Flight Experiments Studying the Growth of Disturbances in the Laminar Boundary Layer

Current laminar flow flight research conducted by NASA Langley Research Center is focused on understanding details about the dominant instabilities responsible for initiating the transition process from laminar to turbulent flow. This paper presents results from recent flight experiments conducted on a gloved surface of a Lear Model 28/29 airplane to study in detail the growth of disturbances in the laminar boundary layer. The gloved wing section incorporated closely-spaced, flush-mounted, streamwise-located instrumentation for measuring instability frequencies and pressure distributions. These experiments were the first of their kind to measure streamwise growth patterns along the wing chord. The airplane flight envelope included Mach numbers up to 0.81, allowing for compressible speed measurements to be obtained. In addition to exploring chord Reynolds number conditions from 10×10⁶ to 20×10⁶, the introduction of sweep via sideslip maneuvers and the effect of cloud particles on laminar flow were also explored. T-S disturbance frequencies were measured from 5000-12,000 Hz, depending on the flight conditions and crossflow instability frequencies were measured in the range of 550-800 Hz. These data provide information on disturbance growth phenomenon and transition mode which is essential to the development of practical design limits for application of laminar flow technology.