Performance measurement and wake characteristics of a model rotor in axialflight

This paper studies the concept of extracting hover performance from model r otor climb data by extrapolating to the limit of zero climb speed, A 2-blad ed rotor was mounted horizontally and tested in the 30' x 31' settling cham ber of the Ames 7' x 10' #1 wind tunnel. The collective pitch and tunnel sp eed were varied. Video cameras were used to visualize the flowfield illumin ated by pulsed white light sheets. Facility recirculation effects were elim inated at all but the lowest rates of climb. With a steady, non-recirculato ry flow, the rotor wake was seen to be fully periodic, with little diffusio n of the vortices for several rotor cycles. The variation of the computed f igure 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 fo r the hover figure of merit The transition to the far wake was seen to occu r 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 coe fficient and rate of climb. The climb-extrapolation method appears to be a reliable and practical approach to obtaining performance data which are fre e of facility recirculation effects.