Characterizing Sailplane Yaw String Deviation and Inaccuracy

This study investigates the inaccuracy of the rear yaw string on sailplanes, a critical flight instrument for assessing sideslip during low-speed maneuvers. While the front yaw string reliably measures the relative wind vector, the rear yaw string often provides erroneous readings, posing risks for rear-seat instructors unable to observe the front yaw string. This research characterizes flight conditions where deviations occur, identifies potential causes, and explores solutions. Using wind tunnel testing (WTT) and computational fluid dynamics (CFD), the study quantified the span-wise pressure gradient along the canopy under varying angles of attack (α) and sideslip angles (β). Initial experiments revealed challenges with Reynolds number (Re) matching due to tunnel constraints, necessitating compromises in model design. CFD simulations validated the use of a scaled wind tunnel model despite limitations. Results demonstrated that the rear yaw string deviates significantly at high angles of attack and sideslip, with CFD confirming the front yaw string's accuracy across conditions. It is hypothesized that vortex shedding caused by the larger rear fuselage diameter disrupts the local flow field, though causality remains unproven. The study concludes that pilots must prioritize the front yaw string near stall speed and recommends avoiding low-speed flight at low altitudes when its visibility is compromised. Future work will enhance Re matching, investigate vortex shedding via water tunnel experiments, and refine theoretical flow models. The ultimate goal is to identify practical solutions, such as optimizing yaw string placement, to ensure reliable sideslip measurement for safe sailplane operation. Approved for public release: distribution unlimited. PA#: USAFA-DF-2025-29