Results are presented of an experimental investi gation of the effects of a second harmonic fea thering device on a two-bladed, semirigid rotor helicopter. Flight tests were conducted with an Army UH-1A helicopter modified to incorporate a Bell-designed experimental Second Harmonic Con trol (SHC) mechanism. Prior theoretical studies indicated that second harmonic feathering would be beneficial in delaying retreating blade stall and in reducing oscillatory loads and vertical vibrations in the cockpit. Air load data of this program are presented which show that pulsations in rotor thrust can be reduced with second har monic feathering. Reductions in cockpit vertical vibrations and structural loads are shown for certain settings of the SHC; however, simultane ous reduction of vibrations measured at different locations in the fuselage and of loads in the ro tor blades and the control system was not real ized. A complete experimental evaluation of sec ond harmonic feathering for the mitigation of re treating blade stall was hampered by unexplained, abnormally high vibrations and control loads en countered at high speeds. Analyses of such a system show that second harmonic feathering can reduce stall and the accompanying high drag on the retreating blade; however, increased drag from compressibility effects occurs at other points around the rotor azimuth, thus offsetting the effects from reducing retreating blade stall.