Abstract:
A gas turbine engine includes a turbomachinery core operable to generating a flow of pressurized combustion gases; a rotating fan adapted to extract energy from the core and generate a first flow of pressurized air; a fan stator assembly connected in flow communication with the fan and operable to vary the first flow of pressurized air while the fan operates at a substantially constant speed; a fan outer duct surrounding the core; and a flade stage comprising a supplementary fan disposed in the fan outer duct and driven by the fan for generating a pressurized bleed air flow.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to gas turbine engines and more particularly to a turbofan engine in which the fan flow can be modulated independent of the fan speed. 
         [0002]    It is known to extract bleed air from a turbine engine to perform functions such as flap blowing, boundary layer control, and lift enhancement in an aircraft. In particular, Short Takeoff and Landing (STOL) aircraft can utilize engine bleed air for wing lift enhancement during the take-off and landing phases of flight. Such aircraft require that the bleed air flow and pressure levels remain essentially constant, even though the engine thrust will vary over a band of about 20% to 100% of maximum, depending on the phase of flight. Bleed pressure levels must also be sufficiently high to keep pipe sizes reasonable for a given bleed energy level. Also, in a multi-engine aircraft, for one engine out operation, the engine system must be capable of generating the equivalent bleed energy of that needed with all engines operating. Immediate operating engine response to an engine out failure is also desired. These requirements present several problems for conventional engine systems since engine bleed air source pressures and flows vary widely over the operating thrust band and, during approach and landing, LP spool speeds are greatly reduced which can result in unacceptable spool-up times. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    The above-mentioned shortcomings in the prior art among others are addressed by the present invention, which according to one aspect provides a gas turbine engine, including: a turbomachinery core operable to generating a flow of pressurized combustion gases; a rotating fan adapted to extract energy from the core and generate a first flow of pressurized air; a fan stator assembly connected in flow communication with the fan and operable to vary the first flow of pressurized air while the fan operates at a substantially constant speed; a fan outer duct surrounding the core; and a flade stage comprising a supplementary fan disposed in the fan outer duct and driven by the fan for generating a pressurized bleed air flow. 
         [0004]    According to another aspect of the invention, a method of operating a gas turbine engine includes burning a fuel in a turbomachinery core to produce a first flow of pressurized combustion gases; extracting energy from the first flow of pressurized combustion gases and using the energy to generate a first flow of pressurized air with a rotating fan; selectively varying a flow area through the fan to vary the first flow of pressurized air while the fan rotates at a substantially constant speed; and using the fan to mechanically drive a flade stage comprising a supplementary fan disposed in a fan outer duct so as to generate a pressurized bleed air flow having a magnitude independent of the first flow of pressurized air. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0006]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine constructed according to an aspect of the present invention; 
           [0007]      FIG. 2A  is a graph depicting a thrust vs. fan stator setting characteristic of the gas turbine engine of the present invention; 
           [0008]      FIG. 2B  is a graph depicting a thrust vs. speed characteristic of a prior art gas turbine engine; 
           [0009]      FIG. 3  is a schematic cross-sectional view of a gas turbine engine constructed according to another aspect of the present invention; 
           [0010]      FIG. 4  is a schematic cross-sectional view of a gas turbine engine constructed according to another aspect of the present invention; and 
           [0011]      FIG. 5  is a schematic cross-sectional view of a gas turbine engine constructed according to yet another aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates a portion of an exemplary gas turbine engine, generally designated  10 . The engine  10  has a longitudinal center line or axis A and an outer stationary annular casing  12  disposed concentrically about and coaxially along the axis A. The engine  10  has a fan  14 , compressor  16 , combustor  18 , high pressure turbine  20 , and low pressure turbine  22  arranged in serial flow relationship. In operation, pressurized air from the compressor  16  is mixed with fuel in the combustor  18  and ignited, thereby generating pressurized combustion gases. Some work is extracted from these gases by the high pressure turbine  20  which drives the compressor  16  via an outer shaft  24 . The combustion gases then flow into the low pressure turbine  22 , which drives the fan  14  via an inner shaft  26 . The fan  14 , inner shaft  26 , and low pressure turbine  22  are collectively considered portions of a “low pressure spool” or “LP spool” (not labeled in the Figures). 
         [0013]    A portion of the fan discharge flows through the compressor  16 , combustor  18 , and high-pressure turbine  20 , which are collectively referred to as the “core”  28  of the engine  10 . Another portion of the fan discharge flows through an annular bypass duct  30  which surrounds the core  28 . The illustrated fan  14  includes, in flow sequence, a row of non-rotating fan inlet guide vanes or “IGVs”  32 , a first stage of rotating fan blades  34 , a row of non-rotating interstage vanes  36 , and a second stage of rotating fan blades  38 . The inlet guide vanes  32  may have their angle of attack with respect to the airflow and their open flow area selectively changed by using an actuator  40  of a known type. Optionally, the interstage vanes  36  may have their angle of attack with respect to the airflow and their open flow area selectively changed by using an actuator  42  of a known type. Collectively, the fan IGVs  32  and the interstage vanes  36  are referred to as a fan stator assembly. The principles of the present invention are equally applicable to other engine configurations. 
         [0014]    The engine  10  also includes a supplementary fan, referred to as a “FLADE” stage  44  in the form of a ring of airfoils extending radially outwardly from an annular shroud  46  and driven by the fan  14  (in this case the second stage  36 ). The FLADE stage  44  is positioned in a fan outer duct  48  which surrounds the bypass duct  30 . The FLADE stage  44  provides an additional flow stream at a different flow and pressure ratio that than of the fan  14 . Other fan stage counts with possibly FLADE stages on more than one fan blade could also be used, depending on the final selection of fan and FLADE pressure ratios. The FLADE stage flow is sized to provide sufficient bleed air pressure and flow for a selected aircraft bleed-air powered system of a known type (not shown). A row of variable-angle FLADE inlet guide vanes  50 , operated by an actuator  52 , are moveable between open and closed positions to vary the flow through the FLADE stage  44 . 
         [0015]    The fan outer duct  48  includes one or more bleed air outlets  54  which direct flow to the aircraft bleed air system. Bleed air valves  56  may also be provided to selectively close off the bleed air outlets  54  and direct the FLADE stage flow downstream through the fan outer duct  48 . 
         [0016]    An exhaust duct  58  is disposed downstream of the core  28 , and receives the mixed air flow from both the core  28  and the bypass duct  30 . A mixer  60  (for example a lobed or chute-type mixer) is disposed at the juncture of the core  28  and bypass duct  30  flow streams to promote efficient mixing of the two streams. 
         [0017]    In operation, the engine  10  generates thrust for aircraft propulsion in a known manner, while the FLADE stage discharges bleed air flow through the bleed air outlets  54 .  FIG. 2A  shows how the thrust of the fan  14  is modulated or varied. The speed of the LP spool (and thus the fan  14 ) are kept constant, or nearly so, at a maximum or near-maximum RPM, in all flight conditions. To reduce the thrust to the required levels during approach and landing, shown in zone “L”, the fan IGVs  32  and optionally the variable interstage vanes  36  are selectively closed to lower the fan flow and pressure ratio, which lowers the engine thrust (the larger numbers along the horizontal axis of  FIG. 2A  indicating increasing closure of the stator assembly). This ability to keep the LP spool at full speed at reduced thrust allows the discharge of the FLADE stage  44  to remain at constant flow and pressure ratio levels across the entire thrust band of interest. Thus, the aircraft can operate with high levels of lift enhancement or other bleed flow energy-dependent functions, while the engine thrust is low. In contrast, the percentage of sea-level static (SLS) thrust in a prior art engine is generally proportional to the speed of the LP spool, as shown in  FIG. 2B . To reduce the thrust to the required levels during approach and landing, the speed “N 1 ” of the low pressure turbine and fan must necessarily be reduced, reducing flow and pressure levels available for bleed air flow. 
         [0018]    The FLADE inlet guide vanes  50  are used to modulate the bleed air flow, and are nominally at some partially-closed setting. In a multi-engine installation, if one engine fails, FLADE inlet guide vanes  50  would move from the nominal setting to a full open setting. Flow and pressure will increase to keep the total bleed air flow energy level constant. Mach numbers in the internal aircraft ducting will not change since flow and pressure will change along a fixed area operating line. Since the LP spool is already at maximum RPM, response to an engine out emergency will be very fast. The FLADE stage flow and pressure levels can be selected to match the wing lift enhancement or other bleed air flow needs, while the fan  14  and core system can be optimized for the specific in-flight mission needs. 
         [0019]      FIG. 3  illustrates an engine  110  similar to engine  10  and having a fan  114 , a core  128 , a FLADE stage  144  positioned in a fan outer duct  148 , and an exhaust nozzle  162 . In the illustrated example, the exhaust nozzle  162  is a so-called “2-D” design having moveable flaps  164  that may be used to change a throat area and/or an exit area, denoted “A8” and “A9” respectively in accordance with conventional practice, in order to accommodate changes in the operating cycle of the engine  110 . The flaps  164  can also be used to provide thrust vectoring. The exhaust nozzle  162  also includes reverser cascade vanes  166  which generate reverse thrust when the flaps  164  are moved to a fully closed position, shown in phantom lines. The present invention may also be used with a conventional axisymmetric nozzle design (not shown). A FLADE nozzle  168  is disposed around the exhaust nozzle  162  and is connected to the fan outer duct  148 . When bleed air valves  156  are closed, the bleed air outlets  154  are shut off and the FLADE stage discharge exits the FLADE nozzle  168 . This flow could be used for cooling the exhaust nozzle  162  or for in-flight performance enhancements. In such cases it may be desirable to size the FLADE stage  144  with more flow than is needed for wing lift enhancement or other bleed air functions. 
         [0020]    With proper selection of flade pressure ratio it would be possible to introduce any excess FLADE flow into the primary engine flow stream to eliminate the need for a separate FLADE nozzle. This concept is depicted in  FIG. 4 , which shows an engine  210  similar to engine  10  and having a fan  214 , a core  228 , a FLADE stage  244  positioned in a fan outer duct  248 , and an exhaust nozzle  262 . FLADE injector doors  270  are positioned upstream of the exhaust nozzle  262 . They are moveable between a closed position in which the aft end of the fan outer duct  248  is blocked, and an open position in which the FLADE stage discharge is injected or dumped into the mixed flow stream upstream of the exhaust nozzle  262 . This flow dump will tend to cause an accompanying fan operating line change, which may be corrected by a change in the throat area of the exhaust nozzle  262 . If this operating line change can be tolerated in a particular application, a fixed area exhaust nozzle (not shown) may be used. 
         [0021]      FIG. 5  depicts yet another engine  310  similar to engine  10  and having a fan  314 , a core  328 , a FLADE stage  344  positioned in a fan outer duct  348 , and an exhaust nozzle  362 . The fan outer duct  348  terminates upstream of the core exit. When the bleed air valves  356  are open, the FLADE stage discharge flows through the bleed air outlets  354 . When the bleed air valves  356  are closed, the FLADE stage discharge mixes with the bypass flow from the fan  314 . As with the engine  210  described above, no separate FLADE nozzle is required. 
         [0022]    The foregoing has described a gas turbine engine having a modulated flow fan. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.