Yaw-driven sail effect generates negative body-axis drag in a streamlined ground vehicle exhibiting positive base pressure

Negative body-axis drag, i.e., net forward thrust from the free stream, has been reported only once for a ground vehicle body (Salari and Ortega, PNAS 2021). Complementing this, we demonstrate for the first time, through wind tunnel experiments and numerical simulations on a streamlined vehicle, two distinct mechanisms: at yaw, unusually strong pressure recovery yields positive base pressure, while at finite yaw (⁠ ⁠), a sail effect drives ⁠, marking a yaw-induced transition from positive to negative body-axis drag. Particle Image Velocimetry confirms the wake topology; combined experiments and simulations establish Reynolds number independence; and wind tunnel measurements with different surface modifications on the vehicle indicate bulk boundary-layer insensitivity. Our findings reveal a new aerodynamic pathway in which streamlining produces a previously unobserved positive base-pressure state at zero yaw and a yaw-driven thrust state at finite yaw, with the latter resulting in a drag-to-thrust transition.