The power requirements measured in static thrust and in level forward flight are presented for two helicopter rotor configurations. One is a coaxial rotor arrangement having the rotors spaced approximately 19 percent of the rotor radius; the other is a tandem configuration in which the rotor-shaft spacing is 3 percent greater than the rotor daimater and in which the rotors lie in the same plane. The experimental meausrements are compared with the results of calculations based on existing NACA single-rotor theory.