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A flight test investigation was conducted to evaluate an infrared (IR) imaging technique to visualize off-surface flow phenomena. A single-engine, general-aviation airplane was equipped with an IR imaging system that viewed the region around the left wingtip. Vortical flow at the wingtip was seeded with sulfur hexafluoride, a gas with strong infrared absorbing and emitting characteristics. Different terrain and sky backgrounds were evaluated for their effect on IR images of vortical flow. The best IR images were obtained with a clear sky background. The results of the investigation indicate that IR flow visualization compliments existing smoke generator methods for off-surface flow visualization.

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.

Correlation of in-flight boundary-layer transition experiments with linear boundary-layer stability theory contributes both to the validation of the numerical methods as well as the analysis of the measured transition process. Transition results obtained in a recent flight experiment, in which the extent of laminar flow and the transition process on the wing of a business-jet fitted with an instrumented glove section were determined, are analyzed. The experiment was conducted at freestream Mach numbers from 0.55 to 0.82, chord Reynolds numbers from 10 to 20x106, and leading-edge sweep angles of 17° to 20°. The growth of both Tollmien-Schlichting and crossflow instabilities are predicted using the en method for several flight conditions and the calculated “n-factors” at transition onset are correlated. Comparison of the measured dominant boundary-layer disturbance frequencies and the predicted unstable frequencies shows fair agreement for several of the flight conditions studied.

The visualization of boundary-layer transition from laminar to turbulent flow plays an important role in flight and wind tunnel aerodynamic testing. Visualization aids in the understanding of specific causes of transition. In the past, the most popular boundary-layer visualization methods for flight and wind tunnel applications included the sublimating chemical technique and the oil flow technique. However, each method has certain advantages and limitations which constrain the applications. This paper discusses a new method for visualizing boundary layer flows including transition, separation, and shock locations, by the use of liquid-crystal coatings. For flight applications, liquid crystals provide transition visualization capability throughout almost the entire altitude and speed ranges for subsonic aircraft flight envelopes. The method is also applicable to supersonic flow visualizaton and for general use in high- and low-speed wind tunnel and water-tunnel testing.

In recent years, natural laminar flow (NLF) has been proven to be achievable on modern smooth airframe surfaces over a range of cruise flight conditions representative of most current business and commuter aircraft. Published waviness and boundary-layer transition measurements on several modern metal and composite airframes have demonstrated the fact that achievable surface waviness is readily compatible with laminar flow requirements. Currently, the principle challenge to the manufacture of NLF-compatible surfaces is two-dimensional roughness in the form of steps and gaps at structural joints. This paper presents results of recent NASA investigations on manufacturing tolerances for NLF surfaces, including results of a flight experiment. Based on recent research, recommendations are given for conservative manufacturing tolerances for waviness and shaped steps.

Two major concerns have inhibited the use of natural laminar flow (NLF) for viscous drag reduction on production aircraft. These are the concerns of achieveability of NLF on practical airframe surfaces, and maintainability in operating environments. Previous research in this area left a mixture of positive and negative conclusions regarding these concerns. While early (pre-1950) airframe construction methods could not achieve NLF criteria for waviness, several modern construction methods (composites for example) can achieve the required smoothness. This paper presents flight experiment data on the achieveability and maintainability of NLF on a high-performance, single-propeller, composite airplane, the Bellanca Skyrocket II. The significant contribution of laminar flow to the performance of this airplane was measured. Observations of laminar flow in the propeller slipstream are discussed, as are the effects of insect contamination on the wing.