Boundary-Layer Stability Analysis of NLF and LFC Experimental Data at Subsonic and Transonic Speeds 871859

Boundary-layer stability analyses of experimental data obtained in two recent NASA-Langley wind tunnel experiments and a Cessna flight test are presented. These experiments were designed to test the state-of-the-art in natural-laminar-flow (NLF) and laminar-flow-control (LFC) airfoils for subsonic and transonic applications. The NLF(1)-0414F was tested at subsonic speeds in the Langley Low-Turbulence Pressure Tunnel and in flight on a modified Cessna 210 aircraft. The SCLFC(1)-0513F airfoil was tested on a swept wing at both subsonic and transonic speeds in the Langley 8-Foot Transonic Pressure Tunnel. These experiments have demonstrated that an appreciable amount of laminar flow is attainable on general aviation and transport-type lifting surfaces with a consequential reduction in drag. The amplification of small disturbances by linear processes on such surfaces can be due to Tollmien-Schlichting (TS) and/or crossflow (CF) mechanisms. This study consists of the examination of these two instabilities by both the commonly used incompressible (SALLY and MARIA) method and the more involved compressible (CQSAL) method. These experiments provide a wide range of experimental test conditions for transition correlation. Hot-film gages were used to determine the transition locations for the range of test cases. It was found that the incompressible TS transitional n-factors were generally in the range of 9 to 12 In agreement with earlier correlation studies. The TS instability was determined to be the dominant instability mode on the swept LFC model over the entire range of test conditions. The incompressible TS correlated n-factors remained essentially constant as Mach number was increased up to 0.82, but the compressible TS n-factors were found to decrease with Mach number from 9 to 12 at low Mach numbers down to approximately 5 to 7 at M_∞ = 0.82.