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, with the aircraft gas turbine engine operating at maximum take-off conditions, an eccentricity ratio, E, of each journal bearing is a function of a temperature T of oil flowing into the journal bearing, such that E>AT+B where A is 0.0015/° C. and B is 0.69, the eccentricity ratio being defined as 1-2H min /c, where H min is a minimum oil film thickness between the internal and external sliding surfaces and c is the diametral clearance of the journal bearing.
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 E is less than around 0.98.
4. The gearbox of claim 1 , wherein E<A′T+B′ where A′ is 0.00033/° C. and B′ is 0.94.
5. The gearbox of claim 1 , wherein E>A″T+B″ where A″ is 0.00083/° C. and B″ is 0.84.
6. The gearbox of claim 1 , wherein T is greater than around 60° C.
7. The gearbox of claim 6 , wherein T is greater than around 100° C.
8. The gearbox of claim 1 , wherein T is less than around 120° C.
9. The gearbox of claim 1 , wherein the gearbox has a gear ratio of 3.2 to 4.5 or 3.2 to 4.0.
10. The gearbox of claim 1 , wherein the gearbox is in a star configuration.
11. 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.
12. The gas turbine engine of claim 11 , 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.
13. The gas turbine engine according to claim 11 , 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.
14. The gas turbine engine according to claim 11 , wherein:
the fan has a moment of inertia of between around 5.5×10 7 and 9×10 8 -kg m 2 .
15. A method of operating a gas turbine engine according to claim 11 , the method comprising operating the aircraft gas turbine engine at maximum take-off conditions, an eccentricity ratio, E, of each journal bearing being a function of a temperature T of oil flowing into the journal bearing, such that E>AT+B where A is 0.0015/° C. and B is 0.69.