GAS TURBINE ENGINE WITH LOW STAGE COUNT LOW PRESSURE TURBINE
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.