Gearboxes for aircraft gas turbine engines
Gearboxes for aircraft gas turbine engines, in particular arrangements for journal bearings such gearboxes, and related methods of operating such gearboxes and gas turbine engines. A gearbox for an aircraft gas turbine engine includes: a sun gear; a plurality of planet gears surrounding and engaged with the sun gear; and a ring gear surrounding and engaged with the plurality of planet gears, each of the plurality of planet gears being rotatably mounted around a pin of a planet gear carrier with a journal bearing having an internal sliding surface on the planet gear and an external sliding surface on the pin.
1. A gearbox for an aircraft gas turbine engine, the gearbox comprising:
a sun gear;
a plurality of planet gears surrounding and engaged with the sun gear; and
a ring gear surrounding and engaged with the plurality of planet gears, each of the plurality of planet gears being rotatably mounted around a pin of a planet gear carrier with a journal bearing having an internal sliding surface on the planet gear and an external sliding surface on the pin,
wherein a ratio of a length, L, of the internal and external sliding surfaces to a diameter, D, of the journal bearing is between around 1.1 and 1.4 and,
wherein the journal bearing has a diametral clearance that is between 0.1% and 0.3%.
2. The gearbox of claim 1 , wherein the ring gear has a pitch circle diameter of around 550 mm or greater.
3. The gearbox of claim 1 , wherein the ratio is between around 1.1 and 1.3.
4. The gearbox of claim 1 , wherein each journal bearing has a maximum operating specific load and a maximum operating sliding speed, wherein the maximum operating specific load multiplied by the maximum operating sliding speed is around 240 MPa m/s or greater.
5. The gearbox of claim 4 wherein the maximum operating specific load multiplied by the maximum operating sliding speed is less than around 720 MPa m/s.
6. The gearbox of claim 1 , wherein the pitch circle diameter of the ring gear is no greater than 1200 mm.
7. The gearbox of claim 1 , wherein the gearbox has a gear ratio of 3.2 to 4.5 or 3.2 to 4.0.
8. The gearbox of claim 1 , wherein the gearbox is in a star configuration.
9. The gearbox of claim 1 , wherein the journal bearing has a diametral clearance that is between 0.1% and 0.2%.
10. A gas turbine engine for an aircraft, comprising:
an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;
a fan located upstream of the engine core, the fan comprising a plurality of fan blades; and
a gearbox according to claim 1 , the gearbox configured to receive an input from the core shaft and provide an output drive to the fan so as to drive the fan at a lower rotational speed than the core shaft.
11. The gas turbine engine of claim 10 , wherein:
the turbine is a first turbine, the compressor is a first compressor, and the core shaft is a first core shaft;
the engine core further comprises a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor; and
the second turbine, second compressor, and second core shaft are arranged to rotate at a higher rotational speed than the first core shaft.
12. The gas turbine engine according to claim 10 , wherein the gas turbine engine has:
a specific thrust from 70 to 90 N kg −1 ; and/or
a bypass ratio at cruise conditions of 12.5 to 18 or 13 to 16.
13. The gas turbine engine according to claim 10 , wherein:
the fan has a moment of inertia of between around 5.5×10 7 and 9×10 8 kg m 2 , optionally between around 7.4×10 7 and 7×10 8 kg m 2 , optionally between around 8.3×10 7 and 6.5×10 8 kg m 2 .
14. The gas turbine engine according to claim 10 , wherein the fan has a diameter on the order of 220 cm.
15. The gas turbine engine according to claim 10 , wherein the gas turbine engine has an overall pressure ratio at cruise conditions in the range of 45 to 60.
16. The gas turbine engine according to claim 10 , wherein:
the fan has a diameter in the range of 220 cm to 240 cm;
the gas turbine engine has an overall pressure ratio in the range 45 to 50; and
a bypass ratio at cruise conditions is in the range 12.5 to 16.0.
17. The gas turbine engine according to claim 16 , wherein the gearbox is in a star configuration.
18. A method of operating a gas turbine engine according to claim 10 , the method comprising operating the engine at maximum take-off conditions, wherein for each journal bearing in the gearbox a specific loading multiplied by a sliding speed is greater than around 240 MPa m/s.
19. The method of claim 18 wherein the specific loading multiplied by a sliding speed for each journal bearing is less than around 720 MPa m/s.
20. The gas turbine engine of claim 10 , wherein the gas turbine engine has:
a bypass ratio at cruise conditions of from 10 to 11.5; and
a fan tip loading at cruise conditions of 0.30 to 0.34.