On Monday, June 12, 2017 at 3:29:43 PM UTC-6, wrote:
If there is an effect on performance or handling, this will be different from glider to glider. It may do nothing, improve performance or even be needed to prevent flutter. There were a lot of quantitative studies done for various gliders, especially the newer ones. For which type do you need this information?

I have a PIK20E (1985 last year they were made). The manual says nothing about seals. Guys on our forum say that there is an internal fabric seal in the aileron joints. My glider has them on the wings and rudder, but not the elevator.

From a search of Soaring Mag, it seems external mylar curved lip seals date from about that time or later, and became popular later, and common about 10 years later. In fact in February 1986 Dick Johnson was reviewing the new Discus, and actually removed the external mylar seals because he didn't trust them. He wrote: "The ailerons are hinged near their top surfaces and they did have the normal tape seal bridging the span wise gaps between the wing and the aileron leading edges. However, a curved piece of Mylar plastic strip was attached to the wing portion by an adhesive layer at its leading edge. It curved downward in its aft direction to maintain a sliding contact with the aileron top surface. W ith the aileron in neutral the Mylar strips created a small but perhaps significant upward bulge in the critical airfoil profIle. Also, these Mylar strips cannot be observed from the cockpit in flight, and it is possible that airstream suction force over their cambered top surfaces caused the strip trailing edges to ride high and above the aileron, thereby adding drag. For the above reasons the Mylar aileron seals were removed and Ceconite polyester airseals were installed at the
wing root aileron control rod openings."

In a article a few years later about improving the 1-26, I think, Dick wrote that external mylar seals might provide a little improvement.