Experimental Boundary Layer Study on Hovering Rotors

Abstract

A study has been conducted in an effort to gain basic knowledge about the boundary layer of rotors in the hovering condition. The results were obtained using evaporative chemical films, pressure probes and hot wire anemometers. Tuft and oil film studies were also conducted for comparison with the chemical film technique. Information is presented on the location of transition from laminar to turbulent flow, on the forming of tip vortices and other aerodynamic features. A slightly inward flow of the boundary layer over a large part of the airfoil surface was noted, indicating that, for the hovering rotor, the pressure forces along the blade and the effects of the contraction of the wake have to be taken into account besides the centrifugal acceleration. Also presented are preliminary velocity measurements in the boundary. Results using the chemical film and pressure probe measurement techniques on rotating rotors are compared to non‐rotating tests conducted in a wind tunnel.