Global Journal of Researches in Engineering, A: Mechanical & Mechanics, Volume 22 Issue 1

unmanned aircraft system using computational fluid dynamics. Khalid and Golson (18) varied the winglet height to wing span ratio parameter from 5% to 25%. The Study finds that a 15% winglet to wingspan ratio gave the highest lift to drag ratio while a taper ratio of 0.4 provided the highest lift to drag ratio. Khalid and Kumar (19), however find that varying the airfoil, winglet height and aspect ratio resulted in a significant increase in lift to drag ratio relative to the baseline design. Specifically, the model with a 30% winglet to wing span ratio generated the highest increase in aerodynamic efficiency, equivalent to 15% increase in lift to drag ratio, when compared to a cantilever model. Barcala et al. (20) studied the aerodynamics of an unmanned aircraft system of Box-Wing configuration at low Reynolds numbers through a wind tunnel experiment. By varying the positions of the wings along the fuselage and the sweepback angles of the wings, significant differences in aerodynamic efficiency were found. This result indicates that the relative positions of the wings affect the aerodynamic efficiency of the Box- Wing configuration (21). Another observation from this Study is the late separation of flow on the fore-wing at high angles of attack as the angle of attack is increased (21). Nonetheless, the flow separates at a higher angle of attack on the rear-wing relative to the fore wing as highlighted in Frediani’s (5) work. Gagnon and Zingg (22) undertook a study to minimize the drag of a Box-Wing aircraft configuration using high-fidelity aerodynamic optimization. The study finds that Box-Wing aircraft with a tip fin height-to-wing span ratio of about 0.2 creates up to 43% less induced drag than its conventional counterpart. This aerodynamic benefit was derived from the inherent characteristics ‘of Box Wing Aircraft to redistribute its optimal lift distribution with almost no performance degradation’ (22) . Balaji et al (23) explored different aerodynamic issues in the design of the Box-Wing aircraft using a wind tunnel. Experimental results revealed a decrease in drag due to ‘the overall reduction in the downwash of the complete system’ (23) . In addition, the study established that adding an endplate to a lifting system further reduces the downwash thereby increasing the effective span and thus the aerodynamic efficiency of the Box-Wing aircraft (23). Bagwill and Selberg (24) investigated twist and cant angles of the tip fins of Box-Wing aircraft. The results from the study conformed to Wolkovitch’s (1) findings. These studies suggest that careful selection of twist and cant angles of a Box Wing aircraft, at higher aspect ratio, provides a greater increase in the lift to drag ratio compared to a conventional cantilever wing aircraft (24) This discovery was corroborated by Nangia et al. (25) in a study to investigate the effect of high aspect ratio on Joined-Wing aircraft. Nangia et al. (25) find that Joined/Box-Wing aircraft generate lower induced drag as well as higher wing stiffness compared to conventional cantilever aircraft. In terms of stalling characteristics, Bell (26) study revealed that the rear wing of a Joined-Wing aircraft induces an upwash on the forward wing which then initiates a downwash on the rear wing. According to Bell (26), the higher angle of attack on the fore-wing of a Joined/Box-Wing aircraft ensures that it stalls before the rear wing. This prevents deep stall thereby improving stalling characteristics of the Box-Wing Aircraft. Accordingly, the Joined/Box-Wing configuration exhibits safer stall characteristics than a conventional aircraft. V. E ffect of O ptimization on A erodynamic C haracteristics of J oined/ B ox W ing A ircraft Gallman et al (27) performed a synthesis and optimization for a medium-range Joined-Wing transport aircraft. They developed a program to model joined- wing transport aircraft and measured their overall performance in terms of direct operating cost. The program predicted the aerodynamic interaction between the lifting surfaces and the stresses in the statically indeterminate structure. Aerodynamic forces were determined using a vortex lattice model of the complete aircraft in a LinAir program. Viscosity and compressibility were then added to compute compressibility drag while inextensible theory was used to simulate fully stressed lifting surface structures. The Study revealed that Joined/Box-wing aircraft is deficient in field performance owing to a low maximum lift capability. Gallman et al (27) showed that Joined Wing aircraft is cheaper to operate than an equivalent conventional transport. Additionally, they opined that an in-depth study of wing sweep, flap span, and elevator span provides further gains in the aerodynamic performance of a Joined-Wing performance aircraft. Gallman et al. (27) posit that any design changes that reduce the tail sweep angle would likely improve the performance of a Joined Wing Aircraft. They identified take-off field length and horizontal-tail buckling as the critical design constraints for Joined/Box-Wing aircraft. Gallman et al. (27) attributes the significant increase in direct operating cost of Joined/Box-Wing aircraft to the poor field performance characteristics of the configuration. The Box Wing aircraft exhibits poor field performance characteristics due to its limited capacity to generate maximum lift in take-off mode. VI. C onclusion The investigation of aerodynamic design issues of the Joined/Box-Wing aircraft highlights the aerodynamic efficiency of the concept and the complex interactions of several disciplines within the configuration. The Joined/Box-Wing aircraft shows improved aerodynamic efficiency compared to a Global Journal of Researches in Engineering (A ) Volume XxXII Issue I Version I 33 Year 2022 © 2022 Global Journals An Analysis of Aerodynamic Design Issues of Box Wing Aircraft