Gearboxes for aircraft gas turbine engines

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 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, 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 1.1 and 1.4, and
• wherein the aircraft gas turbine engine is configured so that, operating at maximum take-off conditions, a Sommerfeld number of each journal bearing is greater than around 4.