An innovative variable stiffness spar (VSS) approach is proposed for the enhancement of aircraft roll performance. F/A-18 pre-roll-modification aircraft (F/A-18 PRM) is selected as the baseline aircraft for its low torsional wing stiffness and available baseline data. The F/A-18 PRM, when integrated with VSS (F-18/VSS), requires only the outbound trailing-edge aileron for roll maneuver. The present objective is to explore when and how to best apply the F-18/VSS concept and to access its payoff in terms of weight impact, maximum performance, and ease of structural integration. The success of the F-18/VSS concept relies upon optimal design of the VSS stiffness satisfying strength, flutter and gust response constraints.
The feasibility study of the F-18/VSS concept will be performed using a proven engineering tool called ASTROS*. ASTROS* is the enhanced version of the MDO software ASTROS with the integrated ZAERO (unified unsteady/steady aerodynamics covering subsonic to hypersonic Mach numbers) and ASE (aeroservoelastic optimization/analysis) modules. In addition, the impact of dynamic instability on roll performance will be investigated where a trade-off study between flutter suppression by passive and active means will be performed. 
