Structural design and optimization of a tiltrotor aircraft to enhance whirl flutter stability

In order to augment the aeroelastic stability of a tiltrotor aircraft, structural optimization framework has been developed using a two-level optimization approach. Each level of optimization has a different purpose, and therefore, different optimizers are used. An increase of the flutter speed was selected as an object for the upper-level optimization by changing the structural properties of the wing such as its flapwise, edgewise, and torsional stiffness. As an optimizer, the response surface method (RSM), one of the general statistical methods, was utilized. The XV-15 tiltrotor aircraft was the target aircraft used for the experiments. For the aeroelastic analysis, an existing in-house analysis was used. The object of the lower-level optimization was to replace the structural properties used in upper-level optimization with composite materials by adding design parameters, such as layup angles, layer thickness, spar positions, etc. In order to avoid manufacturing difficulties, discrete ply orientation angles and an integral number of plies was considered as constraints. A genetic algorithm (GA) was used as an optimizer. In order to analyze composite wing cross-sections, UM/VABS (University of Michigan Variational Asymptotic Beam Section analysis) was used. The results obtained from the upper-level optimization show approximately a 10% and 16% increase in the flutter speed when compared to those in the baseline configuration using unsteady aerodynamics and quasi-steady aerodynamics models, respectively. At the lower-level optimization, two different design cases were obtained by changing the composite materials. In those cases, detailed results about the discrete orientation angles, integral number of plies, and the shear web positions were obtained.