Two gliders of the same typ may have quite different performance. This is a known fact and that's why so many pilots spend long hours and a lot of money to tune their gliders. A well tuned glider can easily be 10% better than a not-so-good glider of the same type.
In this article I will stay within proven tuning that makes a difference. This is valid for any and all gliders.
The gap between the control surface and the air frame(or stabiliser) must be air tight. This is done by using a teflon tape either internally or externally at the control surface. Mylar seals are mainly used to just make the surface smooth but not air tight. In this video you can see the air seals on my Standard Cirrus.
Start by reading this. Our goal is a gap of less than 1mm with no out flow or preferably suction. Now days air extractors are becoming more available and a well designed one will give suction thru the gap. On a club class glider the performance gain can easily be 5%. The aft arc is common to seal using mylar like in this picture.
The wings give the biggest contribution to drag and thus it have the biggest impact on performance. The most important part is the forward half of the wing and especially the leading edge. These areas should be perfectly smooth. Indentions, holes, deep scratches, paint chips or other surface imperfections will reduce the performance. An often overlooked part is the competitions signs and registrations on the bottom side of the wing. These are often protruding from the surface and tripping the laminar flow much to early. Paint them on or use the thinnest plastic sticker you can find.
Use mylar seals and tape to seal the gaps.
Tesa 4104: Thin white covertape for holes and mylar
Tesa 4965: Double sided for fixating mylars
Teflon tape: air seal for control surfaces(becomes white after 1-2 years in the sun)
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