Tollmien-Schlschfing Instabilities in Laminar Flow In-Flight Detection of 871016

The ability of modern airplane surfaces to achieve laminar flow over a wide range of subsonic and transonic cruise flight conditions has been well-documented in recent years. Current laminar flow flight research conducted by NASA explores the limits of practical applications of laminar flow drag reduction technology. Past laminar flow flight research focused on measurements of transition location, without exploring the dominant instability(ies) responsible for initiating the transition process. Today, it is important to understand the specific causes(s) of laminar to turbulent boundary layer transition. This paper presents results of research on advanced devices for measuring the phenomenon of viscous Tollmien-Schlichting (T-S) instability in the flight environment. In previous flight tests, T-S instability could only be inferred from theoretical calculations based on measured pressure distributions. In addition, hot-film sensors used in those tests only provided indications of the presence of laminar, transitional, and turbulent boundary layers. In a recent NASA flight experiment, surface-mounted hot-film sensors were used to detect T-S wave behavior. The results of that experiment are presented, including the design and operation of these sensors and flight data to provide information on disturbance growth and transition mode.