Froude Similarity and Flying Qualities Assessment in the Design of a Low-Speed BWB UAV

In the development of a blended wing body (BWB) planform for low-speed unmanned aerial vehicles (UAV), the present design approach relies on rather cost-effective fidelity tools in aerodynamic modeling and testing. This enables efficient analysis management of the huge parametric studies from airfoil to wing definitions, and proves thereby to be appropriate for conceptual design. Here, emphasis is put on the dynamic stability analysis and flight quality criteria assessment as early design tasks. This turned out to be a challenging task in the conceptual design, since there are no established standards criteria for small to medium scales UAVs. However, design criteria are often derived from the existing larger manned aircraft test data and adjusted for these UAVs. In the present work, on the one hand, it is first shown how existing criteria on short period and phugoid frequencies and damping can be applied to UAVs as inherent longitudinal stability characteristics. On the other hand, the control anticipation parameter (CAP) criterion is used to assess the inherent maneuverability. To overcome the lack of precise subsonic BWB data, adjustment of these criteria through Froude scaling and parameters tuning are used for the definition of the upscaled reference aircraft as a candidate for CAP criterion assessment. The proposed procedure is built on existing class I light aircraft data. It is thus shown how mass-based scaling provides a more realistic approach compared to other scaling techniques. Consequently, the flight quality level evaluated for the virtually upscaled aircraft is then assigned to the designed mini UAVs. It is then believed that the suggested study based on Froude similarity for predicting flight quality in the early design phase contributes to more efficient conceptual design.