Flightpath
Yeah - fascinating but difficult subject!
What I noticed with winglets is a movement of the turbulent air towards the wingtip.
One thing that seems common to all the various configurations is that the local lift coefficients are increased in the tip region. This actually means that the vortex strength is
increased there (keeping the wing total constant of course). So the argument about reduced drag from reduced tip vortex strength is
totally wrong!
First off, the radical differences in winglet designs, the particular models of aircraft they are installed on, and the outward flap configurations of those aircraft. Noting that there is little performance difference associated with the Airbus winglet vs sharklet, and the general differences between the Boeing and Airbus configurations.
If this was simply a span factor, then wouldnt there be significant differences in the winglets performances (Airbus vs Boeing)?
Its not just a span factor of course; the detailed design of the wing outboard aerodynamics, the local structural strength all come into it. There is also a considerable difference in winglet performance inside one manufacturer's range of products - from the A300 through to the A350 for example. That may be due to increased understanding of how the things behave, understanding developed over several decades.
Secondary, but very important, is the winglet performance differences during phases of flight. There is a marked difference when the aircraft is at the beginning of a long haul (very heavy and the highest AoA) vs the performance when the aircraft is light, with a decreased AoA.
That is certainly true, but the reasons for the differences may be quite subtle. On one aircraft I was associated with we flew back to back performance tests with and without winglets. We found that the benefits were indeed weight dependent, but at least some of that dependency was because the wing aeroelastic twist was greater at the higher weights so that the actual span loading at the tip varied with weight and the winglets were working from a different datum.
Boeing notes that the benefit from the winglet decreases with decreased AoA, and is negated by the friction drag of the winglet itself.
True and of course consistent with the benefits being associated with lift dependent drag.
Finally, I must remind you that I only deal with the measurements from aircraft on final approach. There is significant influence from landing gear, AoA, and especially flap settings/flap configuration, and atmospheric conditions at this phase of flight...
I was aware of your interest in approach conditions, so permit me a couple of thoughts....
It seems to me to be obvious from the pictures you have posted from time to time that the vortices springing from the outer edge of the flaps must be much stronger that those at the wing tip. I say this because you often see condensation in the 'flap' vortices but nothing at the tip. This would suggest that the pressure drop/vortex velocity/strength is much greater in the flap system.
This in turn is entirely consistent with the expression I quoted earlier - that lift/unit span is vortex strength*airspeed - the lift per unit span being much greater where the flaps are of course.
It also suggests that the danger to following aircraft may be linked more to these flap vortices than to those at the tip. Paradoxically then winglets may reduce wake vortex problems (ever so slightly) because they transfer vorticity from flaps to tip (keeping the total vorticity constant) ie because they increase the tip vortex strength!