Mechanism analysis of hysteretic aerodynamic characteristics on variable-sweep wings
Lifang Zeng, Liu Liu, Xueming Shao, Jun Li
Abstract
Variable-sweep wings have large shape-changing capabilities and wide flight envelops, which are considered as one of the most promising directions for intelligent morphing UAVs. Aerodynamic investigations always focus on several static states in the varying sweep process, which ignore the unsteady aerodynamic characteristics. However, deviations to static aerodynamic forces are inevitably caused by dynamic sweep motion. In this work, first, unsteady aerodynamic characteristics on a typical variable-sweep UAV with large aspect ratio were analyzed. Then, deep mechanism of unsteady aerodynamic characteristics in the varying sweep process was studied. Finally, numerical simulation method integrated with structured moving overset grids was applied to solve the unsteady fluid of varying sweep process. The simulation results of a sweep forward–backward circle show a distinct dynamic hysteresis loop surrounding the static data for the aerodynamic forces. Compared with the static lift coefficients CL, at the same sweep angles, dynamic CL in sweep forward process are all smaller, while dynamic sweep backward CL are all larger. In addition, dynamic deviations to static CL are positively related with the varying sweep speeds. Mechanism study on the unsteady aerodynamic characteristics indicates that three key factors lead to the dynamic hysteresis loop in varying sweep process. They are the effects of additional velocity caused by varying sweep motion, the effects of flow hysteresis and viscosity. The additional velocity induced by sweep motion affects the transversal flow direction along the wing and the effective angle of attack α at the airfoil profile. The physical properties of flow, the hysteresis and viscosity affect the unsteady aerodynamic characteristics by flow separation and induced vortexes.