Conceptual design and optimization methodology for box wing aircraft

A conceptual design optimization methodology was developed for a medium range box wing aircraft. A baseline conventional cantilever wing aircraft designed for the same mission and payload was also optimized alongside a baseline box wing aircraft. An empirical formula for the mass estimation of the fore and aft wings of the box wing aircraft was derived by relating conventional cantilever wings to box wing aircraft wings. The results indicate that the fore and aft wings would use the same correction coefficient and that the aft wing would be lighter than the fore wing on the medium range box wing aircraft because of reduced sweep. As part of the methodology, a computational study was performed to analyze different wing/tip fin fixities using a statically loaded idealized box wing configuration. The analyses determined the best joint fixity by comparing the stress distributions in finite element torsion box models in addition to aerodynamic requirements. The analyses indicates that the rigid joint is the most suitable. Studies were also performed to investigate the structural implications of changing only the tip fin inclinations on the box wing aircraft. Tip fin inclination refers to the angle the tip fin makes to the vertical body axis of the aircraft. No significant variations in wing structural design drivers as a function of tip fin inclination were observed. Stochastic and deterministic optimization routines were performed on the baseline box wing aircraft using the methodology developed where the variables were wing area, average thickness to chord ratio and sweep angle. The conventional aircraft design showed similar performance and characteristics to the equivalent in-service aircraft thereby providing some validation to the methodology and the results for the box wing aircraft. Longitudinal stability investigations showed that the extra fuel capacity of the box wing in the fins could be used to reduce trim drag. The short period oscillation of the conventional cantilever wing aircraft was found to be satisfactory but the box wing aircraft was found to be unacceptable hence requiring stability augmentation systems. The field and flight performance of the box wing showed to be better than the conventional cantilever wing aircraft. Overall, the economic advantages of the box wing aircraft over the conventional cantilever wing aircraft improve with increase in fuel price making the box wing a worthy replacement for the conventional cantilever wing aircraft.