Abstract:
(A2) A turbofan engine control system for managing a low pressure turbine speed is provided. The turbofan engine control system includes a low spool having a low pressure turbine that are housed in a core nacelle. The low pressure turbine is adapted to rotate at a speed and includes a maximum design speed. A turbofan is coupled to the low spool. A fan nacelle surrounds the turbofan and core nacelle and provides a bypass flow path. The bypass flow path includes a nozzle exit area. A controller is programmed to command a flow control device adapted to effectively decrease the nozzle exit area in response to a condition. Reducing the nozzle exit area, either physically or otherwise, maintains the speed below the maximum design speed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a turbofan engine, and more particularly, the invention relates to managing the maximum speed of a low pressure turbine. 
         [0002]    A typical turbofan engine includes low and high spools. The low spool is coupled to a turbofan and typically supports a low pressure turbine and compressor. The high spool typically supports a high pressure turbine and compressor. The spools, turbines and compressors are housed in a core nacelle. The turbofan is arranged upstream from the core nacelle. A fan nacelle surrounds the turbofan and core nacelle to provide a bypass flow path having a nozzle area through which bypass flow from the turbofan exits. 
         [0003]    The turbines are designed to accommodate a maximum operating speed plus a margin. The engine becomes heavier and more costly as the maximum speed increases for a given turbine design. For example, the turbofan, low pressure turbine, low spool and fan containment system must be designed more robustly for a higher low pressure turbine maximum speed. Current commercial turbofan engines use fixed area nozzles that limit the ability to operate the engine to a fixed characteristic, for example maximum low pressure turbine speed. As a result, the engine must be designed for the condition requiring the maximum turbine speed during a flight envelope, even though the condition may rarely occur during typical aircraft usage. 
         [0004]    Low bypass ratio turbofan engines are used in military fighter aircraft. These turbofan engines use variable area nozzles to balance thrust requirements, maximum rotor speed and fan stability requirements. Military fighter engines encounter maximum rotor speeds at different flight conditions compared to commercial engines. For example, military aircraft fly at maximum speed exceeding the speed of sound whereas the maximum flight speed of commercial aircraft is below the speed of sound, for example around Mach 0.8. The thrust requirement of commercial and military fighter aircraft are also significantly different. 
         [0005]    What is needed is a turbofan engine capable of providing the needed thrust throughout the flight envelope without increasing the maximum speed of the low pressure turbine. 
       SUMMARY OF THE INVENTION 
       [0006]    A turbofan engine control system for managing a low pressure turbine speed is provided. The turbofan engine control system includes a low spool having a low pressure turbine that are housed in a core nacelle. The low pressure turbine is adapted to rotate at a speed and includes a maximum design speed. A turbofan is coupled to the low spool. A fan nacelle surrounds the turbofan and core nacelle and provides a bypass flow path. The bypass flow path includes a nozzle exit area. A controller is programmed to command a flow control device adapted to effectively decrease the nozzle exit area in response to a condition. Reducing the nozzle exit area, either physically or otherwise, maintains the speed below the maximum design speed. 
         [0007]    In operation, the turbofan engine control system detects a condition affecting a speed of a component mounted on the low spool, such as the low pressure turbine. In one example, the controller determines if the condition would cause the speed to reach an undesired speed relative to the maximum design speed for the low pressure turbine. If the condition would result in at least the undesired speed, then the nozzle exit area is reduced, which creates a backpressure on the turbofan thereby counteracting an increase in low spool speed. In this manner, the maximum design speed of the low pressure turbine is avoided. 
         [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]      FIG. 1  is a cross-sectional view of an example turbofan engine. 
           [0010]      FIG. 2  is a partially broken perspective view of the turbofan engine shown in  FIG. 1 . 
           [0011]      FIG. 3  is a flow chart illustrating an example turbofan engine control system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    A geared turbofan engine  10  is shown in  FIG. 1 . A pylon  38  secures the engine  10  to an aircraft. The engine  10  includes a core nacelle  12  that houses a low spool  14  and high spool  24  rotatable about an axis A. The low spool  14  supports a low pressure compressor  16  and low pressure turbine  18 . In the example, the low spool  14  drives a turbofan  20  through a gear train  22 . The high spool  24  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 . Compressed air from compressors  16 ,  26  mixes with fuel from the combustor  30  and is expanded in turbines  18 ,  28 . 
         [0013]    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  is an epicycle gear train, for example, a star gear train, providing a gear reduction ratio of greater than  2 . 5 . 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 engines. 
         [0014]    Airflow enters a fan nacelle  34 , which surrounds the core nacelle  12  and turbofan  20 . The turbofan  20  directs air into the core nacelle  12 , which is used to drive the turbines  18 ,  28 , as is known in the art. Turbine exhaust E exits the core nacelle  12  once it has been expanded in the turbines  18 ,  28 , in a passage provided between the core nacelle and a tail cone  32 . 
         [0015]    The core nacelle  12  is supported within the fan nacelle  34  by structure  36 , which are commonly referred to as upper and lower bifurcations. A generally annular bypass flow path  39  is arranged between the core and fan nacelles  12 ,  34 . The example illustrated in  FIG. 1  depicts a high bypass flow arrangement in which approximately eighty percent of the airflow entering the fan nacelle  34  bypasses the core nacelle  12 . The bypass flow B within the bypass flow path  39  exits the tan nacelle  34  through a nozzle exit area  40 . 
         [0016]    For the engine  10  shown in  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 structure associated with the nozzle exit area  40  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 may be effectively altered by other than structural changes, for example, by altering the boundary layer, which changes the flow velocity. Furthermore, it should be understood that any device used to effectively change the nozzle exit area is not limited to physical locations near the exit of the fan nacelle  34 , but rather, includes altering the bypass flow B at any suitable location. 
         [0017]    The engine  10  has a flow control device  41  that is used to effectively change the nozzle exit area. In one example, the flow control device  41  provides the fan nozzle exit area  40  for discharging axially the bypass flow B pressurized by the upstream turbofan  20  of the engine  10 . A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The turbofan  20  of the engine  10  is designed for a particular flight condition, typically cruise at 0.8M and 35,000 feet. The turbofan  20  is designed at a particular fixed stagger angle for an efficient cruise condition. The flow control device  41  is operated to vary the nozzle exit area  40  to adjust fan bypass air flow such that the angle of attack or incidence on the fan blade is maintained close to design incidence at other flight conditions, such as landing and takeoff. This enables desired engine operation over a range of flight condition with respect to performance and other operational parameters such as noise levels. In one example, the flow control device  41  defines a nominal converged position for the nozzle exit area  40  at cruise and climb conditions, and radially opens relative thereto to define a diverged position for other flight conditions. In one example, the flow control device  41  provides an approximately 20% change in the exit nozzle area  40 . 
         [0018]    In one example, the flow control device  41  includes multiple hinged flaps  42  arranged circumferentially about the rear of the fan nacelle  34 . The hinged flaps  42  can be actuated independently and/or in groups using segments  44 . In one example, the segments  44  and each hinged flap  42  can be moved angularly using actuators  46 . The segments  44  are guided by tracks  48  in one example. 
         [0019]    Referring to  FIGS. 1 and 2 , the engine  10  includes a controller  50  that commands the flow control device  41  to limit the speed of the low pressure turbine  18 . However, limiting the low pressure turbine speed in a conventional turbofan engine reduces the available thrust. This can be particularly problematic for high altitude take-off conditions, which typically require the maximum thrust from the engine. Thus, reducing the low turbine speed is not possible unless the needed thrust can be achieved. The example turbofan engine and control system provides the needed thrust with a slower low pressure turbine speed than would be needed otherwise. 
         [0020]    The controller  50  communicates with, for example, a speed sensor, altitude sensor and throttle position sensor  52 ,  54 ,  56 . In the example, the speed sensor  52  provides the speed of the low pressure turbine  18 , which corresponds with the speed of the low spool  14 . The low pressure turbine speed can be determined directly or indirectly. The altitude sensor  54  provides information relating to the altitude of the aircraft, which is particularly relevant for take-offs from high altitude runways. The throttle position sensor  56  can communicate, for example, a full throttle position indicative of a take-off. Additional and/or different sensors can also be used. 
         [0021]    In one example, the above information is used by the controller  50  to determine if the low pressure turbine  18  speed is approaching its maximum design speed, which is illustrated at  60  in  FIG. 3 . In one example, it is desirable to maintain a safety margin relative to the maximum design speed such that the operating speed of the low pressure turbine  18  is less than the maximum design speed, shown at block  62 . The controller  50  monitors for various conditions using the sensors  52 ,  54 ,  56 , shown at block  64 . When the controller  50  determines that conditions exist for a desired thrust that would result in an undesired speed, shown at block  66  (for example, high altitude take-offs), the controller  50  commands the flow control device  41  to effectively reduce the nozzle exit area  40  (block  68 ). For the example shown in  FIG. 2 , the controller  50  commands the actuators  46  to close the flaps  42  (moving them radially inward from the position shown) to physically reduce the area of the nozzle. This control scheme can be used to limit the low pressure turbine speed, or the speed of any other component coupled to the turbofan, for any conditions desired. 
         [0022]    Effectively reducing the nozzle exit area  40  has two effects. First, a backpressure on the turbofan  20  is increased providing resistance to its rotation. This counteracts an increase in low pressure turbine speed since the low pressure turbine  18  and turbofan  20  are coupled to one another via the low spool  14 . Second, the thrust provided by the bypass flow path  39  is increased from the throttling provided by the effectively smaller nozzle exit area  40 . In this manner, the example turbofan engine  10  is capable of providing the needed thrust at a reduced low pressure turbine speed as compared to conventional turbofan engines. 
         [0023]    Although an example embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.