Enhancements to the Inverse Design of Low-Speed Natural-Laminar-Flow Airfoils

Abstract

The objective of inverse airfoil design has traditionally been the determination of the airfoil shape that results in desired aerodynamic characteristics. Under this general classification, inverse methods have progressed a great deal over the past few decades. With modern inverse methods, it is possible to prescribe velocity and/or boundary-layer characteristics along with desired geometric constraints in the design of airfoils. In spite of these advances, inverse airfoil design still involves a certain amount of trial and error when a designer attempts to fine tune the drag polar of the airfoil or when attempting to tailor the airfoil for a particular application. The research presented in this thesis makes two specific advances to the state of the art in inverse design. The first part of the research describes the development of an approach by which a desired boundary-layer transition curve can be specified as an input to inverse design. The second part presents an approach for incorporating aircraft performance considerations in the inverse design process. The two advances can help reduce the design cycle time for airfoil and aircraft design by reducing the amount of trial and error in the design process.

The motivation factor for the first part of the research (inverse design via specification of the boundary-layer transition curve) was the strong connection between the transition curve and the airfoil drag polar. In the approach developed, a multidimensional Newton iteration is used to adjust the velocity distribution until the transition lift coefficient at several locations on the airfoil are within a given tolerance of the specifications. It is shown that the shape of the drag bucket as well as the camber and extents of laminar flow on the airfoil can be controlled through the specification of the transition-curve. This method represents an enhancement over previous inverse airfoil design methods since it allows for a single specification that spans multiple operating points.

The second part of the research (incorporation of aircraft performance considerations in inverse airfoil design) was driven by a motivation to incorporate airfoil-aircraft matching considerations in the airfoil design process. Two aircraft performance parameters are considered in this work: level-flight maximum speed and maximum range. Through the use of a multidimensional Newton iteration, the method adjusts the lift coefficients for the corners of the low-drag region of the drag polar to tailor the airfoil for the two flight conditions. The results from the design method are validated using a post-design aircraft performance simulation. This method results in the next level of sophistication in inverse airfoil design technology since system-level performance considerations are used to drive the airfoil design.