Gas turbine engine operating point

A method of designing a gas turbine engine, comprising:

• selecting an engine core comprising a turbine, a compressor, a combustor, and a core shaft connecting the turbine to the compressor;
• locating a fan upstream of the engine core, the fan comprising a plurality of fan blades;
• defining a bypass duct located radially outside the engine core and radially inside a nacelle, such that a proportion of a fan flow flows through the bypass duct as bypass flow, and a further proportion of the fan flow flows through the engine core as core flow;
• providing a gearbox that receives an input from the core shaft and outputs drive to the fan so as to drive the fan at a lower rotational speed than the core shaft;
• defining a reference operating point of the gas turbine engine at cruise conditions for the gas turbine engine, the reference operating point defining a fan reference rotational speed, the cruise conditions corresponding to one of:
• atmospheric conditions defined by the International Standard Atmosphere at an altitude of 11582 m and a forward Mach Number of 0.8;
• and
• atmospheric conditions defined by the International Standard Atmosphere at an altitude of 10668 m and a forward Mach Number of 0.85;
• defining a bypass efficiency as an efficiency of a fan compression of the bypass flow, the bypass efficiency being a function of a fan bypass inlet mass flow rate;
• designing the fan such that at the fan reference rotational speed and the cruise conditions, a ratio of a theoretical fan bypass inlet mass flow rate that would give a peak bypass efficiency to a theoretical maximum possible fan bypass inlet mass flow rate that the fan could support at the fan reference rotational speed and the cruise conditions is no greater than 0.96, where the fan bypass inlet mass flow rate at the fan reference rotational speed and the cruise conditions is determined by a fixed throat area of the bypass duct;
• and
• choosing the fixed throat area of the bypass duct throat that is greater than a throat area that would give the theoretical fan bypass inlet mass flow rate that would give a peak bypass efficiency at the fan reference rotational speed and the cruise conditions.