Transition Control on High-Lift Low Reynolds Numbers UAV Wings

摘要

Formation, location and size of laminar separation bubbles is arndominant aerodynamic phenomena at domain of low Reynolds numbers.rnThis is especially relevant for high-lift, highly cambered, thick UAV wingsrnwith their strong adverse pressure gradients and resulting tendency forrnformation of large laminar separation bubbles. The burst of laminarrnbubble at certain flight conditions produces an abrupt stall of the wing andrndevelopment of strong hysteresis loop. This is unacceptable for the safernflight of air vehicles, especially for operation of small and medium sizernTactical UAV flying at reduced airspeeds in windy air. Until this problemrnis solved, the race for high maximum lift at low Reynolds numbers isrnmeaningless. This is important for the recently developed high-lift, mildstallrnwings (MS-wings) with plateau of lift at extended range of post-stallrnangles of attack and remarkable capabilities to ensure safe flight up to highrnpost-stall angles of attack. Without prevention/delay of the burst ofrnlaminar bubble and elimination of hysteresis, the feature of mild-stallrncannot be realized at the flight of small UAV. This was demonstrated inrnwind tunnel testing by evaluation of abrupt stall pattern of single-element,rnhigh-lift, mild-stall MS/DTE airfoil at domain of low Reynolds numbers.rnRecovery of lift characteristics for this airfoil was achieved byrnimplementation of multi-strip transition control methodology. Thisrnprevented the burst of laminar bubble at plateau of lift and ensured highlift,rnmild-stall up to high post-stall angles of attack without development ofrnhysteresis phenomena. For operational UAV, this delay of abrupt stall tornhigh post-stall angles of attack is equivalent to elimination of hysteresis,rnallowing safe flight at plateau of lift at post-stall. The obtained results werernin contrast to previous experience with transition control technique thatrnconcentrated only on recovery of maximum lift at low Reynolds numbers.rnThe developed methodology of high-lift flight at post-stall angles of attackrnat low Reynolds numbers is based on the combination of the concept ofrnmild-stall airfoils and transition control technique. The achieved progressrnon this issue allows completely different approach to design of small UAVrn(W = 5-20kg) with reversal of design priorities and formulation of new andrnunconventional objectives in development of air vehicles. Elimination ofrnstandard speed safety margin, extension of usable lift up to the maximumrnlift, safe flight in windy air, post-stall flight capabilities are among thernexpected benefits of new design methodology. Implementation of developedrnconcept in design of small UAV has a potential to produce a new class ofrnair vehicles with enhanced performance and extended flight capabilities.