Longitudinal Flight Modes Natural Frequencies of Glider Type Small UAVs

Geometry and mass of an aircraft are important factors in flight mechanics and in the calculations of stability and natural frequencies of its flight modes, which are of great importance in controller design process. In this paper, comparison between the formulae of the large aircraft applied on small UAVs scale, XFLR5 as a numerical program and experimental data obtained from flight tests is made to investigate their accuracy. A method to validate natural frequencies for UAVs is presented. It is found that for the phugoid mode, methods of Roskam (exact), Ostoslavsky, and XFLR5 estimate the frequency within range of the experimental results while the methods of Roskam (approximation) and Hull overestimate it and should not be applied in case of small UAVs. Considering the short-period mode, all methods can predict the actual frequency with acceptable accuracy.