Aspects of Flow Transition Detection When Flight Testing Engine Nacelles

Abstract

Some lessons learned as regards flow transition detection in the course of flight-testing carbon fiber composite conventional and laminar flow nacelles are presented. The key objective of the flight trials was to examine the influences of environmental phenomena and of the engine nacelle structure regarding the interpretation of the nacelle infrared images and of the nacelle surface temperature measurements, both with reference to determining the location of the flow transition front on the nacelles. As regards the application of the infrared thermography, the study has yielded the result that the quality of the infrared images is influenced by heat radiation to space and from Earth, as well as by reflection of sunlight from the airframe. For good quality infrared images, only carbon fiber composites should be used as structural material for the nacelle fan cowls. In addition, the mechanical design of the fan cowls should not contain any stiffening frames in the substructure, but the should be designed as a uniform honeycomb skin structure. Concerning the surface temperature measurements, for good-quality results,a uniform honeycomb carbon fiber composite skin structure is just as essential as for the infrared imaging system.