Three doubtful issues occurred in several references of helicopter aerodynamics are pointed out: (1) whether the global momentum theory for helicopter rotors can be applied to the differential analysis; (2) whether the distribution of blade induced velocities along the radius can be assumed to be independent of the blade pitch angle of rotors in blade element theory, even for vertical flight; (3) how does the blade induced velocity distribute over rotor azimuth in forward flight. This paper tries to clarify the doubtful issues and gives some conclusions.