Abstract:
To meet the performance demands of modern UAV operations, hybrid air vehicles that incorporate the speed and efficiency of fixed wing vehicles and the VTOL capabilities of rotorcraft have come to the forefront. This paper presents work being done to expand the performance of one such hybrid air vehicle, the quadrotor biplane. This vehicle takes flight in tail-sitter configuration and uses differential thrust vectors to pitch forward into forward flight configuration. This control method does not require conventional control surfaces, therefor making the system mechanically simple. The vehicle design and its package delivery functionality have been validated experimentally, in both hover and forward flight. Flight data was collected demonstrating the decreased power consumption in forward flight mode. Next, wind tunnel studies were conducted to explore measures to improve vehicle performance. Wind tunnel testing with variable pitch proprotors demonstrated a marked advantage over fixed pitch propellers at high airspeeds. To minimize drag and thus further improve vehicle performance, two different approaches to favorably modify lift distribution on the wings were considered. The method evaluated the effects of rotor spin direction and placement along the span. The second approach explored the implementation of various boxed wing geometries. Experimental results from the boxed wing wind tunnel test demonstrated that the coefficient of lift (CL) could be increased by up to 50% with a NACA 0005 box wing at Reynolds number (Re) of 108,000. The wing-propeller wind tunnel testing showed that having the rotors further outboard improved performance for the higher Reynolds numbers tested (Re of 60,000-100,000). Additionally, a significant increase in the CL was observed when the propeller direction was switched from inwardly rotating to outwardly rotating at Re = 100,000.