i can't think of any aircraft flying that uses a winglet facing downwards, but i'm sure somebody tried it out. the only comparable thing i can come up with is the wing fence used on some airbus models and the MD11
OPNLguy From , joined Dec 1969, posts, RR: Reply 2, posted (6 years 9 months 4 days 18 hours ago) and read 1515 times:
Notwithstanding the aerodynamic factors, installing blended winglets that point downward might as well come equipped with big bullseyes painted on them, since they'd be subject to being hit by any/every type of vehicle operating on the airport ramp...
The effect of the winglet is to produce a vertical diffusion of the vorticity in tip region. This diffusion process is also realized as an expansion of the wake in the far field due to induced velocities from the nonplanar components of the
winglet. The out of plane bound vortex on an upward winglet induces horizontal velocities on the free wake that cause a spanwise spreading of the wake field.
When referenced to the actual span, the resulting efficiency can be greater than that of an elliptical loading, emulating the effect of a span increase. It should be noted that, while still beneficial, a winglet downward oriented would produce a contraction of the wake and is not as effective in reducing the induced drag as is a winglet oriented upward.