Wake Interaction of Tandem Tilt-Wing UAV with No Vertical Stager Wings

A tandem tilt-wing vertical take-off and landing (VTOL) aircraft is one of the most promising configurations offering high endurance in cruise since it does not have additional devices that cause a drag penalty as in a lift-and-cruise type VTOL aircraft. The authors has been developing a tandem tilt-wing VTOL unmanned aerial vehicle (UAV). The UAV is expected to be applied for disaster monitoring missions and as a member of manned unmanned teaming (MUMT) in a search and rescue. These missions require a long endurance and precise navigation and control capabilities. Therefore, the aircraft configuration is designed to have front wing and rear wings installed with no vertical stager. This design provides a simple structure to manufacture and small frontal projected area of the aircraft to reduce aerodynamic drag. In contrast, a complicated aerodynamic interference between a front wing with proprotor to a rear wing with proprotors exists and this should be managed to realize a high performance navigation and control capabilities. A high-performance flight controller design requires an elaborate mathematical flight model including aerodynamic interference features of the tandem tilt-wing aircraft. In this study, we investigated the multifactorial interference on two configurations with varying deflections of the front flaperon using wind-tunnel test with particle image velocimetry (PIV) measurement. Moreover, simulations based on a three-dimensional panel method are conducted as well. One configuration included a 0-degree tilted front and rear wings and another had 15-degree tilted wings. The aerodynamic forces and moments measured in the wind-tunnel test and the simulation showed a good agreement, and the ratio of the pitching moment contributed by the rear wing overwhelmed that contributed by the front wing for the tilted-wing configuration despite deflections of a front flaperon. By introducing a simple analysis based on horseshoe vortex model, the dynamic pressure and lift slope on the rear wing were identified as a key to this interference. An accelerated air flow owing to higher thrust encountered the rear wing in the tilted configuration and non-degraded lift slope on the rear wing owing to downwash from the front wing maintained a lift generation. A data table including information used in the simple analysis could enhance the quality of the mathematical model.