FIG. 1 shows a conventional ducted fan gas turbine engine 10 comprising, in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by interconnecting shafts 38, 40, 42.
In current gas turbine engines, electrical power is typically generated by a wound field synchronous generator 44, although it will be appreciated that other electrical machines could be used subject to reliability and electrical performance requirements. The generator 44 is driven via a mechanical drive train 46 which includes an angle drive shaft 48, a step aside gearbox 50 and a radial drive 52 which is coupled to the intermediate pressure spool shaft 40 via a geared arrangement. Thus, due to the choice of generator 44 and transmission 46, the rotational speed of the generator's rotor and the electrical frequency which is outputted into the electrical system of the aircraft is proportional to the speed of the engine. It will be appreciated that other arrangements are possible, including the use of constant speed drives which help provide a fixed frequency electrical supply.
The electrical power provided by the engines supplies the various loads of the airframe and gas turbine engines themselves. These loads typically increase in size with each new generation of civil aircraft to the point where the electrical loading on current aircraft and aircraft in development accounts for a significant portion of the fuel consumption.
The present invention seeks to improve the known electrical architectures used in conventional airframes to increase the overall efficiency of an aircraft.