Performance measurement and wake characteristics of a model rotor in axial flight

This paper studies the concept of extracting hover performance from model rotor climb data by extrapolating to the limit of zero climb speed, A 2-bladed rotor was mounted horizontally and tested in the 30' x 31' settling chamber of the Ames 7' x 10' #1 wind tunnel. The collective pitch and tunnel speed were varied. Video cameras were used to visualize the flowfield illuminated by pulsed white light sheets. Facility recirculation effects were eliminated at all but the lowest rates of climb. With a steady, non-recirculatory flow, the rotor wake was seen to be fully periodic, with little diffusion of the vortices for several rotor cycles. The variation of the computed figure of merit,vith climb rate compared very well with the experiments and it is shown that extrapolation is a good method to get an accurate value for the hover figure of merit The transition to the far wake was seen to occur through a periodic pairing of the tip vortices, Followed by their merging into a single diffuse vortex for each rotor cycle, The number of discrete vortex turns in the near wake before the pairing varied with the thrust coefficient and rate of climb. The climb-extrapolation method appears to be a reliable and practical approach to obtaining performance data which are free of facility recirculation effects.