AN INVESTIGATION ON THE IMPROVEMENT FOR TANDEM WING PITCHING MOMENT CHARACTERISTICS

Foldable unmanned aerial vehicles employing tandem wings can equip twice as much wing area as the traditional configuration with a similar folded length and magnitude of wing bending moment, which makes it preferred for tube-launched long-endurance UAVs. However, the tandem wing exists a defect of nonlinear pitching-moment caused by wing-wing interaction in some degree, which has been rarely studied. Our investigation concentrates on the flow phenomenon between the front and rear wing and aims to achieve some improvement on pitching moment characteristics. First, the analysis of the aerodynamic characteristic differences between the two airfoils and the three-dimension configuration is conducted, and the different consequences of flow interactions are discussed respectively. Following that, a verification simulation that enlarges the front wing span is conducted to reveal the different influences of downwash and wingtip vortex. Finally, two modified wingtip shapes are studied and verified to meet the wingspan constraints. The conclusion is the wingtip vortex of the front wing and the induced spanwise flow of the rear wing causes the poor pitching moment characteristic. The downwash of the front wing is a secondary effect. Declining the aerodynamic efficiency loss of the rear wing can relieve the pitching moment nonlinearity caused by a forward tendency of the aerodynamic center at a negative angle of attack. Optimizing the front wing wingtip shape improves the linearity of the pitching-moment, and a better rear wing aerodynamic performance is accompanied. This article provides a reference to the wingtip design and aerodynamic optimization of the tandem wing.