On the effect of infrared camera positioning for in-flight boundary layer transition measurements

Abstract

Infrared thermography is an optical measurement technique that is attractive for in-flight measurements of the boundary layer transition location on aircraft wings because it requires minimal instrumentation of the test object. A known drawback of this technique is however that the position of the infrared camera with respect to the analysed surface can influence the measurement accuracy. The positioning of the infrared camera on the test aircraft for in-flight measurements is a complex design trade-off that is influenced by many different factors. In this paper, measurement results from a wind tunnel experiment where adverse infrared camera positions were emulated are presented. Relatively simple corrections for the perspective distortion and the directional emissivity effects were subsequently applied to the image data. The results show that the boundary layer transition location can be extracted accurately and reliably under relatively steep viewing angles, with up until 75 degrees between the principal viewing axis and the surface normal direction. These results are useful for the design of in-flight experiments, where the demonstrated feasible range limit for the orientation between the infrared camera and the test surface can be used as a design constraint.