Manufacturing Tolerances for Natural Laminar Flow Airframe Surfaces

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.