting the development of its
XCH-62 heavy-lift helicopter (HLH) program for the U.S. Army, using the Allison 501-M62B turboshaft engine. The 501-M62B had a 13-stage compressor based on the 501-M24 demonstrator engine, which was a fixed single-shaft engine with an increased
overall pressure ratio and a variable-geometry compressor, and it had an
annular combustor based on the T56-A-18 and other development programs. The turbine was derived from the fixed single-shaft T56, which had a four-stage section in which the first two stages provided enough power to drive the compressor, and the other two stages offered enough power to drive the propeller shaft. For the double-shaft 501-M62B engine, it was split into a two-stage turbine driving the compressor, where the turbine stages had air-cooled blades and vanes, and a two-stage free power turbine driving the propeller through a gearbox. The 501-M62B also incorporated improvements proven by Allison's GMA 300 demonstrator program, which allowed for an airflow of . After DSTR testing was successful, the 501-M62B engine was further developed into the
XT701-AD-700 engine for use on the HLH. The XT701 passed the tests required to enter ground and flight testing on the HLH, but funding of the HLH program was canceled in August 1975, when the triple-turbine, tandem-rotor helicopter prototype had reached 95% completion. Following the HLH program cancellation, Allison decided in early 1976 to apply the XT701 engine technology into a new industrial gas turbine product, the 570-K. The industrial engine, which entered production in the late 1970s, was derated to and adapted for marine, gas compressor, and electrical power generation variants. The only major changes made for the 570-K were the elimination of compressor
bleed air and replacing the XT701's titanium compressor case with a steel case. The 570-K was then adapted to the 501-M78B demonstration engine, which Lockheed flew on a
Grumman Gulfstream II as part of the NASA Propfan Test Assessment Program in the late 1980s. The 501-M78B had the same 13-stage compressor, combustor, 2-stage gas producer turbine, and 2-stage free power turbine used on the XT701 and 570-K, but it was connected through a 6.797 reduction ratio gearbox to a
Hamilton Standard single-rotation
propfan, containing propfan blades that were swept back 45 degrees at the tips. ==Variants==