At the stage of a power unit selection for a multi-purpose aircraft the problem of mutual relations between the dimension of an aircraft and an engine should be solved. Starting from the motion equation of an aircraft and the theory of similarity the criteria and performance were determined which connect in a geometrical and power way the engine and the aircraft. The analysis of the influence of flight conditions and the parameters of an engine comparative cycle on the geometrical dimensions was conducted. In the paper it was shown that the fundamental flight stage which determines the relations between the geometrical parameters of the aircraft and the engine is the take-off or supersonic flight on the big altitude. Usually the parameters selection of the turbine engine thermal cycle is done on the basis of the internal characteristics of the engine, such as specific thrust and specific fuel usage. In case of the turbofan engine model with the mixer, afterburner, and the aircraft model (with simplified aerodynamic and mass characteristics) the influence of the cycle parameters on the performance and aerodynamic lift/drag ratio, the agreed range and the theoretical range was described.