Allison Model 250

The Allison Model 250, now known as the Rolls-Royce M250, (US military designations T63 and T703) is a highly successful turboshaft engine family, originally developed by the Allison Engine Company in the early 1960s. The Model 250 has been produced by Rolls-Royce since it acquired Allison in 1995.

Development
In 1958, the Detroit Diesel Allison division of General Motors was chosen by the US Army to develop a new light turbine engine to power a "Light Observation Aircraft" (LOA), to replace the Cessna O-1A Bird Dog. At this stage the US Army was unsure whether to have a fixed- or rotary-wing aircraft, so Allison was instructed to consider both applications. Design studies undertaken considered a wide range of possible mechanical configurations for the turboprop/turboshaft. These studies culminated in the testing of the first prototype engine, designated YT63-A-3, in April 1959. In 1960, the US Army settled for a rotary wing platform. The YT63-A-3 first flew in a variant of the Bell 47 helicopter in 1961. A modified version of the engine (YT63-A-5) with the exhaust pointing upwards (to avoid grass fires) soon followed. This version, rated at 250 hp, passed the Model Qualification Test in September 1962. The Hughes OH-6 design, powered by the T63, was selected for the US Army LOH in May 1965.

The Model 250 powers a large number of helicopters, small aircraft and even a motorcycle (MTT Turbine Superbike). As a result, nearly 30,000 Model 250 engines have been produced, of which approximately 16,000 remain in service, making the Model 250 one of the highest-selling engines made by Rolls-Royce.

Design
Allison adopted a reverse-airflow engine configuration for the Model 250: although air enters the intake/compression system in the conventional fashion, the compressed air leaving the centrifugal compressor diffuser is ported rearwards via two transfer pipes, which go around the outside of the turbine system, before the air is turned through 180 degrees at entry to the combustor. The combustion products expand axially forward through the two-stage (single-stage on early engines) high-pressure turbine section, which is connected to the compressor via the HP shaft. The combustion products continue to expand through the two-stage power turbine which generates shaft horsepower for the aircraft. A coaxial stub shaft connects the power turbine to a compact reduction gearbox, located inboard, between the centrifugal compressor and the exhaust/power turbine system. The exhaust stream then turns through 90 degrees to exit the engine in a radial direction through twin exhaust ducts, which form a V-shape seen in the front elevation.

An important design feature of the Model 250 engine is its modular construction which greatly simplifies maintenance and repair activity. Also the unique reverse-flow design provides for ease of hot section maintenance. There are four modules:
 * compressor module, at the front of the engine
 * gearbox module (including accessory drives)
 * turbine module (including V-shaped exhaust ports)
 * combustion module (including twin compressed air transfer ducts) at the rear

Earlier versions have seven axial compressor stages mounted on the HP shaft to supercharge a relatively low-pressure-ratio centrifugal compressor. The -C20B is typical, with an overall pressure ratio of 7.2:1, at an airflow of 3.45 lb/s (1.8 kg/s), with a power output, at the shaft, of 420 hp.

One of the latest versions of the Model 250 is the -C40, which has only a centrifugal compressor producing a pressure ratio of 9.2:1, at an airflow of 6.1 lb/s (2.8 kg/s), and develops, at the shaft, 715 hp.

Variants

 * 250-B15
 * 250-B15A
 * 250-B15C
 * 250-B15G
 * 250-B17
 * 250-B17B
 * 250-B17C
 * 250-B17D
 * 250-B17Fg
 * 250-B17F/1
 * 250-B17F/2
 * 250-C10D
 * 250-C18 :317 hp
 * 250-C18A :317 hp
 * 250-C20
 * 250-C20B
 * 250-C20F
 * 250-C20J :420 hp
 * 250-C20R
 * 250-C20R/1
 * 250-C20R/2
 * 250-C20R/4
 * 250-C20S
 * 250-C20W
 * 250-C22B
 * 250-C28
 * 250-C28B
 * 250-C28C
 * 250-C30
 * 250-C30G
 * 250-C30G/2
 * 250-C30M
 * 250-C30P
 * 250-C30R
 * 250-C30R/3
 * 250-C30R/3M
 * 250-C30S
 * 250-C30U
 * 250-C34
 * 250-C40B
 * 250-C47B
 * 250-C47E
 * 250-C47M
 * 250-E3: Experimental engine containing a regenerative heat exchanger. First regenerative engine to fully power a VTOL aircraft in flight. Ran on a Hughes YOH-6A Light Observation Helicopter in 1967. 185 lb engine delivering 280 hp.
 * 250-KS4: Used aboard marine ships as a starter system for larger gas turbines. RIMSS, Redundant Independent Mechanical Start System
 * T63-A-5
 * T63-A-5A
 * T63-A-700 :317 hp
 * T63-A-720:420 hp
 * T703-AD-700
 * Soloy Turbine-Pac:Typically 2x 250-C20S driving a single propeller via a combining gearbox, able to operate individually.
 * Mitsubishi CT63: Licence production for Kawasaki-Hughes 500 / OH-6A helicopters.

Fixed-wing
• Aermacchi M-290 RediGO

• BAE Systems Mantis

• Beechcraft Bonanza (Prop-jet conversions)

• Britten-Norman BN-2T Turbine Islander

• Cessna P210 Silver Eagle (conversion)

• Extra EA-500

• Fuji T-5

• Fuji T-7

• GAF Nomad

• Gippsland GA10

• Grob G 120TP

• Partenavia AP.68TP variants - Spartacus & Viator

• RFB Fantrainer

• SIAI-Marchetti SF.260TP

• SIAI-Marchetti SM.1019

• Soloy Cessna 206 turbine conversion

Rotary-wing
• Agusta A109A

• Bell 206

• Bell 407

• Bell 222SP

• Bell 230

• Bell 430

• Bell OH-58 Kiowa

• Bell YOH-4

• Boeing AH-6

• Cicaré CH-14

• Enstrom 480

• Eurocopter AS355F

• Fairchild Hiller FH-1100

• HESA Shahed 285

• Hughes OH-6 Cayuse

• Kamov Ka-226

• MBB Bo 105

• MBB Bo 108

• MD Helicopters MD 500

• MD Helicopters MD 600

• MD Helicopters MH-6 Little Bird

• Northrop Grumman MQ-8 Fire Scout

• PZL Kania

• PZL SW-4

• Schweizer 330/330SP

• Schweizer S-333

• Sikorsky S-76

Other applications

 * Gerhardt 66, USAC Indycar racing car
 * Loral GZ-22, non-rigid airship
 * MTT Turbine Superbike, motorcycle
 * RIMSS, Redundant Independent Mechanical Start System (use on gas turbine ships)

Engines on display

 * A partially sectioned Allison 250-C20B is on public display at the City of Norwich Aviation Museum in Horsham St Faith, Norfolk.