Atmospheric turbulence is encountered frequently in flight. It creates oncoming flow disturbances for aircraft passing through turbulent zones. For natural laminar flow airfoils such conditions are potentially detrimental since their goal of maximizing laminar flow may be counteracted by increased disturbance levels. In this study the flow behavior of a natural laminar flow wing section is investigated in gliding flight experiments under calm and light to moderately turbulent conditions.

A comprehensive measurement platform is integrated into a motorized glider to obtain insights into the flow processes acting on a laminar wing glove in cruise flight. Simultaneous measurements of characteristic airfoil quantities enable important correlations with the oncoming flow disturbances. To develop a comprehensive understanding for flight through turbulence, boundary-layer transition is investigated in detail under calm conditions. Differences of the transition behavior between the upper and the lower side of the airfoil are demonstrated. New insight into the weakly nonlinear transition stage in a low disturbance environment is presented.

Due to the random nature of atmospheric turbulence, characteristic results under moderately turbulent conditions are presented as case studies. This enables a complete examination of the time-varying boundary conditions, the inviscid flow effects and the boundary-layer response to the turbulent forcing. It is shown that all these processes interact with each other. Furthermore, it is demonstrated that the unsteadiness of the oncoming flow assumes an important role in the laminar-turbulent transition process of the airfoil boundary layer. On the lower side of the airfoil significant and rapid upstream fluctuations of transition are verified under moderately turbulent conditions. It is shown that these fluctuations are driven by the time-varying pressure gradient and that transition is initiated by Tollmien-Schlichting waves. Indications for a premature transition behavior under such unsteady conditions are presented, which can only partially be explained by unsteady distortions of the boundary layer. The experimental observations are complemented by numerical investigations employing unsteady panel and boundary-layer methods as well as quasi-steady linear stability theory.