# Study of Airfoil Deflections for Unsteady Aerodynamics Optimization in Pitching Airfoils +
###### Abstract
[21,22,23] added state-of-the-art optical laser diagnostics (particle image velocimetry, 'PIV' [24]) to assess the performance by characterizing the velocity fields around the wing designs. In these studies, in which the designs included morphing wings [21], trailing edge parabolic flaps [22], and high-frequency trailing edge actuation [23], particle image velocimetry (PIV) proved to be a much superior diagnostic tool for flow characterization than the previous research had been able to provide. In this investigation, initial assessment steps are taken towards the design of an optimal morphing airfoil by focusing on the study of LE/TE static deflections to provide a set of optimal deflections as a function of the angle of attack during airfoil pitching. Details regarding the airfoil designs, the pitching methodology, and experimental test setup are discussed. The various aspects of the design include details of the manufacturing for a light and robust airfoil, the smooth transition of the wing surfaces with deflection areas, the use of numerical simulations as preliminary input for determining deflection location and shaping conditions, and relevant wind tunnel testing using PIV to assess the deflection aerodynamics effectiveness. The wings were manufactured with a mostly hollow interior design to allow light weight while preserving structural integrity. The experimental campaigns performed with PIV were used to characterize the flow unsteady aerodynamics and dynamic stall on the set of models fabricated with static deflections of the LE and TE in various configurations. The second stage utilized these models in a novel approach for tandem configuration performance studies (in linear arrangement) where a non-deflected baseline model was used upstream of the primary airfoil being evaluated. The tandem design gives flexibility in synchronizing the phases of the two wings. This campaign investigated both in-phase pitching, where both the primary and secondary airfoils reached the maximum and minimum angles of attack at the same time,and out-of-phase pitching, where the primary airfoil reached the maximum angle of attack as the secondary airfoil reached its minimum angle of attack. This design puts forward a method of study of complex rotor/rotor interactions occurring in many engineering applications that not only include rotorcraft, but also turbine engines. In rotorcraft, the rotor encounters ever-changing flow fields that, due to the rotation nature of a rotorcraft's blades, generate a change in velocity from the hub to the tip. Additionally, blade-to-blade interactions occur as one rotor follows in the wake of the one before it. Lastly, whether the rotorcraft is in forward flight or hover flight has direct implications for the flow experienced by the rotor blade, as the direction of flow changes with the change in azimuth about the central hub. The scope of this study is to demonstrate the capabilities of the deflection concepts in two-dimensional wings for a given Re number (250 K) and reduced frequency (0.04), characteristic of a range of rotorcraft conditions based on nominal operation at a cruise with an average load that could be replicated in a low-speed wind tunnel. It is noted that in rotorcraft, the wing-to-wing distance varies significantly from the hub to the tip, and compressibility effects are noteworthy near the tip. This investigation aims to study conditions far from the tip, and testing is performed in a low-speed wind tunnel with spanwise uniform unswept wings. The tandem configuration is linear with a prescribed distance between the two pitching centers of three and a quarter chord that, while limited by the dimensions of the test section, still allows significant wake development of the upstream wing prior to encountering the downstream wing. These conditions are representative of a medium regime of rotorcraft operations and offer a benchmark demonstration of studying deflected pitching wings and their wing/wing interactions.