Abstract:
Electric power from the low spool of a turboshaft engine is transferred to drive the compressor of an other turboshaft engine. This is used to assist in maintaining the other turboshaft idling while a single engine provides flight power or to increase acceleration for instance.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to the management of a multiple turboshaft arrangement in a helicopter, more particularly, involving electrical power transfer between engines. 
       BACKGROUND OF THE ART 
       [0002]    Helicopters are often provided with at least two turboshaft engines. Both engines are connected to the main rotor via a common reduction gearbox, and each of the engines is sized to account for the worst-case scenario of the other engine failing at takeoff. Accordingly, the power of each engine is significantly greater than what is required for cruising. 
         [0003]    In cruising conditions, operating a single engine at a relatively high regime instead of both at a lower regime can allow significantly better fuel efficiency. However, once a turboshaft is stopped, there is a significant delay in starting it back up again. This delay is associated with the required amount of time to get the engine running at a sufficient RPM (and draw in a sufficient amount of air) for engine operation to begin. Henceforth, for safety purposes, the typical approach was not to shut down the second engine completely, but to keep it idling, which limited the gain in fuel efficiency. 
         [0004]    Accordingly, there remains room for improvement in addressing the fuel consumption of helicopters. 
       SUMMARY 
       [0005]    In one aspect, there is provided a method of controlling operation of an arrangement having at least a first turboshaft engine and a second turboshaft engine of a helicopter, each turboshaft engine having a first electric machine on a compressor spool, the compressor spool having both at least one turbine stage and at least one compressor stage, and a second electric machine on a power spool, the power spool having a low-pressure turbine stage, the method comprising : extracting electrical power from the power spool of the first turboshaft engine using the second electric machine of the first turboshaft engine, and the first electric machine of the second turboshaft engine imparting mechanical rotation power to the corresponding compressor spool using a portion of the extracted electrical power. 
         [0006]    In a second aspect, there is provided a system for a helicopter having least two turboshaft engines, the system comprising, for each turboshaft engine, a first electric machine on a compressor spool linking at least one turbine stage to at least one compressor stage, a second electric machine on a power spool having a low-pressure turbine stage, and a controller connected to each electric machine, the controller being operable to transfer at least a portion of electrical power obtained from the second electric machine of either one of the two turboshaft engines to the first electric machine of the other turboshaft engine to impart mechanical rotation power to the corresponding compressor spool. 
         [0007]    In a third aspect, there is provided a helicopter having least two turboshaft engines, each turboshaft engine having a first electric machine on a compressor spool linking at least one turbine stage to at least one compressor stage, a second electric machine on a power spool having a low-pressure turbine stage, and a controller connected to each electric machine, the controller being operable to transfer at least a portion of electrical power obtained from the second electric machine of either one of the two turboshaft engines to the first electric machine of the other turboshaft engine to impart mechanical rotation power to the corresponding compressor spool. 
         [0008]    Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    Reference is now made to the accompanying figures, in which: 
           [0010]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
           [0011]      FIG. 2  is a schematic view showing two turboshaft engines in a twin-pac helicopter arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  illustrates an example of a turbine engine. In this example, the turbine engine  10  is a turboshaft engine generally comprising in serial flow communication, a multistage compressor  12  for pressurizing the air, a combustor  14  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  16  for extracting energy from the combustion gases. 
         [0013]    The turbine engine  10  in this example can be seen to include a high pressure spool  18 , including a multistage compressor  12  and a high-pressure turbine stage  20 , and a low pressure spool  22 , including a low-pressure turbine stage  24 . The low spool  22  leads to a power shaft via a gear arrangement. The high spool  18  can be refer to herein as a compressor spool, given that it contains at least one compressor stage, and the low spool  22  can be reffered to herein as the power spool. 
         [0014]    In this example, the turbine engine  10  is of the more-electric engine type which uses spool-mounted electric machines to power aircraft equipement. A first electric machine  26  is provided on the high spool  18  and a second electric machine  28  is on the low spool  22 . 
         [0015]    The first electric machine  26  can be of the spool-mounted permanent magnet type and be referred to as an integrated starter/generator (ISG). The first electric machine  26  is used in starting the turbine engine  10  to drive the high spool  18 , and hence the multistage compressor  12  to cause a flow of air to enter the combustor, thereby allowing subsequent fuel admission and ignition. The first electric machine  26  may also operate in generator mode. 
         [0016]    The second electric machine  28  can also be of the spool-mounted permanent magnet type. Although the construction of the second electric machine  28  can typically allow its use in either one of generator or motor mode it is typically only used in generator mode and can thereby be referred to as a low spool generator (LSG). The power capacity of the low spool generator is typically at least one order of magnitude higher than the power capacity of the integrated starter/generator. 
         [0017]      FIG. 2  shematically shows an arrangement of two turboshaft engines in a twin-pac arrangement. Both turboshaft engines  10 ,  30  may be the same and are both connected to the helicopter main rotor  32  via a common reduction gearbox  34 . 
         [0018]    When the helicopter is cruising, a first one  10  of the two turboshaft engines is operated to provide flight power at the main rotor  32 , whereas the second one  30  of the two turboshaft engines can be idling. In these conditions, the twin-pac arrangement has a system by which a controller  36  associated with power electronics can transfer electric power extracted by the low spool generator  28  of the operating engine to the integrated starter/generator  38  of the idling engine  30  to assist in driving the high spool thereof, and therefore the compressor. This can allow gaining further fuel efficiency than having the idling engine  30  run entirely on fuel power. It will be understood that the operating engine and idling engine were selected arbitrarily in this example and that the controller can provide the same power transfer function independently of which engine is being operated. 
         [0019]    The system can also be used to transfer electrical energy in a comparable manner (i.e. from the low spool generator of one engine to the integrated starter/generator of the other) in a scenario where there is a significant difference in the RPM of both engines and there is a rapid rise in power requirement, independently of whether the second engine is idling or not. In fact, driving the compressor spool with an external power source such as electrical energy diverted from the low spool generator of the other engine can favourably affect the surge line, thereby increasing the compressor stall margin, and allowing for better acceleration time. This can be particularly useful during landing, for instance. 
         [0020]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, a helicopter can have more than two engines, in which case the power can be transferred between any two engines of the helicopter as desired. Further, although turboshaft engines have only one spool leading to the power shaft (referred to herein as the low spool or power spool), alternate embodiments can have more than one compressor spool (i.e. a spool having at least one stage of compressor blades in addition to turbine blades), such as an intermediate spool and a high spool for instance, in which case, the electric machine to which the electrical power is transferred to can be on either one of the compressor spools, depending on design requirements. Also, the electric machines referred to in the description provided above can either be unitary electric machines, or dual redundant electric machines to provide additional safety. Still other modifications which fall within the scope of the present invention will 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 scope of the appended claims.