| A helicopter rotor in descent flight encounters its own wake, resulting in a doughnut-shaped ring around the rotor disk, known as the Vortex Ring State (VRS). Flight in the VRS condition can be dangerous as it may cause uncommanded drop in descent rate, power settling, excessive thrust and torque fluctuations, vibration, and loss of control effectiveness. As the simple momentum theory is no longer valid for a rotor in VRS, modeling of rotor inflow in the VRS continues to challenge researchers, especially for flight simulation applications.; In this dissertation, a simplified inflow model, called the ring vortex model, is developed for a rotor operating in descent condition. By creating a series of vortex rings near the rotor disk, the ring vortex model addresses the strong flow interaction between the rotor wake and the surrounding airflow in descent flight. Each vortex ring induces normal velocity at the rotor disk. In addition, the momentum theory is augmented by adjusting the total mass flow parameter to create a steady state transition between the helicopter and the windmill branches. The combined effect of the normal velocity from the vortex rings and the baseline induced velocity from the augmented momentum theory provides an improvement in predicting the inflow at the rotor disk in descent flight. With the ring vortex model, the rotor inflow can now be reasonably predicted over a wide range of descent rates.; Validations of the ring vortex model for helicopter rotors are conducted in both axial and inclined descent. Sources of test data are from four selected experiments, including Castles and Gray's wind-tunnel tests, Yaggy and Mort's wind-tunnel tests, Washizu's moving track tests, and ONERA's Dauphin flight tests. The validations focus on rotor induced velocity variation, torque requirement, collective control setting, and changes in rotor thrust and torque. Effects from blade taper, blade twist, and rotor thrust are also discussed with further application of the finite-state inflow model.; The ring vortex model is applied to a full-scale single main-rotor helicopter. The main effort is to establish a VRS boundary based on heave stability criterion. In the dynamic simulation, two important phenomena observed in the descent flight tests are addressed, including uncommanded drop in descent rate and loss of collective control effectiveness.; In addition to being applied to the conventional helicopter, the ring vortex model is further applied to a side-by-side rotor configuration. Lateral thrust asymmetry on a side-by-side rotor configuration can be reproduced through uneven distribution of vortex rings at the two rotors. Aerodynamic interaction between the two sets of vortex rings associated with its two rotors is taken into account. Two important issues are investigated: the impact of vortex rings on lateral thrust deficit and the impact of vortex rings on the lateral AFCS limit. |
