Abstract:
A gas turbine engine has a compressor section, a low spool, and a fan. The fan delivers air into the compressor section and into a bypass duct having a variable area nozzle. The compressor section compresses air and delivers it into a combustion section. The combustion section mixes air with fuel, igniting the fuel, and driving the products of the combustion across a turbine. The turbine drives the low spool. A control for the gas turbine engine is programmed to position the nozzle at startup of the engine to increase airflow across the fan. A variable inlet guide vane is positioned upstream of the compressor section. The control also positions the variable inlet guide vane at start-up to increase air flow across the compressor section.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/367,579, filed Feb. 7, 2012, which claims priority to U.S. Provisional Application No. 61/592,672, which was filed Jan. 31, 2012. 
     
    
     BACKGROUND 
       [0002]    This application relates to a gas turbine engine having an inlet guide vane which has its position controlled to increase windmilling speed of engine components. 
         [0003]    Gas turbine engines are known, and typically include a fan delivering air into a bypass duct outwardly of a core engine, and into a compressor in the core engine. Air in the compressor is passed downstream into a combustor section where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, driving them, and in turn drive the compressor and fan. Recently it has been proposed to include a gear reduction between a low pressure compressor and the fan, such a low pressure turbine can drive the two at distinct speeds. 
         [0004]    A gas turbine engine as used on an aircraft must be able to start under several conditions. First, the gas turbine engine must be able to start when on the ground. A starter can be used on the ground. Second, the gas turbine engine must be able to start in the air. In the air, at lower speeds of the aircraft, the normal starter for the gas turbine engine may be utilized to begin driving the turbine/compressor rotors. However, at higher speeds the starter may not be utilized. At higher speeds so called “windmilling” is relied upon at startup. Windmilling typically occurs as the compressor and fan rotors are driven by the air being forced into the core engine, and the bypass duct, as the aircraft continues to move. 
       SUMMARY 
       [0005]    In a featured embodiment, a gas turbine engine has a compressor section, a low spool, and a fan. The fan delivers air into the compressor section and into a bypass duct having a variable area nozzle. The compressor section compresses air and delivers it into a combustion section. The combustion section mixes air with fuel, igniting the fuel, and driving the products of the combustion across turbine rotors. The turbine rotors drive the low spool. A control for the gas turbine engine is programmed to position the nozzle at startup of the engine to increase airflow across the fan. A variable inlet guide vane is positioned upstream of the compressor section. The control also positions the variable inlet guide vane at start-up to increase air flow across the compressor section 
         [0006]    In another embodiment according to the foregoing embodiment, the compressor section includes a high pressure compressor and a low pressure compressor. The turbine rotors include a low turbine rotor driving the low spool and the low pressure compressor. 
         [0007]    In another embodiment according to the foregoing embodiment, the fan is driven with the low pressure compressor by the low spool. There is a gear reduction between the fan and the low spool. 
         [0008]    In another embodiment according to the foregoing embodiment, the control includes stored desired positions for the nozzle to provide increased airflow into the compressor at startup at various conditions. 
         [0009]    In another embodiment according to the foregoing embodiment, the various conditions include the altitude of an aircraft carrying the gas turbine engine, and an air speed of the aircraft. 
         [0010]    In another embodiment according to the foregoing embodiment, the conditions also include a speed of the low spool when startup is occurring. 
         [0011]    In another embodiment according to the foregoing embodiment, the position of the nozzle is selected to increase airflow across the fan while an aircraft associated with the gas turbine engine is in the air, and to increase windmilling speed of the turbine rotors. 
         [0012]    In another embodiment according to the foregoing embodiment, a starter is also utilized in combination with the windmilling while the aircraft is in the air to start the engine. 
         [0013]    In another embodiment according to the foregoing embodiment, the nozzle is moved toward a full open position to increase windmilling speed. 
         [0014]    In another embodiment according to the foregoing embodiment, the positioning of the variable inlet guide increases air flow across the low pressure compressor. 
         [0015]    In another embodiment according to the foregoing embodiment, the control includes stored desired positions for the nozzle to provide increased airflow into the compressor section at startup at various conditions. 
         [0016]    In another embodiment according to the foregoing embodiment, the various conditions include the altitude of an aircraft carrying the gas turbine engine, and an air speed of the aircraft. 
         [0017]    In another embodiment according to the foregoing embodiment, the conditions also include a speed of the low spool when startup is occurring. 
         [0018]    In another embodiment according to the foregoing embodiment, the position of the nozzle is selected to increase airflow across the fan while an aircraft associated with the gas turbine engine is in the air, and to increase windmilling speed of the turbine rotors. 
         [0019]    These and other features may be best understood from the following drawings and specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows a gas turbine engine. 
           [0021]      FIG. 2  is a schematic of a control logic circuit. 
           [0022]      FIG. 3  is a flowchart. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  22  drives air along a bypass flowpath B while the compressor section  24  drives air along a core flowpath C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
         [0024]    The engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided. 
         [0025]    The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure compressor  44  and a low pressure turbine  46 . The inner shaft  40  is connected to the fan  42  through a geared architecture  48  to drive the fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a high pressure compressor  52  and high pressure turbine  54 . A combustor  56  is arranged between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  57  of the engine static structure  36  is arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  57  further supports bearing systems  38  in the turbine section  28 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is collinear with their longitudinal axes. 
         [0026]    The core airflow C is compressed by the low pressure compressor  44  then the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The mid-turbine frame  57  includes airfoils  59  which are in the core airflow path. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. 
         [0027]    The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than ten (10), the geared architecture  48  is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about 5. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about 5:1. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.5:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
         [0028]    A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (&#39;TSFC&#39;)”—is the industry standard parameter of 1 bm of fuel being burned divided by 1 bf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tambient deg R)/518.7)̂0.5]. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second. 
         [0029]    The gas turbine engine  20  is provided with controls and features to optimize starting. 
         [0030]    A starter  400  (shown schematically) is typically included with a gas turbine engine, and is relied upon to begin driving the high spool when the engine is started. This will typically occur when the airplane is on the ground, and is a relatively simple process at that time. 
         [0031]    On the other hand, there are times when the gas turbine engine is shut down while an aircraft associated with the gas turbine engine is still in the air. At lower air speeds, the starter may be utilized while the aircraft is in the air to begin driving rotation of the spool  32  to begin the restart process. Of course, once the combustion section has begun to ignite and burn the fuel, then the products of combustion will take over driving the turbine rotors and the starter may stop. 
         [0032]    Under certain conditions, use of the starter while the aircraft is in the air is not advised or is not possible. Under those conditions, the force of air being driven into the engine core, and across the fan  42  is relied upon to drive the turbine rotors, and the compressor rotors. This process is called “windmilling.” 
         [0033]    It is desirable to increase the speed of windmilling of the high spool that occurs when it is necessary to restart the engine because higher windmill speeds drive higher airflow. 
         [0034]    The engine is provided with equipment that is controlled to optimize to increase the ability to maximize windmilling of the high spool. Thus, an actuator  180  selectively drives a control to position a compressor inlet guide vane  184  which is just forward of the forward most low compressor rotor  186 . 
         [0035]    An angle of the vane  184  is preferably positioned to maximize the flow of air reaching the rotor  186  while the aircraft is being restarted. In flight, this would be positioning the vane  184  such that the air being forced into the core engine as the aircraft continues to move through the air with engine  20  not being powered, is maximized. 
         [0036]    Also, the bypass airflow B may be maximized by positioning a variable fan nozzle  200 . The variable fan nozzle  200  is controlled by an actuator  204 , shown schematically, to move axially and control the flow area at  202 . Generally, one would open the nozzle to a full open position to maximize this air flow. 
         [0037]    Both the inlet guide vane  180  and the actuator  204  for the variable area fan nozzle  200  are generally as known. However, they have not been utilized at startup to maximize the amount of windmilling which occurs. 
         [0038]    In general, it is desirable to position the vane  184  to maximize airflow through the core engine, and position the variable area nozzle  200  to maximize airflow across the fan  42 . Airflow across the fan  42  will drive the fan to rotate, and air being forced into the core engine will cause the compressor rotor  186  to rotate. 
         [0039]    Applicant has developed a control system as shown in  FIG. 2  which takes in altitude signals  210 , an aircraft speed signal  212 , and a signal  214  which is the windmilling speed of the low spool  30 . 
         [0040]    Lookup tables are stored in control component  216 ,  218  and  222 . Applicant has developed tables which associates particular altitudes engine speed, or Mach number, with a desired position for the vane  184 , and/or the position of the nozzle  200  to maximize the airflow as discussed above. The desired positions can be developed experimentally and will vary by aircraft and engine design. While the two features may be used in combination, it is also within the scope of this application that each could be used individually without the other, where appropriate. 
         [0041]    The control of the variable inlet guide van is disclosed in co-pending application entitled Gas Turbine Engine With Compressor Inlet Guide Vane Positioned for Starting, filed on even date herewith, Ser. No. 13/367,742. 
         [0042]    The signal passes downstream to a block  224 , wherein additional signals come from control elements  218  and  216 . Element  218  and  216  provide an adjustment to the output of element  222  based upon the low spool  30  speed altitude and aircraft airspeed. 
         [0043]    Downstream of the block  224 , a signal passes to the actuators  180  and/or  204 . The  FIG. 2  control can be incorporated into a FADEC  199 . 
         [0044]    Of course, if the aircraft is positioned on the ground, the altitude would be generally the same, and the Mach number would be zero. Further, the low spool speed might be zero. Even so, there would be desired positions for the vane  184  and/or nozzle  200 . If the aircraft is in the air when being restarted and moving at a relatively slow Mach number, it may be possible to utilize a starter  400 , shown schematically, in combination with the windmilling. However, this would all be incorporated into the lookup tables stored in component  216 . Also, as mentioned above, at times the starter  400  cannot be relied upon in some circumstances. Again, this would be anticipated and relied upon at components  216 , , 218  and  222  or in the look-up table. 
         [0045]    Although an 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.