Flow around helicopter blade tip sections using a 2D particle image velocimeter - Part I

The current study is an effort to better understand the viscous flow physics existing near helicopter blade tips at various angle of attack positions. A planar particle image velocimeter (PIV) was successfully integrated into the helicopter blade tip flow measurement system in a model hover stand The current study utilizes a 1200 mm (47.24 in) diameter scaled helicopter rotor blade system with a 'collective pitch control' capability for flow field measurements. Downwash and upwash behavior at the sections on the blade, and re-circulatory flow behavior near near the tip section are investigated. The specific issues related to the PIV technique used are discussed. The flow characteristics for 5 pitch case looks like a mirror image of the characteristics for -5 pitch case since the airfoil shape is very close to a symmetrical airfoil. The flow field is found to be relatively symmetric on upper and lower surfaces of the blade when pitch is 0. Lift is produced at positive pitch case, which leads to a downwash motion in the region by the trailing edge. Similar flow characteristics are observed for negative and positive pitches at different sections away form the tip, but the upwash motion of the flow is more dominant for negative pitch.