Natural laminar flow for quiet and efficient supersonic aircraft

Abstract

This paper summarizes work undertaken in the first phase of a DARPA sponsored program to evaluate the feasibility of natural laminar flow for a low-boom supersonic aircraft. Initial studies suggested that wings with low leading edge sweep are promising candidates for extensive natural laminar flow at supersonic speeds, but simultaneously satisfying the requirements for low sonic boom is particularly challenging with such a configuration. The paper describes methods by which such a configuration was developed and the concepts underlying this design strategy. Introduction and Background Although the desirability of laminar flow for improved aircraft performance has been recognized for decades, the difficulty in achieving extensive laminar flow at subsonic speeds has limited its use for most aircraft. Supersonic aircraft might seem even less well-suited to natural laminar flow because high sweep and high Reynolds numbers make transition more likely. Supersonic design, therefore, generally starts with a configuration consistent with low inviscid drag and accepts the turbulent skin friction associated with this geometry. When this approach is reversed, and one considers wings with low sweep, designed to achieve pressure distributions consistent with extensive natural laminar flow, but at the expense of some inviscid drag, some interesting design options are created. This is the approach suggested by Tracy [1,2]. Related work on supersonic natural laminar flow (SLF) has been described by Gerhardt [3], and at NAL [4]. Results suggest that significant range improvements are possible by exploiting the favorable pressure gradients associated with supersonic leading edges to achieve extensive natural laminar flow. Low wing sweep produces chordwise pressure gradients that suppress Tollmein-Schlichting instabilities while reducing the spanwise gradients that lead to cross-flow instabilities. Although some inviscid drag compromise or structural weight trade-offs are then required, several application studies have suggested that such a concept has significant performance advantages for configurations such as supersonic business aircraft or unmanned reconnaissance vehicles [5]. Figure 1 shows how such a wing configuration might be integrated with a supersonic business aircraft. Figure 1. Artist concept of supersonic business aircraft with natural laminar flow wing. With support from DARPA, NASA, and internal funding by Directed Technologies, Inc., a flight demonstration of this concept was recently completed [6]. Figure 2 shows the small scale test-blade mounted under the NASA F-15 flight testbed. The figure also shows the infrared image taken from an on-board camera, along with the pre-test estimate of transition based on a simplified boundary layer analysis method [7]. At the flight conditions shown, full chord laminar flow was achieved over much of the wing. * Professor, Dept. of Aeronautics and Astronautics, Stanford University, [e-mail], Fellow AIAA. 1 President, Reno Aeronautical Corporation, Member AIAA 8 Reno Aeronautical Corporation f Doctoral Student, Dept. of Aeronautics and Astronautics, Stanford University, Member AIAA. Copyright ©2002 by Ilan Kroo. Published by the American Institute of Aeronautics and Astronautics, Inc., with Permission.