A Winglet Design Study for the Slotted, Natural-Laminar-Flow Strut-Braced Transport Aircraft

A detailed design study for improvements in aerodynamic efficiency for the slotted, natural-laminar-flow strut-braced transport concept is explored. The strut-braced transport aircraft is a conceptual design that uses a strut to alleviate structural bending loads for a high aspect ratio transport aircraft. This configuration has been coupled with slotted, natural-laminar-flow technology, which allows for extensive runs of laminar flow at transport aircraft Reynolds and Mach numbers. The combination of the two concepts yields a complex aerodynamic case study with several novel design challenges. Particular interest is paid to the termination of the slotted wing-tip, the strut-wing juncture, and the current computational ability to capture runs of laminar-flow at transonic Reynolds and Mach numbers. Both multi-element and single-element winglets are explored, where the geometry was optimized for maximum cruise performance using classical drag build-up methods and refined using the computational fluid dynamics solver OVERFLOW. While the dual-element winglet provides a simple solution (both conceptually and computationally), it is thought that the extra wetted area of the slot and the high lift coefficients achievable are not of particular benefit for operation at the tip of the wing, where both Reynolds number, Mach number, and operational lift coefficient are all lower than that of the main wing. With a dual-element winglet an increase of 6.5% in cruise performance is attainable. A single-element winglet was not found to have any significant benefit aerodynamically when compared to the dual-element, though is significantly less complex structurally. Further exploration around the strut-wing attachment point is also highlighted, where a "cleaning" of the flow around the wing-strut juncture are observed with the addition of both winglet types.