The statement attributed to Dr Maughmer that winglets don't stall is baloney. They do stall and it is easy and interesting to observe by simply tufting them and watching as you increase yaw or slip in a turn.

I found on the Duo Discus XL the inboard winglet stalls at a yaw string angle of about 15-20 degrees (not sure how that corrolates to actual aircraft yaw angle). But the amount of slip required to cause the yarn tufts to blow backward was a little more than a pilot would naturally use. So Schempp has chosen a mounting angle that, while not optimum in perfectly straight flight, is tolerant of pilot "abuse." A stalled winglet is effectively a mini speed brake. Not welcome.

KS