Gas turbine engine with low stage count low pressure turbine

Abstract

A gas turbine engine comprises a gear train defined along an engine axis. A spool along the engine axis drives the gear train, and includes a low stage count downstream turbine. A fan rotates at a fan speed about the engine axis and drives the downstream turbine through the gear train. The fan speed is less than a speed of the downstream turbine. A core is surrounded by a core housing defined about the engine axis. A fan nacelle is mounted 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 engine axis;   a spool along said engine axis which drives said gear train, said spool includes a low stage count downstream turbine;   a fan rotatable at a fan speed about the engine axis and driven by the downstream turbine through the gear train, wherein the fan speed is less than a speed of the downstream turbine;   a core surrounded by a core housing defined about the engine axis;   a fan nacelle mounted 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 , wherein said gear train defines a gear reduction ratio of greater than or equal to about 2.3. 
     
     
         13 . The engine as recited in  claim 1 , wherein said gear train defines a gear reduction ratio of greater than or equal to about 2.5. 
     
     
         14 . The engine as recited in  claim 1 , 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 downstream turbine defines a turbine pressure ratio that is greater than about five (5). 
     
     
         16 . The engine as recited in  claim 1 , wherein said downstream turbine defines a turbine pressure ratio that is greater than five (5). 
     
     
         17 . The engine as recited in  claim 1 , wherein there are three turbine rotors, and said downstream turbine is a most downstream of said three turbine rotors.