Tandem Rotor Helicopters

Flight measurements of some longitudinal static stability characteristics with an overlapped-type tandem-rotor helicopter at low airspeeds are presented. The data show that a critical amount of longitudinal control is required to maintain trimmed flight at certain combinations of airspeed and rate of descent. Estimates based on theoretically predicted interference effects indicate that the front rotor downwash acting on the rear rotor could cause variation of the longitudinal moments of the same order of magnitude as the measured variation.