Annotation: The process of creating modifications of aircraft in the transport category is a very relevant and widespread phenomenon in modern aircraft construction. A separate group of military transport aircraft has been distinguished in connection with the specific character of their mission: – the need to formulate the characteristics “cargo – range” for light, medium, operational tactical and strategic military transport aircraft, since it is precisely according to this characteristic that they are positioned by their purpose; –specific requirements are imposed on military transport aircraft cargo compartment not only with respect to its geometrical dimensions and usable volume, but also with respect to the possibility of simultaneous accommodation of military equipment and people, as well as the placement of a stretcher with t he wounded during their evacuation from the war zone; – the possibility of airborne landing of military equipment and paratroopers, which requires specific hatches and means of maintaining weight balance in flight; – the possibility of basing on poorly prepared sites with a runway length of less than 800 m in the short take-off and landing (STL) mode, especially for operational tactical military-technical vehicles, which significantly expands their use in combat zones; – the possibility of conversion into a civilian aircraft: for the delivery of goods to areas of the far north, when fighting fires, when evacuating victims from disaster zones, etc. The article shows that creation of modifications of expensive military transport aircraft is the main direction of their development. All leading aircraft manufacturing companies in the world use modification procedures as the way to most quickly meet constantly changing requirements for military transport aircraft. Along with the traditional methods of designing the modifications, the domestic school proposed a new methodology for determining the necessary parameters for “deep” modifications in wing geometry and propulsion system. The methodology is based on the use of three principles: – ensuring growth of carrying capacity and the required range of modifications of military transport aircraft of various purposes; – geometric re-arrangement of wing and system of carrying surfaces “wing + tail units” according to the criterion of minimum inductive resistance when lifting forces are equal to basic version; – coordination of modifications in wing with the required parameters of propulsion system as a condition for ensuring the required fuel efficiency.

Annotation: The main parameters are understood as: carrying capacity mг, range L and fuel efficiency qт, which largely determine the competitiveness of aircraft of this type, including military transport aircraft. The reason for the creation of modifications of transport category aircraft is the requirement for a constant increase in their flight performance by increasing the carrying capacity and range. Among the main goals of implementing such decisions there is an indispensable increase in the fuel efficiency of modifications, since the cost of fuel reaches 80 % of the cost of an airplane hour during operation. There are a number of models that make it possible to assess the influence of the weight and aerodynamic parameters of the airframe of the aircraft and the fuel performance of the power plant (specific engine consumption) on the integral indicator of the fuel efficiency of the modification at cruising mode and the average hourly fuel consumption at the certification stage, when all parameters of the airframe and engine are fixed and consideration of options is not possible. A new model is proposed for the stage of designing modifications, in which deep modification changes are made in the geometry of wing and in the power plant with various variants of their correlation and coordination. The parameters of new model: specific fuel efficiency – specific route productivity, in order to form the relative carrying capacity and relative range of action for the required specific fuel efficiency. An analysis of such dependencies showed: – with an increase in relative range L , fuel costs per flight also increase; – the adequacy of changes in route performance is observed only at L < 0.5. At L > 0.5 productivity is constantly decreasing, while the specific indicator of fuel consumption per unit of work increases exponentially; – if in the analysis we take into account the specific value of transport efficiency, that is, the characteristic “load – range” ( mп.н  f L ), it becomes obvious that the most favorable (from the point of view of fuel efficiency) are relative ranges of 0,3 < L < 0,5. In this range L , not only acceptable fuel efficiency values are realized, but also the maximum value of the route performance, that is the main parameters for which modifications are developed.