Allison Model 250
The Allison 250 military designation Allison T63 is a turboshaft engine of the U.S. Allison Engine Company, which was intended as a military development from about 1965 primarily for use in military helicopters. From about 1967 it was included in the civil helicopter market. Later, a modification was developed as a turboprop engine for use in fixed wing aircraft. Following the acquisition of Allison in 1995 by the Rolls- Royce PLC, the engine is ( civil ) or Model 250 ( military) now known as the Rolls- Royce M250.
It is a two-shaft engine that was designed and built from 1958 due to a military request from Allison. Surprisingly similar looks to the Russian draft Isotow GTD -350, which was completed about five years later.
The first flight of the engine took place in a Bell UH -13R in January 1961. It was used from 1965 in the Hughes OH -6A series, first with a power of 317 hp. The compressor consisted of a combination of axial and radial compressor stages to the variant C30. The engine is a major commercial success. To date, over 30,000 units were sold. From 1979 to 1983 the engine in the variant 250 - MTU - C20B was also manufactured in Germany at MTU for the military version of Bölkow Bo 105 under its own military approval.
The engine is still produced today ( 2013). Current model variants are the Rolls-Royce M250 C40 and C47, which have a FADEC and are used for example in the Bell 430.
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 even with the extra 500.
Description Allison T63
Designation
The engine Allison 250 was originally developed to military equipment and the designation system according to the U.S. forces was then called the T63. In the following civilian use after it was given the name " Allison 250 ".
Modular design
The engine is modular and consists of four main modules:
- Compressor
- Combustion chamber
- Transmission
- Turbine
For these four modules each with its own lifetime limits are defined; they can be exchanged in the context of maintenance procedures as required by the operator. That is, if a module is defective, does not need to be sent in the entire engine for repair.
Gas flow
The air is compressed in the front -mounted compressor, and then guided laterally by means of two pressure lines at the gearbox and the turbine module by the rear-mounted individual reverse combustor. The compressed air is deflected at the tip end 180 degrees before it enters into the actual combustion chamber. There fuel is injected through a single, centrally located nozzle. Laterally offset by a single spark plug, the fuel- air mixture is ignited. From the combustion chamber, the gas passes into the turbine. After the gas has passed through the turbine section, the exhaust gases are expelled through a V-shaped flue gases upwards. Between compressor and turbine section is the gearbox, which is the central supporting element of the engine. In this and the flanges for receiving the engine sit in the aircraft structure.
Compressor
Until the model Allison 250 - C20B is the compressor of six axial compressor stages and one centrifugal stage. The axial compressor stages are surrounded by an axially split stainless steel housing, which is provided to the planes of the wheels with a special plastic liner in order to achieve an optimum clearance between the tips of the compressor blades and the housing. The axial rotor stages represent an early form of blisk, ie is running wheels with integral blades, which were made in a special investment casting process. The individual sheets are held together with the aid of an axial clamping bolt. On later models has been waived, the axial compressor stages. The compressor has a total of five bleed air exits. At the compressor inlet housing is a powered hot compressor bleed air anti-icing system. The hot air required for this is taken after the centrifugal compressor. In Axialverdichterstufe five and six is the so-called compressor bleed valve. This valve controls the amount of air continuously in dependence on the operating mode ( partial load or full load), in order to unload the compressor during certain operating conditions.
Transmission
The gearbox is the central part of the engine. In him both engine shafts are mounted concentrically. It houses the gear reducer for power extraction, most of the components of the lubrication and a transfer case for the auxiliary drives. The gear housing is made of magnesium. When C20 or C20B also the electric starter generator and the fuel control system ( controlled mechanical- pneumatic models of the Bundeswehr ) are installed on it. For the engine Allison 250 engine controllers from different manufacturers were offered, for example, are ( mechanical and pneumatic) and CECO ( hydromechanical ) for type C20 and C20B equipment manufactured by BENDIX in use.
Turbine
The turbine module contains two separate sections: The two-stage gas generator turbine and the two-stage labor or power turbine. The hollow shaft is designed as a wave trains of the two portions are arranged coaxially and are not mechanically connected together. The expanding gas flows through the first gas generant portion, and thereafter, the power turbine, in which the wave power is generated (so-called two-shaft design ).
Lubrication
Lubrication is provided by a dry sump lubrication. Oil reservoir and oil cooler are located outside of the turbine housing and include the installation fullest extent of the airframe. The exiting from the turbine central sump return oil amount is collected in an external, corresponding to the engine return oil tank. The oil pump drive is controlled from the gas generator shaft. The bearings are supplied by a Sprühölschmierung. An exception to the bearings of the main output shaft having an oil mist lubrication. The oil pump is a gear pump with a pressure pump and four return pumps.
The oil pressure is limited by a control valve to nine bar. The main flow oil filter has a bypass valve. There are four screens in the oil circuit; two sensors which respond to metal chips, chips are used as warning transmitters.
The oil pump and the oil filter located within the Getriebmoduls, is provided by means of the oil lines and the main bearing etc..