http://www.aeros.com.ua/structure/hg/vipers_en.php

The Viper-S has an optional Matrix cloth outer skin for drag reduction. This cloth has been proved in the wind tunnel to be the most 'slippery' available at the moment. With the Viper-S, same as with all Viper series you are able to replace the entire outer matrix cloth covering at anytime with a new one.