On the measurement, characterization and control of static-wing tip vortex flow and its extension to flapping wing

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Lift increment and drag reduction of aircraft have been persistently pursued by aerodynamicists and aircraft designers. As such, the reduction of wingtip vortices-generated lift-induced drag, including flight hazards, is therefore of great importance. The sharklet winglets and split winglets installed on A320NEO and B737MAX reinforce the significance of lift-induced drag reduction and the ensuing fuel-burn savings. Intensive study of the wingtip vortex and its control has been carried out at McGill University by the applicant (via 2009-2013 NSERC DG). In addition to the measurements and characterization of the tip vortex at low Reynolds numbers, the lift-induced drag is also computed, based on the cross-flow velocity measurements, which not only allows the evaluation of its contribution to the total drag but also serves as an indicative of the effectiveness of tip vortex flow control (alternative to the changes in the strength and size of the tip vortex). Meanwhile, different tip vortex control methods were also attempted. Among them, the tip-mounted non-slender half-delta wing (HDW), with a root chord smaller than 50% of the main wing, was found to have the greatest potential for effective vortex attenuation and induced-drag reduction. Surprising enough, the proposed HDW tip-vortex control is also capable of producing concurrent lift increment and stall delay. Further investigations are, however, needed to materialize its practical application. **The objectives of this proposed research, therefore, have two folds. The first part is to continue the study of the promising on-going tip-mounted HDW tip vortex control scheme. Extensive wind-tunnel measurements of the size, location and deflection (relative to the main wing) of the HDW, and the Reynolds number will be acquired. These measurements will enable us to realize a new and practical tip vortex control technique. The knowledge and findings learned from the static-wing tip vortex is then extended to the second part of this proposal; i.e., the study and control of the tip vortex generated by flapping wings. The flapping wing study has been an increased interest in micro-air vehicles (MAVs) due to their extraordinary maneuverability. However, in comparison with fixed-wing MAVs, the flapping-wing MAVs are more difficult due to the extraordinary complexity of the flapping motions (which produce both lift and thrust) and the ensuing flow phenomena. Furthermore, to add to the complexity, tip effects must be carefully considered. Fortunately, the leading-edge vortex (LEV), similar to that developed on oscillating or pitching wings, has been identified recently to dominate the unsteady lift generation. The LEV formation and growth are, however, still not fully understood. In this study, the behavior of the LEV (under different flapping frequencies, amplitudes and phases) is studied first by using the unique multi-element hot-film sensor (MHFS) arrays, designed and fabricated at McGill. These on-surface MHFS and surface pressure measurements will be used to supplement the off-surface particle-image-velocimetry measurements of the flapping-wing tip vortex generation and its control (e.g., via the HDW tip-vortex control concept). It is our belief that these previously unavailable joint on- and off-surface flowfield measurements will not only advance our understanding of the complex flapping-wing MAV aerodynamics and propulsive efficiency, but also enable effective flapping-wing tip vortex control. The proposed experiments will also provide a good baseline for computation-fluid-dynamic validation.