Effect of Retrofitted Winglets on 767 Handling

[WeekendFlyer]

Winglets and Vortices

Uplinker,

as I understand it, tip vortices take a lot of energy from the aircraft because previously still air becomes rapidly spinning air. That increase in Kinetic energy has to come from somewhere, and ultimately it manifests itself as induced drag. Force * velocity = power = energy per second, so the induced drag force * TAS gives you an idea of the rate of energy transfer from the aircraft to the air. This energy transfer creates the vortices, the upwash ahead of the leading edge and outside the wing tips, and the downwash behind the wing.

The vortices are caused by high pressure air beneath the wing forcing its way around the wingtips to try and fill the low pressure area above the wing. This causes airflow towards the wingtips from underneath, and away from the wingtips on the top of the wing. Adding this span-wise velocity component with the velocity of the air coming towards the wing and you get tip vortices.

Winglets exploit the vortices by taking the flow from the top and fuselage-facing parts of the vortex and using it just like a normal wing uses the relative airflow - i.e to generate a lift force. However, for a winglet this force acts mostly horizontally towards the fuselage but also has a forward facing component. This "thrust" component cancels out some of the induced drag force, resulting in lower overall induced drag and thus could be argued to be a form of energy recovery from the vortices. The resulting vortices are therefore smaller and less energetic and the downwash is thus reduced. These changes are small, but enough to lead to significant fuel savings in the long run.


Regarding directional stability - yes, I suspect winglets have sufficient area in the vertical plane to cause a non-trivial sideforce at high sideslip angles. As this will be acting behind the aircraft CG it will cause a small increase in directional stability, which in turn could lead to a slightly lower crosswind limit for takeoff and landing to ensure directional control can be retained.

Hope this clarifies things a bit!