Maneuver load control for flexible delta wing using multidisciplinary design optimization

This paper addresses the Maneuver Load Control (MLC) for a generic fighter aircraft using optimization techniques to augment its performance. Multidisciplinary Design Optimization (MDO), an important application of which is in the field of aircraft design, is applied to the Maneuver Load Alleviation of a generic fighter aircraft, in order to reduce the wing loading and also to improve the performance. The aircraft considered is a typical generic multi role combat aircraft, with a delta wing and no horizontal tail. It is assumed that there are no individual control surfaces for pitch (elevator) and roll (aileron) control. Instead, both these controls are achieved by having two independent control surfaces viz. Inboard and Outboard ELEVONS (ELEVator+ailerON) on each wing. An optimized combination of differential elevon deflections are generated, which, in turn, would redistribute the loading on the wing so that that the wing root bending moment is reduced. The optimization problem has been formulated for (i) minimizing the wing root bending moment (ii) improving the flight envelope, by considering the angle of attack and the differential elevon deflections as design variables, subject to equality and inequality constraints. These constraints would arise from various interdisciplinary areas such as Aerodynamics, Structures and Controls. In the optimization cycle, for an initial guess, a global search is done using Genetic Algorithm (GA) and with this as initial value, the actual optimization is performed using Gradient Based Technique coupled with Method of Feasible Directions (MFD) for constraint handling, to generate the final optimized solution. The static aero elastic analysis is done using NASTRAN/PATRAN, while aerodynamic analysis is done using a Cartesian grid based EULER solver. The load computations are carried out for a few identified maneuvers of the aircraft, considering the inertial parameters such as vertical acceleration, roll rate and roll acceleration. The maximum bending and torque moment values on the wing are computed for various maneuver load cases and the critical maneuver loads are identified as corner points of the flight envelope boundaries. A few of these critical maneuver load cases have been identified for the present study and the MLC benefits achieved - structural as well as performance are presented. Further aerodynamic analysis is done for the optimized geometry for the cases studied, to compute the centres of pressure of the optimized solutions to confirm the reduction in the bending moments. Copyright © 2008 by Dr(MS) Indira Narayanaswamy.