The Numerical Investigation on the Aerodynamic Characteristics of a Long-Endurance UAV with Low Aspect-Ratio

Nowadays, the requirement of long-endurance for the civilian or military UAV is increasingly prominent and its importance exceeds that of the requirement of the high cruise speed in some cases. With the purpose of developing a long-endurance UAV, which may be used for long-time works of spraying pesticides or reconnaissance and which does not emphasize the requirement of high cruise speed, the delta wing configuration with low aspect-ratio and structure simplicity is used and the winglets with the shape of trapezoid are used for this configuration to suppress the flow separation at the wingtips and increase the course stability. And, both the dual-swept and single-swept delta wings are considered, with the purpose of obtaining as high aerodynamic efficiency as possible by simple comparison of computational results. Under the cruise conditions that the cruise Mach number Ma is equal to be 0.15 and the cruise height H is 1 km, the aerodynamic characteristics of the UAV are estimated by solving the steady compressible Reynolds-averaged Navier-Stokes (RANS) equation with the usage of S-A turbulence model and unstructured mesh. The unstructured mesh for the computation of the UAV is generated by the software Pointwise and the typical mesh contains about 13000000 cells. The mesh convergence analysis has been conducted to ensure the reliability of the computational results. The results show that the dual-swept and sing-swept delta wings are both longitudinally stable and the linearities of the lift curves are good, when AoA (angle of attack) is less than 12°. Beneficial from the usage of winglets, the stall angles for both delta wings are larger than 20°, which is the upper limit of AoA in our computation. The maximum lift-drag-ratio for the single-swept delta wing with the aspect ratio of 1.95, which arises at the AoA value of 6°, is slightly larger than the one for the dual-swept delta wing with the aspect ratio of 2.05, for which the maximum lift-drag-ratio arises at the AoA value of 7°, implying that the configuration of single-swept delta wing is a better choice for the UAV with the requirement of long-endurance. Further, for the chosen single-swept delta wing, the linearity of the pitching moment curve is good with a static margin of about 4.6%. The linearity of the elevator efficiency is also good, and it will be reduced by about 50% when the deflection angle of the elevator exceeds 20°.