Aeronautics and astronautics – Aircraft power plants – Auxiliary
Reexamination Certificate
2000-05-04
2002-10-22
Barefoot, Galen L. (Department: 3644)
Aeronautics and astronautics
Aircraft power plants
Auxiliary
C060S039163
Reexamination Certificate
active
06467725
ABSTRACT:
The present invention relates to an aircraft generator, and in particular to a generator for supplying electrical power to an aircraft when one or more engines thereof has lost propulsive power, or there has been a failure of the main electrical power system.
Aircraft electrical power requirements have increased over the years. This trend is expected to continue as the number of electrical devices, and electrically operated loads increase within aircraft. It is expected that flight control surfaces will increasingly be driven directly by electrical devices, or indirectly, wherein an electrical device is used to provide a supply of hydraulic pressure which may then be used by hydraulic actuators to operate flight control surfaces. With this greater dependence on electrical power, it is becoming more important to ensure that there is an electrical supply available at all times that the aircraft is in service, the main concern is the loss of electrical power in flight in the event of a failure of combustion in the aircraft engines. Such total “flame-out” conditions have been known to occur as a result of air turbulence or flying through airborne debris, such as volcanic dust.
Traditionally emergency electrical power in the event of engine failure has been provided by a ram air turbine, RAT, which comprises an electrical generator equipped with a propeller. The RAT is normally stored within the fuselage of an engine and provides no output. However, in emergency conditions, the RAT may be deployed by causing an arm to extend the RAT into the air stream surrounding the aircraft. This flow of air causes the propeller of the RAT to rotate thereby generating electrical power.
It is predicted that it will soon be necessary to provide two RATs on each aircraft in order to ensure sufficient power is available in the event of total engine flame out. This is expected to incur a weight penalty in excess of 250 kg for the RATs plus associated airframe reinforcements.
There are many problems associated with RAT operation. The device itself is a relatively heavy piece of equipment which is carried at all times and which is very rarely deployed, thus it involves a fuel penalty on every flight. Furthermore, the aircraft structure must also be reinforced in the region of the RAT mounting in order to ensure that it can stand the loading experienced in the event that the RAT is deployed. Furthermore, because the RAT is operated only very occasionally and it is not regularly tested for functionality, faults may remain latent for some considerable period of time before being detected. It should be noted that satisfactory operation and deployment of a RAT is not always achieved in practice. It would therefore be advantageous to dispense with the RAT completely.
It is known that, in a multistage high bypass gas turbine engine, the low pressure shaft (LP) or low speed spool which drives the low pressure compressor and the bypass fan will continue to rotate in the event of engine failure because the bypass fan is caused to rotate due to the airflow resulting from the motion of the aircraft as it glides to earth. This is known as the “windmill” effect. The energy of the fan could be extracted by a generator connected to the low pressure shaft which could then supply electrical power to the aircraft during periods of flame-out.
GB 2216603 discloses a gas turbine in which the low speed spool is coupled to an emergency generator via a coupling unit in the event of loss of propulsive drive of the engine. The power take off from the low speed spool may be applied to a gear box which drives an hydraulic pump and an electrical generator. Thus the connection between the low speed spool and the generator is broken when the engine is functioning.
WO93/06007 discloses an arrangement in which the down stream end of the low speed spool is connected to a first gearbox which has an output shaft
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which extends perpendicularly to the spool and into the interior of the engine pylon. The shaft engages with a second gear box which has various engine accessories coupled to it, such as the main engine fuel pump, a hydraulic pump and one or more electrical generators. This document discloses that the ram air turbine can be dispensed with since power in the event of flame-out conditions can be derived from the windmilling of the bypass fan.
EP 0798454 discloses a multi-spool aero engine in which each of the spools independently and directly drive an electrical generator. The primary source of electrical power is an electric motor/generator positioned within the down stream bearing support structure at the down stream end of the innermost engine shaft. This document goes on to describe that the advantage of this configuration is the elimination of the main engine gearbox and two electrical generators driven by that gearbox. This document also goes on to disclose that the bearings for the shaft may be electromagnetic bearings and that “electricity generated by the bearing
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constitutes the primary source of electricity for the aircraft upon which the engine
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is mounted”. In order for the magnetic bearing to function as a generator, the magnetic bearing must be directly connected to the shaft, that is there is no intermediate gearbox provided.
EP 0659234 relates to an arrangement in which motor generators are connected to at least two of the engine spools and power transfer can be provided between them. Inductive electrical machines are described as being connected to the low pressure spool via gearing, or alternatively a switched reluctance machine may be provided such that the rotor of the switched reluctance machine is an integral part of the engine shaft, as described in column 10 lines 45 to 50. Thus the switched reluctance machine is directly coupled to the engine shaft.
Although these documents disclose the provision of emergency power by utilising the windmilling of the engine, and two of them disclose providing generators within the structure of the engine itself, none of these documents addresses the issue of obtaining a reasonable amount of power from the generator under the windmilling conditions. The amount of power required should be sufficient to power essential systems until the aircraft descends to an altitude where the auxiliary power unit can be started to initiate an engine restart sequence, and preferably to allow flight control surfaces to be actuated, either directly from an electrical power source or indirectly via an intermediate load transfer system, such as an hydraulic circuit.
According to a first aspect of the present invention, there is provided an electrical generator for use with a gas turbine engine having a low speed spool, comprising a generator driven from a low speed spool, and in which the generator is a switched reluctance generator coupled to the low speed spool via a step up gearbox.
It is thus possible to provide an electrical generator which, by virtue of the step up gearbox, provides a greater electrical output during windmilling conditions of the engine than could be obtained from a similarly sized direct coupled generator.
The maximum available output from a switched reluctance generator is, to a fair approximation, directly related to speed of rotation the mass of the magnetic material forming the rotor and stator of the generator. The step up ratio of the gearbox can be traded against weight of the generator to select a predetermined power output for a given rate of rotation of the low speed spool corresponding to windmilling. Preferably the generator should provide in excess of 10 kW during windmilling, and advantageously should provide around 25 kW or more. However, the step up ratio is limited by the requirement that the generator should still be within safe operating speeds when the low speed spool of the aircraft engine has reached its maximum operating speed. Typically the speed of the low speed spool will vary between 200 and 250 rpm for windmilling and 3000 rpm or so as its maximum operating speed. In an embodiment of the present invention, the gearbox has a step u
Coles Jeffrey Ronald
Doyle James Patrick
Holme Marc
Baker & McKenzie
Barefoot Galen L.
Lucas Industries Limited
LandOfFree
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