The process of creating free flying dynamically similar models (FDSM) has a number of features that distinguish it from the similar process for aircraft for other purposes. The main one is the necessity to satisfy the accepted (taking into account the modelling tasks) similarity conditions at all stages of design and production, ground and flight tests of FDSM, as well as flight studies on them. Even minor deviations from these conditions can raise doubts about the reliability of the results obtained, the solution of which requires additional labour-intensive research. In addition, a negative decision on the operation of FDSM with parameters that cannot be adjusted (at the final stages of development of FDSM) may exclude the possibility of obtaining advanced data on the actual aircraft and make it impractical to carry out the entire research programme on free-flying dynamically similar models. From these positions, the authors of the paper justify the use of numerical modelling methods and the mathematical apparatus known in the theory of automatic control to investigate the quality of the automatic control system (ACS) in the task of determining the functional capabilities of FDSM aircraft. The degree of similarity was assessed by comparing the behaviour of the real designed (hereafter we will write simply “real”, although it may differ markedly from the really real one) and the benchmark FDSM. In doing so, a literature review was performed regarding the selection and comparison of quality indicators to determine the functionality of FDSM aircraft. The theoretical foundations for determining the suitability of a FDSM aircraft as a research instrument is developed. As an illustration of the developed theoretical foundations, calculations are performed to determine the functionality of a FDSM aircraft in a real-world design.