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 in communication with the fan. The speed reduction device includes a planetary fan drive gear system with a planet gear ratio of at least 2.6. A fan blade tip speed of the fan is less than 1400 fps. A low pressure turbine section is in communication with the speed reduction device. The low pressure turbine section includes three or four stages. A bypass ratio is between 11.0 and about 22.0. A method of improving performance of a gas turbine engine comprising

What is claimed is: 
     
         1 . A gas turbine engine comprising:
 a fan section including a fan rotatable about an axis;   a speed reduction device in communication with the fan, wherein the speed reduction device includes an epicyclic gear system with a gear ratio of at least 2.6, wherein a fan blade tip speed of the fan is less than 1400 fps; and   a low pressure turbine section in communication with the speed reduction device, wherein the low pressure turbine section includes three or four stages; and a bypass ratio that is between 11.0 and about 22.0.   
     
     
         2 . The gas turbine engine of  claim 1 , wherein the epicyclic gear system is a planetary gear system. 
     
     
         3 . The gas turbine engine of  claim 2 , wherein the gear ratio is less than or equal to 4.1. 
     
     
         4 . The gas turbine engine of  claim 3 , including a fan pressure ratio that is below 1.7. 
     
     
         5 . The gas turbine engine of  claim 3 , including a fan pressure ratio that is below 1.48. 
     
     
         6 . The gas turbine engine of  claim 2 , wherein the planetary 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 include 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 2 , wherein the fan blade tip speed of the fan is greater than 1000 fps. 
     
     
         10 . The gas turbine engine of  claim 1 , wherein there are three turbine sections, with said low pressure turbine section driving the fan through said speed reduction device, and an intermediate turbine section and a high pressure turbine section each driving compressor rotors. 
     
     
         11 . 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; and   wherein a bypass ratio is greater than about 6.0.   
     
     
         12 . The method of  claim 11 , 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 and no more than about 4.1. 
     
     
         13 . The method of  claim 11 , wherein a fan pressure ratio is below 1.7. 
     
     
         14 . The method of  claim 11 , wherein a fan pressure ratio is below 1.48. 
     
     
         15 . The method of  claim 11 , wherein the bypass ratio is greater than about 11. 
     
     
         16 . The method of  claim 11 , wherein a fan blade tip speed of the at least one fan blade is less than 1400 fps. 
     
     
         17 . The method of  claim 11 , 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. 
     
     
         18 . The method of  claim 11 , 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. 
     
     
         19 . A gas turbine engine comprising:
 a fan section including a fan rotatable about an axis;   a speed reduction device in communication with the fan, wherein the speed reduction device includes an epicyclic gear system with a gear ratio of at least 2.6; and   a bypass ratio that is between about 11.0 and about 22.0, wherein a fan blade tip speed of the fan is less than 1400 fps.   
     
     
         20 . The gas turbine engine of  claim 19 , wherein there are three turbine rotors, with a fan drive turbine rotor driving said fan, and an intermediate and a high pressure turbine rotor each driving a compressor rotor. 
     
     
         21 . The gas turbine engine of  claim 19 , wherein said epicyclic gear system is a planetary gear system.