This report presents test results on the 8/3l-scale McDonnell Model 82 Convertiplane in the Massie Memorial Wind Tunnel, Wright Air Development Center. The purpose of these tests was to determine the high-speed potential of the McDonnell rotor at a zero rotor angle of attack and in an approximately zero rotor lift condition. The main results are as follows: The critical advance ratio, a defined by the development of unstable blade flapping of the model rotor, decreases from approximately 1.6 at 400 rpm to approximately 1.3 at 800O rpm. The Wright Field result at 400 rpm agrees with previous tests with the same rotor and at the same rotor speed. The considerable effect of rotor speed on the critical advance ratio was unexpected and no definite explanation can be given for it either from these tests or from the available theoretical treatments of the rotor blade motion. The theoretical investigations on rigid rotor blades indicate stable blade motion up to much higher advance ratios than measured during these tests. However, these theoretical) investigations were made under simplifying assumptions which do not represent the true conditions on the rotor blades. Some of the discrepancies between these assumptions and the true blade conditions are discussed in this report. The discussion of the test results and their comparison with the results of References 3 and 4 show that the knowledge about the nature of the observed blade instability is still insufficient and that more experimental and theoretical work is required to clarify the problem.