Abstract:
A turbofan engine starting system includes a core nacelle housing ( 12 ), a compressor ( 16 ) and a turbine ( 18 ). The core nacelle is disposed within a fan nacelle ( 34 ). The fan nacelle includes a turbofan ( 20 ). A bypass flow path (B) downstream from the turbofan is arranged between the two nacelles. A controller ( 50 ) is programmed to manipulate the nozzle exit area (A) of the bypass flow path to facilitate startup of the engine. In one example, the controller manipulates the nozzle exit area using hinged flaps ( 42 ) in response to an engine shutdown condition. The flaps open and close to adjust the nozzle exit area and the associated bypass flow rate, the mass flow rate of the air through the cave nacelle and the rotational speed of the compressor rotar ( 14 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to starting gas turbine engines, and, more particularly, to facilitating gas turbofan engine restarts by effectively altering the nozzle exit area. 
         [0002]    Gas turbine engines are widely known and used for power generation and vehicle (e.g., aircraft) propulsion. During in-flight propulsion of a multi-engine aircraft, certain problems may occur with one engine causing the engine to shut down. For example, inclement weather, non-optimum trimming of engine idle, fuel nozzle coking, fuel contamination, loss of electric power, fuel mismanagement, pilot error, or the like may, under certain conditions, warrant voluntary or automatic shut down of an engine. Although the remaining engines can typically fly the aircraft, it is ordinarily desired to restart the shut down engine while the aircraft is still in-flight. 
         [0003]    An engine restart envelope includes combinations of aircraft altitude and airspeed that provide a suitable air supply to the engine sufficient for restarting. When traveling outside of the engine restart envelope, the air supply to the engine may not contain enough oxygen to support combustion during ignition. In some instances, starter-assistance may be used to increase the rotational speed of a fan section of the engine, which increases altitude and airspeed combinations suitable for restarting the engine. Increasing the rotational speed of the fan section draws additional airflow into the engine and, in so doing, augments the supply of oxygenated air supporting combustion. 
         [0004]    Disadvantageously, at certain combinations of altitude and airspeed, increasing the rotational speed of the fan section is not alone sufficient to generate adequate airflow to support combustion. As a result, aircraft experiencing in-flight shutdown may have to rapidly adjust altitude and/or airspeed to move within the engine restart envelope or starter-assisted engine restart envelope. As an example, if an engine requires restart in aircraft traveling at an altitude unsuitable for engine restarts, the aircraft may rapidly decrease elevation to move to an altitude suitable for restarting the turbofan engine. Alternatively, the aircraft may be forced to continue flying, without propulsion from the shutdown engine. 
         [0005]    What is needed is a method capable of restarting the turbofan engine through an increased number of altitudes and airspeeds. 
       SUMMARY OF THE INVENTION 
       [0006]    An example turbofan engine starting system includes a core nacelle housing a compressor and a turbine. The core nacelle is disposed within a fan nacelle. The fan nacelle includes a turbofan. A bypass flow path downstream from the turbofan is arranged between the two nacelles. A controller is programmed to manipulate the nozzle exit area to facilitate startup of the engine. In one example, the controller manipulates the nozzle exit area using hinged flaps in response to an engine shutdown condition. The hinged flaps open and close to adjust the nozzle exit area and the associated bypass flow rate. 
         [0007]    An example method for starting the engine includes detecting an engine shutdown and changing the effective nozzle exit area during a restart procedure to facilitate restarting the engine. In one example, the method includes adjusting the nozzle exit area to increase the windmilling speed of a fan section of the turbofan engine and decreasing the nozzle exit area to increase the mass flow rate of air through the core nacelle. 
         [0008]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows. 
           [0010]      FIG. 1  illustrates selected portions of an example gas turbine engine system. 
           [0011]      FIG. 2  illustrates a variable air nozzle and coolant passage within the gas turbine engine system shown in  FIG. 1 . 
           [0012]      FIG. 3  illustrates an example turbofan engine restart envelope without assistance from a variable area nozzle. 
           [0013]      FIG. 4  illustrates an example turbofan engine restart envelope with assistance from a variable area nozzle. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    A geared turbofan engine  10  is shown in  FIG. 1 . The engine  10  includes a core nacelle  12  that houses a low rotor  14  and high rotor  24 . The low rotor  14  supports a low pressure compressor  16  and low pressure turbine  18 . In this example, the low rotor  14  drives a fan section  20  through a gear train  22 . The high rotor  24  rotationally supports a high pressure compressor  26  and high pressure turbine  28 . A combustor  30  is arranged between the high pressure compressor  26  and high pressure turbine  28 . The low and high rotors  14 ,  24  rotate about an axis X. At least a portion of the core nacelle  12  is disposed within a fan nacelle  34 . 
         [0015]    In the examples shown, the engine  10  is a high bypass turbofan arrangement. In one example, the bypass ratio is greater than 10, and the turbofan diameter is substantially larger than the diameter of the low pressure compressor  16 . The low pressure turbine  18  has a pressure ratio that is greater than 5, in one example. The gear train  22  can be any known suitable gear system, such as a planetary gear system with orbiting planet gears, planetary system with non-orbiting planet gears, or other type of gear system. It should be understood, however, that the above parameters are only exemplary of a contemplated geared turbofan engine. That is, the invention is applicable to other types of engines. 
         [0016]    For the engine  10  shown  FIG. 1 , a significant amount of thrust may be provided by the bypass flow B due to the high bypass ratio. Thrust is a function of density, velocity, and area. One or more of these parameters can be manipulated to vary the amount and direction of thrust provided by the bypass flow B. In one example, the engine  10  includes a nozzle structure  38  associated with the nozzle exit area A to change the physical area and geometry to manipulate the thrust provided by the bypass flow B. However, it should be understood that the nozzle exit area A may be effectively altered by other than structural changes, for example, by altering a boundary layer of the bypass flow B. Furthermore, it should be understood that effectively altering the nozzle exit area A is not limited to physical locations approximate to the exit of the fan nacelle  34 , but rather, includes altering the bypass flow B by any suitable means at any suitable location of the fan section  20 . 
         [0017]    In the example shown in  FIG. 2 , an engine restart system  54  includes multiple hinged flaps  42  arranged circumferentially about the rear of the fan nacelle  34 . The hinged flaps  42  form a portion of a control device  41 , which further includes a controller  50  in communication with actuators  46  used to manipulate the hinged flaps  42 . A detector  52  communicates information about the engine  10  to the controller  50 , for example, information that the engine  10  has shut down or relating to the startup state of the engine  10 . In one example, the detector  52  monitors the rotational speed of the low rotor  14 , which is indicative of the state of the engine  10 . The controller  50  interprets rotational speeds of the low rotor  14  below a certain level as a condition of the engine  10  indicting the engine  10  has shut down. In another example, the detector  52  monitors fuel consumption of the engine  10 . If the engine  10  experiences a drop in fuel consumption, the detector  52  communicates this information to the controller  50 , which interprets the information as a shutdown of the engine  10 . The detector  52  may be located proximate or apart from the engine  10 . The controller  50  also communicates with a driver  56 , which may be controlled by an aircraft operator. Thus, the controller  50  may operate automatically using information from the detector  52 , or may operate manually based on signals from the driver  56 . A starter  58 , such as a hydraulic starter, may be used to boost the rotational speed of the fan section  20 . 
         [0018]    The hinged flaps  42  can be actuated independently and/or in groups using segments  44 . The segments  44  and individual hinged flaps  42  can be moved angularly using actuators  46 . The control device  41  thereby varies the nozzle exit area A ( FIG. 1 ) between the hinged flaps  42  and the engine  10  by altering positions of the hinged flaps  42 . In a closed position, the hinged flap  42  is closer to the core nacelle  12  for a relatively smaller nozzle exit area A. In an open position, the hinged flap  42  is farther away from the core nacelle  12  for a relatively larger nozzle exit area A. 
         [0019]    When the engine  10  shuts down during flight, the fan section  20  will continue to rotate, or windmill, as the engine  10  moves, either by gliding or powered by additional engines. Restarting the engine  10  requires adequate compressed air to support combustion. Changing the nozzle exit area A influences the mass flow rate of airflow over the fan section  20  as a function of radial distance from the axis X. For example, increasing the size of the nozzle exit area A increases the bypass flow B. This decreases the mass flow rate of the airflow over the fan section  20  at radial distances near to the axis X and increases the mass flow rate of the airflow over the fan section  20  at radial distances away from the axis X. The increased mass flow rate exerts more force on radially outward portions of the fan section  20  to accelerate rotation of the fan section  20 . Thus, by controlling bypass flow B the rotational speed of the fan section  20  is controlled. 
         [0020]    As an example, it is estimated that moving the hinged flaps  42  from a location suitable for aircraft cruising operations to an open position increases the windmilling speed of the fan section  20  about 10-20%. Increasing the windmilling speed of the fan section  20  also increases the rotational speed of the low rotor  14 , the low speed compressor  16 , and the low pressure turbine  18 . 
         [0021]    Inversely, decreasing the size of the nozzle exit area A increases the mass flow rate of the air through the core nacelle  12 . As a result, after increasing the fan section  20  windmilling speed, the hinged flaps  42  move to a closed position to decrease the nozzle exit area A and thereby increase airflow through the core nacelle  12 . Rotational inertia of the fan section  20  forces airflow into the core nacelle  12 . The rotational inertia also contributes to rotating the low pressure compressor  16 , which compresses air in preparation for ignition. In this example, the controller  50  monitors the rotational speed of the low rotor  14  to determine an appropriate time to decrease the size of the nozzle exit area A. 
         [0022]    In an example method of restarting the engine  10 , communications from the controller  50  open the hinged flaps  42  to maximize the windmilling speed of the rotating fan section  20 , which also increases the rotational speed of the low rotor  14 . Next, communications from the controller  50  direct the hinged flaps  42  to close, which increases the mass flow rate of airflow through the core nacelle  12 . Rotational inertia remaining in the windmilling fan section  20  helps to compress the increased airflow through the core nacelle  12 . If not for the rotational inertia in the windmilling turbofan, airflow would only move through the engine  10  at a rate corresponding to the closed position of the hinged flaps  42 . The rotational inertia in the windmilling fan section  20  increases airflow above this rate increasing the supply of oxygenated air available for combustion. Actuating the hinged flaps  42  in this way during the engine  10  restart increases the combinations of altitudes and airspeeds suitable for restarting the engine  10 . After reaching a sufficient level of compressed air, fuel flow is introduced to the compressed air, and the mixture is ignited, thereby restarting the engine  10 . 
         [0023]    Referring now to  FIG. 3  with continued reference to  FIG. 1 , illustrated is a typical flight envelope  60  for the engine  10 , that is, those combinations of altitude and airspeed suitable for operating the engine  10 . Within the flight envelope  60 , an area  64  represents combinations of altitude and speed suitable for restarting the engine without effectively altering the nozzle exit area A.  FIG. 4  represents an increased area  68  illustrating the combinations of altitude and speed suitable for restarting the typical engine when altering the nozzle exit area A. Formerly, the engine  10  may have needed starter assistance to restart at some of the altitudes and speeds included in area  68 . Of course, starter assistance may increase the likelihood of restarting the engine  10  at altitudes and airspeeds beyond those included in area  68 . 
         [0024]    In the disclosed examples, the ability to control the amount of airflow through the nozzle exit area A provides the benefit of restarting the engine  10  while in flight at increased combinations of altitudes and airspeeds. Restarts in prior designs may have required starter assistance for similar restarts. Further, although described in terms of restarts while in the air, adjusting nozzle exit area A ( FIG. 1 ) may also be used to facilitate starting the engine  10  while on the ground. 
         [0025]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art may recognize certain modifications falling within the scope of this invention. For that reason, the following claims should be studied to determine the true scope of coverage for this invention.