An analytical and experimental investigation was conducted to acquire systematic model rotor performance and wake geometry data and to evaluate the accuracy of various analytical methods in predicting the effects on performance of changes in helicopter rotor design and operating parameters. Both classical hover performance analyses and analytical methods recently developed at the United Aircraft Research Laboratories were evaluated. Of primary concern in the study was the assessment of assumptions in the analyses regarding the geometry of the rotor wake. Of particular interest was the prediction by the analysis of an instability of the tip vortex helix at moderate distances from the rotor which appeared to be substantiated by available experimental results.