Aircraft Design Optimization with Multidisciplinary Performance Criteria

By minimizing a cost function consisting of both conventional performance criteria and a measure of aircraft handling qualities, a design with maximum performance for a specified level of handling can be achieved. Handling qualities are measured using a quadratic cost function similar to that used in the design of optimal feedback control systems. This function is proportional to the difference between the unforced response of the aircraft and a "model" case with dynamics that are considered acceptable. The variables to be optimized may include both aircraft design parameters (e.g. span, tail area, skin thickness) and control system feedback gains. The design variables are determined by an unconstrained numerical optimization procedure, using penalty functions to enforce both explicit and implicit constraints. The method is most useful in the simultaneous design of airframe and flight control systems to achieve improved handling, and for cases in which the handling and performance are highly coupled by the design parameters. In certain cases results obtained by this simultaneous design procedure are substantially better than those obtained by I the usual sequential design methods. The presence of multidisciplinary performance criteria and constraints complicates the process of aircraft design and optimization. This paper addresses some of the problems that arise as performance measures from very different fields are compared and tradeoffs are made. The work focuses on the interaction between structural weight, aerodynamic performance, and handling qualities. A method for dealing with such design problems is described and several example cases are considered I ~ Three example design problems using this methodology are discussed: tail sizing for minimum trimmed drag with longitudinal handling qualities constraints; wing weight minimization with aeroelastic constraints; and oblique-wing aerodynamic design for dynamic mode decoupling.