Abstract:
The method and apparatus for in-flight relighting of a turbofan engine involve in one aspect selectively controlling an accessory drag load on one or more windmilling rotors to permit control of the windmill speed to an optimum value for relight conditions.

Description:
TECHNICAL FIELD 
     The invention relates to the in-flight relighting of an aircraft turbofan engine. 
     BACKGROUND 
       FIG. 1  schematically illustrates a typical turbofan engine  10  for subsonic flight. The engine  10  generally comprises in serial flow communication a fan  12  through which ambient air is propelled, a multi-stage compressor  14  for pressurising the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating a stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. The engine  10  also comprises an auxiliary or accessory gearbox (AGB)  20  on which are located mechanical and electrical systems, such as fuel pumps, oil pumps, generators and a starter/generator. The main rotating parts of the engine  10  are connected in two subgroups, the low pressure (LP) spool and the high pressure (HP) spool, which are coaxially disposed. In use, the engine  10  is started by the starter which is mechanically connected to the HP spool using a set of gears and a tower shaft  22 . Once the desired HP spool speed is reached, fuel is provided into the combustor  16  and is ignited to start or “light” the engine  10 . 
     When the engine  10  is mounted on an airplane, in the unlikely event of a flame out or engine shutdown, dynamic pressure due to forward speed of the airplane creates a windmill effect to spin the LP and HP spools. This spinning is then further assisted by the starter to spin the HP spool up to the starting speed so that relight can successfully occur. In other arrangements, a shaft power transfer arrangement is provided to transfer windmilling energy from the LP spool to the HP spool to assist acceleration of the HP spool to relight speed. However, there is a continuing need for simpler and better systems. 
     SUMMARY 
     In one aspect, the present invention provides a method for in-flight relighting a turbofan engine of an aircraft, the engine having at least two shafts, one of which is a high-pressure shaft mounted to a high-pressure compressor and a high-pressure turbine, the high-pressure shaft drivingly connected to an accessory load, the method comprising the steps of: disconnecting the accessory load from the high-pressure shaft to substantially eliminate a parasitic drag load on the high-pressure shaft; permitting ram air to rotate the high pressure shaft; and relighting the engine. 
     In another aspect, the present invention provides a method for in-flight relighting an aircraft turbofan engine, the engine having at least two shafts, one of which is a high-pressure shaft mounted to a high-pressure compressor, a high-pressure turbine and an electrical generator, the generator electrically driving an accessory load, the method comprising the steps of: determining the presence of an engine-out condition of the engine; using the generator to reduce the rate of rotation of the high-pressure shaft to a desired rate within a relight envelope; and relighting the engine. 
     In another aspect, the present invention provides a method for in-flight relighting an aircraft accessory gearboxless turbofan engine, the engine having at least two shafts, one of which is a high-pressure shaft mounted to a high-pressure compressor, a high-pressure turbine and a concentrically-mounted electrical generator, the generator electrically driving an accessory load, the method comprising the steps of: using exclusively ram air through the engine to rotate the high-pressure shaft; and then relighting the engine. 
     In another aspect, the invention provides a method of relighting a gas turbine engine of a fixed-wing aircraft after an in-flight engine-out condition, the engine having at least one electromagnetic bearing apparatus and at least a bladed propulsor mounted to a first shaft and a compressor and turbine mounted to a second shaft, the first shaft drivingly connected to an electric generator, the method comprising the steps of: using windmill rotation of the bladed propulsor to drive the generator; using electricity from the windmill-driven generator to provide power to the electromagnetic bearing apparatus; and relighting the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present method, and to show more clearly how it may be carried into effect, reference will now be made by way of example to the accompanying figures, in which: 
         FIG. 1  schematic view of a typical turbofan gas turbine engine according to the prior art; 
         FIG. 2  is a schematic side view of an example of a turbofan gas turbine engine for use with the present method; and 
         FIG. 3  is block diagram illustrating the present method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a turbofan gas turbine engine  20  which generally comprises a low-pressure (LP) spool  21  supporting at least a fan and a turbine, and a concentric high-pressure (HP) spool  24  supporting at least a compressor and a turbine. An embedded or integrated generator or starter/generator  22  is coaxially mounted on the HP spool  24  of the engine  20 , and preferably a second generator or motor/generator  23  is mounted on the LP spool  21  of the engine  20 . Starter-generator  22  may be operated as a motor to light engine  20 , and also preferably as a generator to generate electricity, which a controller  26  may then provide in form suitable for driving accessories  28  such as electrically-driven pumps and other engine and aircraft services. Generator  23  may be used likewise to generate electricity for controller  26  to provide to accessories  28  (but are not necessarily the same controller or accessories/services as driven by generator  22 ), and if a motor/generator, may be used to selectively drive the LP spool  21 . Consequently, the need for an accessory gearbox is obviated, and is thus not present in engine  20 . The design of engine  20  is not new, however the present invention offers new functionality to the engine  20  to provide improved in-flight relighting, as will now be described. 
     After a flame-out or other shut-down of engine  20  occurs requiring the engine to be relit, in-flight windmilling causes the LP spool  21  and HP spool  24  to rotate, which thus rotates starter-generator  22 . During in-flight windmilling, controller  26  preferably partially or completely disconnects or stops supplying electricity to accessories  28 , so there is substantially no electrical load drawn from starter-generator  22 , and thus there is substantially no parasitic drag on the HP spool  24  caused by starter-generator  22 . For example, in one embodiment shown in  FIG. 3 , a flame-out (or other engine-off) condition is initially detected by the controller  26 , which controls the fuel and oil pumps  28 . The controller also monitors electrical output from the generator(s), and includes suitable means to prevent power output to the aircraft electrical bus (also represented by  28 ) which does not meet the specification requirements—i.e. the controller  26  has control over whether the starter-generator  22  is connected to the bus in the ‘generate’ and ‘start’ modes. In a flame-out condition, an appropriate sensor signals the controller to stop the fuel pump from pumping fuel, and preferably also stops the oil pump, and the electrical output of the starter-generator  22  is also disconnected from the aircraft bus. Thus, electromagnetic drag on the HP spool  24  is reduced, and preferably effectively eliminated. Consequently, unlike the prior art, the accessories  28  are disconnected from the HP spool  24 , preferably prior to relight. 
     Referring again to the engine  10  of  FIG. 1 , during in-flight windmilling AGB  20  remains drivingly connected to the HP spool, and thus a plurality of gears and accessories continue to be driven by the HP spool, which creates a parasitic mechanical drag on the HP spool which tends to decelerate the HP spool windmilling speed. As previously described, another energy source is required to overcome this drag and accelerate the engine to its relight speed. However, by disconnecting the load from the HP spool  24  of engine  20 , the parasitic drag of the accessory system is virtually eliminated and, in the right conditions, windmill speed alone becomes sufficient to spin the HP spool  24  at a desired starting speed, using only aircraft attitude if necessary to control windmill speed. Another external power source is not required, thereby simplifying the engine system. This greatly facilitates relighting of the engine  20  by extending the in flight relight envelope of the engine. 
     Therefore, the windmilling effect of ram air though the high spool may be used to rotate the engine to relight speed, particularly in very small turbofans having low inertia. Thus relight is achieved by disconnecting accessories and then using windmilling power, preferably alone and without the input of additional rotation energy from the starter-generator  22 , or any other power transfer mechanism, to increase the speed of the HP spool. 
     In fact, conversely to the prior art, in some situations such as when descending rapidly on flame out conditions, the rotor may tend to spin too quickly, and thus prevent optimum relight conditions (e.g. lean blow out may occur if there is too much speed at the low fuel flows generally desired for starting), adjustable “drag” may be provided to the high rotor, e.g. by providing a braking force to slow the HP spool speed down. In one approach, this is achieved by operating the starter/generator  22  as a sort of electromagnetic brake, for example by controlling the current of the starter-generator via the controller  26 . In another aspect, a mechanical braking arrangement may be employed to retard spool rotation. This may be used to put an upper limit on windmill speed under conditions requiring a specific relight speed, without requiring the pilot to set a different decent rate than was required for other reasons (for example, in the case where both engines flame out, descending to an altitude where there is air to breathe is often high on the pilot&#39;s list of priorities). Thus, controlling the windmill speed to an optimum value for relight, whether increasing or decreasing the rotor speed as necessary, is available with the present concept. 
     In another aspect of the present invention, in the case of flame-out, generator  23  may provide self-contained back-up electrical to power during windmilling to a magnetic bearings power system (indicated as among the elements of  28 ) to support the required shafts or spools during power-out situations. The LP spool generator does not induce parasitic drag on the HP spool, and thus no hamper relighting of the HP spool. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that other changes may also be made to the embodiments described without departing from the scope of the invention disclosed. For instance, the starter-generator can be any suitable design, and may in fact be provided by two different units (e.g. separate starter and generator). Although it is desirable to adjust parasitic drag (e.g. by disconnecting accessories and/or reducing rotor speed) prior to commencing relight procedures, the operations may be performed in any desired order. Although electrically disconnecting of the HP spool from accessory drive systems is preferred, any suitable selectively operable disconnect system may be employed. Still other modifications may be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.