Impact of Control Allocation Methods on the Design of Control Surface Layouts for Box-Wing Aircraft under Flying Qualities Constraints

This paper compares optimum control surface layouts designed and sized to obtain the same Flying Qualities (FQs) performance with different Control Allocation (CA) methods, and proposes novel layouts for staggered box-wing aircraft aimed at transonic commercial flight. Box-wings allow the installation of redundant control surfaces for which no explicit role can be defined a priori, but present challenges related to aerodynamic interaction and interference effects. To evaluate the impact of different CA methods on top-level layout parameters, the cumulative control surface span and the properties of the Attainable Moment Set (AMS) corresponding to each control surface layout are used. A physics-based multi-disciplinary optimization framework is developed to size the control surface layout. FQs are evaluated through non-linear flight dynamics simulation, using a variable-architecture flight control system that allows their assessment as a function of different CA methods. The most traditional Mechanical Gearing and Ganging (MGG) approach, the Constrained Pseudo-Inverse (CPI) method and the Direct Control Allocation (DCA) method are compared. Results show that different optimum layouts exist with comparable cumulative span, for a given CA method and same FQs requirements. The traditional MGG approach requires the largest cumulative control surface span, but retains the best ability to generate coupled roll-pitch moments. DCA requires the smallest cumulative control surface span, with the largest AMS volume. By using this method, a novel layout featuring a mid-wing rear elevon has been discovered, which reduces the total required control surface span by about 13%, results in a 3.7% increase of span available for flaps on the front wing, and avoids detrimental aerodynamic interaction effects near the wing-tail intersection region.