Gas turbine engine with low stage count low pressure turbine

Abstract

A gas turbine engine comprises a gear train defined along an axis. A spool along the axis drives the gear train and includes a low stage count low pressure turbine. A fan i s rotatable at a fan speed about the axis and driven by the low pressure turbine through the gear train. The fan speed is less than a speed of the low pressure turbine. A core is surrounded by a core nacelle defined about the axis. A fan nacelle i s mounted at least partially around the core nacelle to define a fan bypass airflow path for a fan bypass airflow. A bypass ratio defined by the fan bypass passage airflow divided by airflow through the core is greater than about ten (10).

1 . A gas turbine engine comprising:
 a gear train defined along an axis;   a spool along said axis which drives said gear train, said spool includes a low stage count low pressure turbine;   a fan rotatable at a fan speed about the axis and driven by the low pressure turbine through the gear train, wherein the fan speed is less than a speed of the low pressure turbine;   a core surrounded by a core nacelle defined about the axis; and   a fan nacelle mounted at least partially around said core nacelle to define a fan bypass airflow path for a fan bypass airflow, wherein a bypass ratio defined by the fan bypass passage airflow divided by airflow through the core is greater than about ten (10).   
     
     
         2 . The engine as recited in  claim 1 , wherein said low stage count includes six or fewer stages. 
     
     
         3 . The engine as recited in  claim 1 , wherein said low stage count includes three (3) stages. 
     
     
         4 . The engine as recited in  claim 1 , wherein said low stage count includes five (5) stages. 
     
     
         5 . The engine as recited in  claim 1 , wherein said low stage count includes six (6) stages. 
     
     
         6 . The engine as recited in  claim 1 , wherein said spool is a low spool. 
     
     
         7 . The engine as recited in  claim 1 , further comprising:
 a fan variable area nozzle axially movable relative said fan nacelle to vary a fan nozzle exit area and adjust the fan pressure ratio of the fan bypass airflow during engine operation.   
     
     
         8 . The engine as recited in  claim 7 , further comprising:
 a controller operable to control said fan variable area nozzle to vary the fan nozzle exit area and adjust the pressure ratio of the fan bypass airflow.   
     
     
         9 . The engine as recited in  claim 8 , wherein said controller is operable to reduce said fan nozzle exit area at a cruise flight condition. 
     
     
         10 . The engine as recited in  claim 8 , wherein said controller is operable to control said fan nozzle exit area to reduce a fan instability. 
     
     
         11 . The engine as recited in  claim 8 , wherein said fan variable area nozzle defines a trailing edge of said fan nacelle. 
     
     
         12 . The engine as recited in  claim 1   1 , wherein said gear train defines a gear reduction ratio of greater than or equal to about 2.5. 
     
     
         13 . The engine as recited in  claim 1 , wherein said gear train defines a gear reduction ratio of greater than or equal to about 2.3. 
     
     
         14 . The engine as recited in  claim 13 , wherein said gear train defines a gear reduction ratio of greater than or equal to 2.5. 
     
     
         15 . The engine as recited in  claim 14 , wherein said low pressure turbine defines a low pressure turbine pressure ratio that is greater than about five (5). 
     
     
         16 . The engine as recited in  claim 1 , wherein said low pressure turbine defines a low pressure turbine pressure ratio that is greater than five (5). 
     
     
         17 . The engine as recited in  claim 1 , wherein said low pressure turbine is one of three turbine rotors, and said low pressure turbine driving said fan, while the other two of said turbine rotors each driving a compressor section 
     
     
         18 . The engine as recited in  claim 1 , wherein also including a high pressure turbine, with each of said low pressure turbine and said high pressure turbine driving a compressor rotor. 
     
     
         19 . The engine as recited in  claim 18 , wherein said gear train is positioned intermediate a compressor rotor driven by said low pressure turbine and said fan. 
     
     
         20 . The engine as recited in  claim 18 , wherein said gear train is positioned intermediate said low pressure turbine and said compressor rotor driven by said low pressure turbine.