Aerodynamics of an Ogival Double Delta Wing: Control Authority with Split Elevons

This paper presents the steady-state elevon (pitch) behavior of a sub-25 kg drone with an ogival double delta wing. The case study aircraft, "Kingfisher", is designed to be the first of its kind to exceed speeds of Mach 0.75. The delta wing planform minimizes wetted area for high-speed cruise whilst harnessing leading-edge vortices for low-speed, high-alpha maneuvers. Split elevons orient the aircraft in pitch and roll, and their sizing is crucial to its controllability. However, current analytical aerodynamic models fail to accurately capture the interactions between elevons and adjacent vortices. Moreover, the split elevon configuration employed here represents a gap in present experimental elevon studies. The study focuses on the low-speed control challenge during approach and landing, where the elevon-vortex interaction is critical. A 66% scale wind tunnel model was developed and tested in the University of Bath’s closed-loop wind tunnel. The model’s design combined additive manufacturing with a three-axis load cell to measure the lift, drag, and pitching moment at varying angles of attack and elevon deflections. The aircraft’s elevons exhibit linear changes in lift and pitching moment for deflections up to ±20°, beyond which stall occurs. The elevons are more resilient to stall at large angles of attack due to enhancement from the vortices. At these angles, the vortex cores are above the outboard elevons, and the inboard elevons are twice as effective as the outboard elevons despite being 30% smaller. Further analysis using computational fluid dynamics reveals that the outboard elevons experience spanwise flow perpendicular to the freestream, consequently lowering their effectiveness. The results informed a flight performance model that computes the elevon deflection to achieve equilibrium (trim) under varying flight conditions. This identified control reversal at angles of attack below-15° and above 20° because of a change in the pitching moment due to vortex formation on the aircraft’s frontal strake. An outboard elevon failure is the most damaging failure mode, increasing the minimum control speed. Nevertheless, redundancy is improved using split elevons, revealing fail-safe benefits for fixed-wing drone designs.