Method for setting a gear ratio of a fan drive gear system of a gas turbine engine

Abstract

A gas turbine engine has a fan section including a fan rotatable about an axis. A speed reduction device is connected to the fan. The speed reduction device includes a planetary fan drive gear system with a planet gear ratio of at least 2.6. A bypass ratio is greater than about 11.0. A method of improving performance of a gas turbine engine, a fan drive gear module for a gas turbine engine, and a method of designing a gas turbine engine are also disclosed.

What is claimed is: 
     
         1 . A gas turbine engine comprising:
 a fan section including a fan rotatable about an axis; and   a speed reduction device connected to the fan, wherein the speed reduction device includes a planetary fan drive gear system with a planet gear ratio of at least 2.6, wherein a bypass ratio is greater than about 11.0.   
     
     
         2 . The gas turbine engine of  claim 1 , wherein the gear ratio is less than or equal to 4.1. 
     
     
         3 . The gas turbine engine of  claim 2 , including a fan pressure ratio that is below 1.7. 
     
     
         4 . The gas turbine engine of  claim 2 , including a fan pressure ratio that is below 1.48. 
     
     
         5 . The gas turbine engine of  claim 4 , wherein the fan blade tip speed of the fan section is greater than about 1000 ft/sec and less than about 1400 ft/sec. 
     
     
         6 . The gas turbine engine of  claim 1 , wherein the planetary fan drive gear system includes a sun gear, a plurality of planetary gears, a ring gear, and a carrier. 
     
     
         7 . The gas turbine engine of  claim 6 , wherein each of the plurality of planetary gears includes at least one bearing. 
     
     
         8 . The gas turbine engine of  claim 6 , wherein a low pressure turbine is mechanically attached to the sun gear. 
     
     
         9 . The gas turbine engine of  claim 8 , including a low pressure turbine section in communication with the speed reduction device, wherein the low pressure turbine section includes at least three stages. 
     
     
         10 . The gas turbine engine of  claim 1 , wherein the bypass ratio is less than about 22.0. 
     
     
         11 . The gas turbine engine of  claim 1 , wherein there are three turbine rotors with a first fan drive turbine rotor driving said fan through said speed reduction device, and an intermediate turbine rotor and a high pressure turbine rotor each driving a compressor rotor. 
     
     
         12 . A method of improving performance of a gas turbine engine comprising:
 determining fan tip speed boundary conditions for at least one fan blade of a fan section;   determining rotor boundary conditions for a rotor of a low pressure turbine; and   utilizing stress level constraints in the rotor of the low pressure turbine and the at least one fan blade to determine if the rotary speed of the fan section and the low pressure turbine will meet a desired number of operating cycles, wherein a bypass ratio is greater than about 6.0, wherein a speed reduction device connects the fan section and the low pressure turbine and includes a planetary gear ratio of at least about 2.6.   
     
     
         13 . The method of  claim 12 , wherein the planetary gear ratio is less than about 4.1. 
     
     
         14 . The method of  claim 13 , wherein a fan pressure ratio is below 1.7. 
     
     
         15 . The method of  claim 14 , wherein a fan pressure ratio is below 1.48. 
     
     
         16 . The method of  claim 15 , wherein the bypass ratio is greater than about 11 and less than about 22. 
     
     
         17 . The method of  claim 16 , wherein a fan blade tip speed of the at least one fan blade is less than 1400 fps. 
     
     
         18 . The method of  claim 12 , wherein if a stress level in the rotor or the at least one fan blade is too high to meet a desired number of operating cycles, a gear ratio of a gear reduction device is lowered and the number of stages of the low pressure turbine is increased. 
     
     
         19 . The method of  claim 18 , wherein if a stress level in the rotor or the at least one fan blade is too high to meet a desired number of operating cycles, a gear ratio of a gear reduction device is lowered and an annular area of the low pressure turbine is increased. 
     
     
         20 . A fan drive gear module for a gas turbine engine comprising:
 a planetary fan drive gear system with a speed reduction ratio of at least 2.6, wherein the speed reduction device is configured to drive a fan section with a bypass ratio greater than about 11.0.   
     
     
         21 . The fan drive gear module of  claim 20 , wherein the speed reduction ratio is less than or equal to 4.1. 
     
     
         22 . The fan drive gear module of  claim 20 , wherein the planetary fan drive gear system is configured to drive a fan section with a fan blade tip speed greater than about 1000 ft/sec and less than about 1400 ft/sec. 
     
     
         23 . The fan drive gear module of  claim 20 , wherein the bypass ratio is less than about 22.0. 
     
     
         24 . A method of designing a gas turbine engine comprising:
 selecting fan tip speed boundary conditions for at least one fan blade of a fan section of a gas turbine engine;   selecting rotor boundary conditions for a rotor of a fan drive turbine of the gas turbine engine;   determining stress level constraints in the rotor of the fan drive turbine and the at least one fan blade to determine if the rotary speed of the fan section and the fan drive turbine will meet a desired number of operating cycles, wherein a bypass ratio is greater than about 6.0; and   connecting the fan section and the fan drive turbine through a speed reduction device that includes a planetary gear ratio of at least about 2.6.   
     
     
         25 . The method of  claim 24 , wherein the planetary gear ratio is less than about 4.1. 
     
     
         26 . The method of  claim 24 , wherein the bypass ratio of the gas turbine engine is greater than about 11 and less than about 22. 
     
     
         27 . The method of  claim 24 , including lowering the speed reduction ratio of the speed reduction device and increasing a number of stages of the fan drive turbine responsive to determining that a stress level in the rotor or the at least one fan blade is outside a predefined number of operating cycles. 
     
     
         28 . The method of  claim 24 , including lowering the speed reduction ratio of the speed reduction device and increasing an annular area of the fan drive turbine responsive to determining that a stress level in the rotor or the at least one fan blade is outside a predefined number of operating cycles.