High power epicyclic gearbox and operation thereof

A gas turbine engine for an aircraft, the gas turbine engine 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 that receives input from a gearbox input shaft portion of the core shaft and drives the fan via a fan shaft at a lower rotational speed than the core shaft, the gearbox being an epicyclic gearbox comprising (i) a sun gear that receives the input from the core shaft, (ii) a plurality of planet gears, (iii) a ring gear, and (iv) a planet carrier arranged to have the plurality of planet gears mounted thereon,
• wherein, at cruise conditions:
• a product of radial bending stiffness of the gearbox input shaft portion and torque on the core shaft is equal to or greater than 1.2×109 N2 and less than 8.0×1011 N2;
• wherein the radial bending stiffness is determined (i) by treating the gearbox input shaft portion as being a free body at a forward point of the gearbox input shaft portion at which is applied a force perpendicular to an axis of the gas turbine engine and (ii) by considering a connection between a rearward end of the gearbox input shaft portion and a remainder of the core shaft to be rigid;
• and
• wherein the radial bending stiffness is determined based on a displacement of the forward point that is measured perpendicular to the axis of the gas turbine engine along a line of action of the applied force.

A method of operating an aircraft,

• a gas turbine engine of the 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 that receives input from a gearbox input shaft portion of the core shaft and drives the fan via a fan shaft at a lower rotational speed than the core shaft, the gearbox being an epicyclic gearbox comprising (i) a sun gear that receives the input from the core shaft, (ii) a plurality of planet gears, (iii) a ring gear, and (iv) a planet carrier arranged to have the plurality of planet gears mounted thereon;
• and
• the method comprising controlling the aircraft such that:
• a product of radial bending stiffness of the gearbox input shaft portion and torque on the core shaft is equal to or greater than 1.2×109 N2 and less than 8.0×1011 N2;
• and/or
• a product of tilt stiffness of the gearbox input shaft portion and the torque on the core shaft is equal to or greater than 5.5×108 N2 m2/radian and less than 2.0×108 N2 m2/radian, wherein:
• the radial bending stiffness is determined (i) by treating the gearbox input shaft portion as being a free body at a forward point of the gearbox input shaft portion at which is applied a force perpendicular to an axis of the gas turbine engine and (ii) by considering a connection between a rearward end of the gearbox input shaft portion and a remainder of the core shaft to be rigid;
• the radial bending stiffness is determined based on a displacement of the forward point that is measured perpendicular to the axis of the gas turbine engine along a line of action of the applied force;
• the tilt stiffness is determined (i) by treating the gearbox input shaft portion as being a free body at a forward point of the gearbox input shaft portion at which is applied a moment whose axis is perpendicular to the axis of the gas turbine engine and (ii) by considering a connection between a rearward end of the gearbox input shaft portion and a remainder of the core shaft to be rigid;
• and
• the tilt stiffness is determined based on an angle measured between (i) a line parallel to the axis of the gas turbine engine and (ii) a local tangent to the gearbox input shaft portion at the forward point of the shaft at which the moment is applied.