1. A method for determining a rigid value of an aerodynamic coefficient of an aircraft for a given set of conditions, the rigid value of the aerodynamic coefficient is a value the coefficient would assume if the aircraft were perfectly rigid, the method comprising: taking measurements by a processor, in a wind tunnel, on a model representative of the aircraft for the set of conditions involved; and determining by the processor or by a second processor, on the basis of the measurements, at least one flexible model value of the aerodynamic coefficient of the aircraft, wherein the rigid value of the aerodynamic coefficient is calculated on the basis of the at least one flexible model value and a correction taking into account flexibility of the model.