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Price is not a problem in competition gliders, also no problem with in maintenance surfaces like at airliners. It is very easy to wash glider before every race.

A riblet surface is proven (PDF) to have lower frictional drag.

Why do full scale gliders not use riblet technology or even matte the surface with 1000 grit sandpaper as the windsurfing industry has for many years?

Has anyone tried to sand surface and try compete against regular glossy gliders?

Birds wings are rough too, do they also benefit from this phenomenon?

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There are many variables in the aerodynamics which make a surface appropriate for one use but not another. Controlled turbulent boundary layer(rough/textured) surfaces may have lower average drag over a wide range of conditions, but laminar boundary layer (clean glossy) surfaces can have much lower drag in a narrow set of conditions. The airfoil profile also matters greatly to the effects of surface finish. Reynolds number is a large variable too, birds have an extremely low Reynolds number and human carrying gliders have a medium Reynolds number. Reynolds number drastically effects the behavior of the boundary layer.

There is also consideration for strength and ease of manufacture and maintenance. If it harms the strength to weight ratio, if it creates reliability and inspection problems in manufacture, if it is not readily maintainable.

The assumption that cost is not a factor is also completely erroneous. All competitors have limited budgets, it may be a large limit but it is still a limit.

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