In the last decades an intense research activity have been dealing with drag reduction, with the synergy of analytical studies, experimental tests and simulations. One of the most intensively studied solution consists of small surface protrusions aligned with the flow direction, called riblets. Those structures are inspired by nature: the skin of fast-swimming sharks exhibit riblet structures that reduce the skin friction drag in the turbulent-flow regime (Dean and Bhushan 2010 \cite{Dean2010}). The present experimental work aims to take highly accurate measurements in a fully developed turbulent air flow and to extend the available data on the change of turbulent skin friction coefficient due to the presence of riblets.