| Skin friction drag and induced drag together account for more than 80% of the total drag of current subsonic aircraft. A new probabilistic Natural Laminar Flow (NLF) wing design methodology has been established to minimize these two important sources of drag. Using this method, a synthesis and sizing code is used to estimate the benefits of NLF technology at the system level, and to determine optimal wing planform geometry and wing area for a given set of mission requirements. The chordwise pressure distribution of the optimized NLF wing planform from the sizing code is then parameterized carefully on the basis of physical insights. An Inverse Aerodynamic Design Technique is then used to find the corresponding airfoil geometry for a given pressure distribution. A metamodel building method called Response Surface Methodology (RSM) is then used to minimize the total drag of the wing at the design cruise condition given by the sizing code, with respect to the design parameters chosen. In this procedure, the accurate prediction of the onset of laminar-to-turbulent transition is crucial to estimate the skin friction drag. A reasonable approximation is achieved by initially computing the mean flow by means of a Navier-Stokes code, CFL3D, followed by a compressible linen stability analysis code, COSAL3D, to estimate transition location and finally, CFL3D is executed again. At this time, a turbulence model is applied after the predicted transition point for a more accurate skin friction drag prediction. Finally, to account for the uncertainty of the prediction of the transition location, a Monte Carlo Simulation is performed through the use of Response Surface Equations (RSEs). |
