It's probably a combination of both elements. In high yaw the fin will generate a vortex that will re-energize a stalling rudder and generally increase control authority. (Vortex generators are comonly used in commercial avaition) The wignlet will work as an endplate making the fin more effeciant/increasing the effective area. The winglet itself is probably just outside the relative slipstream and may not be influenced in the same way the stabilizer is. As a result in level flight it can be set to 0 incidence and have no impact to overall trim while the wing incidence is at some small positive angle. In a downline we are trying to fly 0 incidence but wing incidence makes a neutral trim impossible, the winglet may add an element of trim to help as it is at negative incidence relative to the wing.

I didnt think it would be effective as our Reynolds numbers are so low. I suppose the aerodynamic forces on 2M planes are low enough to have this work.

There's lots more that can be discussed, theorized etc....but it looks this thing works so lets build Ts.

Colin.
BSc AE ERAU 1990