An arrangement to set up and to ensure an air gap in an electrical machine, like a generator, is provided. The electrical machine includes a stator arrangement and a rotor arrangement. The rotor-arrangement rotates around a longitudinal axis. At least parts of the rotor arrangement interact with parts of the stator arrangement to generate electrical power. An air gap is defined by the distance between the parts of the rotor arrangement and the parts of the stator arrangement. The parts of the stator arrangement are opposite to the parts of the rotor arrangement along a certain length. The cross-section of the air gap changes along the certain length, so that the air gap is not uniform in view to the referred certain length.