Allison 250

Allison 250-C20B manufactured by MTU

The Allison 250 , military designation Allison T63 , is a shaft power engine of the American Allison Engine Company , which was intended as a military development from around 1965 primarily for use in military helicopters . From about 1967 it found its way into the civil helicopter market. Later, a modification was developed as a turboprop engine for use in fixed-wing aircraft. After Allison was taken over by Rolls-Royce PLC in 1995 , the engine is now known as the Rolls-Royce M250 (civil) or Model 250 (military).

It is a twin-shaft engine that was developed and built by Allison from 1958 onwards due to a military requirement. The Russian design Isotow GTD-350 , which was completed about five years later, looks surprisingly similar .

The engine's maiden flight took place in January 1961 in a Bell UH-13R . It was used in series in the Hughes OH-6 A from 1965 , initially with an output of 317 hp. Up to variant C30, the compressor consisted of a combination of axial and radial compressor stages. The engine is economically very successful. Over 31,000 units had been delivered by 2018. From 1979 to 1983, the 250-MTU-C20B variant of the engine was also manufactured in Germany by MTU for the military variant of the Bölkow Bo 105 under its own military approval.

The engine is still in production today (2013). The current model is the Rolls-Royce M250 C40 and C47, which have a FADEC and are used in the Bell 430 , for example.

The turboprop variant is currently used in the Britten-Norman BN-2T turboprop (a Britten-Norman BN-2 islander with turboprops instead of piston engines) and its military version, the Britten-Norman BN-2T Defender or the Extra 500 .

Rolls-Royce PLC is planning a VTOL - air taxi , with hybrid electric propulsion, in which the engine is to be used to charge the batteries, which in turn feed the electric motors.

Description Allison T63

designation

The Allison 250 engine was initially developed as a military device and was then referred to as the T63 according to the designation system of the US armed forces. In the civil service that followed, it was given the designation "Allison 250".

Modular construction

The engine has a modular structure and consists of four main assemblies:

• compressor
• Combustion chamber
• transmission
• turbine

Separate service life limits are defined for each of these four modules ; they can be exchanged by the operator as part of maintenance measures as required. This means that if a module is defective, the entire engine does not need to be sent in for repair.

Gas flow

The air is compressed in the compressor at the front and then fed by means of two pressure lines to the side of the gear unit and the turbine module to the individual reversible canister combustion chamber at the rear. The compressed air is deflected 180 degrees at its head end before it enters the actual combustion chamber. There, fuel is injected through a single, centrally located nozzle . The fuel-air mixture is ignited by a single laterally offset spark plug . The gas enters the turbine from the combustion chamber. After the gas has flowed through the turbine section, the exhaust gases are expelled upwards through a V-shaped exhaust gas duct. The gearbox, which is the central supporting element of the engine, is located between the turbine section and the compressor. The flanges for mounting the engine in the airframe are also located on this .

compressor

Up to the Allison 250-C20B model, the compressor consists of six axial compressor stages and one radial stage. The axial compressor stages are surrounded by an axially split stainless steel housing , which is provided with a special plastic lining on the plane of the impellers in order to achieve optimum clearance between the tips of the compressor blades and the housing. The axial rotor stages are an early form of the blisk ; H. These are running disks with integral blading , which were manufactured in a special investment casting process. The individual disks are held together with the help of an axial tension bolt. The axial compressor stages were omitted in later models. The compressor has a total of five bleed air outlets . There is an anti- icing system operated with warm compressor bleed air on the compressor inlet housing . The hot air required for this is taken after the centrifugal compressor. The so-called compressor blow-off valve is located on axial compressor stages five and six. This valve regulates the air volume steplessly depending on the operating condition (partial load or full load) in order to relieve the compressor in certain operating conditions.

transmission

The gearbox is the central part of the engine. Both engine shafts are mounted concentrically in it. It houses the reduction gear for the power take-off, most of the lubrication components and a transfer case for the auxiliary drives. The gear housing is made of electron . On the C20 and C20B, the electric starter generator and the fuel control system (mechanically and pneumatically controlled in Bundeswehr models) are also installed on it. Engine regulators from various manufacturers were offered for the Allison 250 engine. For example, systems from the manufacturers BENDIX (mechanical-pneumatic) and CECO (hydromechanical) are in use for the C20 and C20B models.

turbine

The turbine module contains two separate sections: the two-stage gas generator turbine and the two-stage power or power turbine. The wave trains of these two sections, designed as hollow shafts, are arranged coaxially and not mechanically connected to one another. The expanding gas first flows through the gas generator section and then through the power turbine, in which the shaft power is generated (so-called two-shaft design).

lubrication

The lubrication is ensured by a dry sump lubrication . The oil reservoir and oil cooler are located outside the turbine housing and are part of the installation scope of the respective aircraft cell. The return oil that emerges from the middle turbine oil sump is collected in an external return oil tank belonging to the engine. The oil pump is driven by the gas generator shaft. The bearings are supplied by spray oil lubrication. An exception are the bearings on the main output shaft, which have oil mist lubrication. The oil pump is a gear pump with a pressure pump and four return pumps.

The oil pressure is limited to nine bar by a control valve. The main flow oil filter has a bypass valve. There are four sieves in the oil circuit; two sensors, which react to metallic chips, serve as chip warning devices.

The oil pumps and the oil filter are located inside the transmission module, from which the main bearings etc. are also supplied via oil lines.

Technical specifications

Engine	Mass in kg	Air flow in kg / s	compression ratio	Starting power (kW / WPS)	Continuous output (kW / WPS)	Height in mm	Width in mm	length in mm
C20B / F / J	73	1.230	6.2: 1	313/420	313/420	571	483	986
C20R	78	1.733	7.9: 1	339/450	339/450	589	528	986
B17D	90	1.624	7.2: 1	/ 420	/ 370
B17F	93	1.733	7.9: 1	331/450	/ 380
C30M	113	2.54	8.6: 1	/ 650	/ 540	638	555	1041
C30S	114	2.54	8.6: 1	/ 650	/ 501	638	555	1041
C30P	114	2.54	8.6: 1	/ 650	/ 501	638	555	1041
C40B	127	2.77	9.2: 1	/ 715	/ 613	638	555	1040
C47B / M	124	2.77	9.2: 1	/ 650	/ 600	638	555	1040

Production figures (December 31, 2005)

		use	number	Flight hours
1st series	B7 / B15G	plane	95	248,900
	C10 / T63-A-3/5	helicopter	2515	6,430,400
	C18 / T63-A-700	helicopter	3895	19,855,700
2nd series	B17 / B17F (all)	plane	1196	6,231,700
	C20 / T63-A-720 (all)	helicopter	15,313	110.981.200
	C20R (all)	helicopter	927	4,307,200
3rd series	C28 (all)	helicopter	869	7,511,300
4th series	C30 (all)	helicopter	3170	15,788,800
	C40	helicopter	273	523.100
	C47 (all)	helicopter	793	1,579,300

Individual evidence

1. ↑ ^{a b} Andreas Donath: Air taxi: Rolls-Royce starts vertically with an electric motor and gas turbine - Golem.de . July 17, 2018 ( golem.de [accessed July 17, 2018]).

Web links

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