Abstract:In this article, the growth of aerodynamic efficiency and the growth of the wing structural stress is studied for DLR-F4 typical transport aircraft wing-body, after installing classical Whitcomb winglets of different configurations and a delta wingtip fence. A new-concept curved-span winglet was mathematically developed and approved through Computational Fluid Dynamics (CFD) and static structural experiments, revealing the interaction of sub-and transonic air flow dynamics with the wingtip device geometry. The design space of the winglet geometry was explored briefly, and an evaluation of the lift-to-drag ratio increment depending on various winglet input parameters was performed. In particular, the winglet cant angle effect on lift and drag was thoroughly analyzed at various flow regimes and angles of attack, revealing an ambiguity and a conflicting character of results between highly canted winglets and nearly vertical ones. As a result of cant angle impact analysis, a curved winglet concept is suggested and mathematically parametrized, that could provide an innovative solution, alternative to a morphing winglet, but much simpler with a fixed structure. In conclusion, a multidisciplinary winglet efficiency estimation criterion is suggested for comparing the aerodynamic efficiency of different wingtip devices with respect to their structural weight penalty in real flight conditions.

In this paper, we addressed the flow patterns over a light boxplane scale model to explain the previously discovered disagreement between its predicted and experimental aerodynamic characteristics. By tuft flow and CFD visualization, we explored the causes yielding a large zero lift pitching moment coefficient, lateral divergence, difference in fore and aft elevator lift, and poor high lift performance of the aircraft. The investigation revealed that the discrepancy in the pitching moment coefficient and lateral stability derivatives can be attributed to insufficient accuracy of the used predictive methods. The difference in fore and aft elevator lift and poor high lift performance of the aircraft may occur due to the low local Reynolds number, which causes the early flow separation over the elevators and flaperons when deflected downward at angles exceeding 10°. Additionally, some airframe changes are suggested to alleviate the lateral divergence of the model.

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