Abstract:
Vortex-ring state (VRS) is hazardous for helicopter flight, which can cause the vehicle being out of control in some cases. It is important to accurately predict the occasion when helicopter getting into this dangerous state, called vortex-ring boundary. The currently existing calculation model for the boundary is considerably precise but still has error being compared with flight test data. Firstly, glide angles of helicopter in model test and real flight were compared to find out that panel obliquity is the main factor to cause the inaccuracy of ideal vortex-ring boundary. The trend of helicopter attitudes with forward speed increase was analysed in helicopter inclined descent. The calculation model of rotor vortex-ring boundary was found in consideration of panel obliquity. Based on helicopter balance equation and Gao-Xin criterion, the algorithm vortex-ring state boundary for real flight was pointed out, in which the panel obliquity was reckoned in. Then, being taken for instance, the revised boundary curve of a helicopter was numerically figured out. Finally, the calculation results were verified and analyzed in contrast with flight test data. Results show that, the revised vortex-ring boundary curve for real flight is located below the ideal one, which has a more favorable agreement with flight test data. Thus, panel obliquity has high influence on rotor VRS boundary. On account of the difference in panel obliquity with different helicopter, however, the level of the influence also depends on the helicopter type.