# Study of Airfoil Deflections for Unsteady Aerodynamics Optimization in Pitching Airfoils +
## 3 Results and Discussion
### Demonstration of Tandem System  
Following the proof of concept for the design functionality with the pitching and tandem configurations and assessing the basic performance of the deflected airfoils, the main relevant results in the tandem design are presented in this section. This campaign investigated both in-phase pitching, where both the primary (downstream) and secondary (upstream) 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 [21]. It is reiterated that the current configuration of the linear tandem prescribes a distance between the two pitching centers of the airfoils to 3.25 chords. This approximately corresponds to a distance of two chords between the TE of the upstream airfoil and the LE of the downstream airfoil when they are at zero angle of attack. These parameters would represent a medium interaction condition occurring at a span that corresponds to this airfoil separation, wake development, and interactions with the downstream wing. The tandem configuration 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. The study sets the basis for studying and incorporating an active morphing design implementing onboard controls that would permit changing and  
Figure 12: Streamline velocity fields streamlined for 16\({}^{\circ}\)\(\alpha\) pitching down (\(\downarrow\)) for deflected airfoil variations showing dynamic stall control: (**a**) baseline, (**b**) LE deflected. Center of pressure and pitching is at x/c = y/c = 0.2.  
adjusting the deflections on demand. This study establishes the methodology to study such configurations, and other separation distances (or spans) could be studied by changing that distance. From the many relative phases incurred during pithing of both airfoils in the tandem configuration, here we focus on the two most extreme, in-phase and out-of-phase pitching. Other interactions from other phase combinations are expected to fall in between these two cases, although the flow patterns can be unpredictable and chaotic; their study is out of the scope of this current investigation. Here, few pertinent performance phases of the two cases are presented using the phase-averaged PIV described above. As stated earlier, an analysis of instantaneous patterns and their variability within and between cycles could be performed using reduced-order models to establish the phase-coherence from the phase-locked data [34] or more readily with high-speed PIV systems that can track time-resolved motion. The discussion here pertains to the two relative phase tandem test cases (in and out of phase) performed from the four statically deformed airfoils to highlight the phase flow distinctions and highlight the benefits from having LE deflection in certain conditions. The relevant cases for in-phase and out-of-phase pitching are shown in Figures 13 and 14 and Figures 15 and 16, respectively, and for the downstream airfoil at the maximum phase (22\({}^{\circ}\)) and at the 16\({}^{\circ}\) phase pitching down. Other phases have been reported by the authors in [PERSON] et al. [21]. Since most effects are expected to be in the downstream wing, the PIV region of interest was only from the TE of the front wing to the TE of the back wing. For illustrative purposes of the phases between the two wings, the single airfoil (baseline) data for the upstream wing are used in the plots. It is shown that the wakes from the single airfoil wake and the upstream wake in the tandem are indeed a good match. We note here that the front wing being the baseline (a limitation of the current set up) still defines the main flow effects on the deflected downstream wing for the current purposes.