Vortex Ring State

The vortex ring state (VRS) is worthy of some special further discussion. This is because in the VRS the rotor can experience highly unsteady flow with regions of concurrent upward and downward velocities, and the flow can periodically break away from the rotor disk. This flow state appears to have been first recognized by de Bothezat (1919) and such flows have been visualized and measured in helicopter rotor experiments by Lock et al. (1926), Castles & Gray (1951), Drees & Hendal (1951), Washizu et al. (1966a, b), Azuma & Obata (1968), Moedersheim et al. (1994), and others. An example is shown in Fig. 2.22. Yaggy & Mort (1963) show that the unsteady airloads produced in VRS depend on rotor disk loading and blade twist, confirming the nonlinearity of the VRS problem. Betzina (2001) has conducted experiments in descending flight conditions with a relatively highly loaded rotor, representative of that used on a tilt-rotor. Some dramatic in-flight flow visualization on a helicopter rotor during vertical descent were performed by Brotherhood (1949). Glauert (1926a, b) and Nikolsky & Seckel (1949a, b) were some of the first to conduct a mathematical analysis of the problem. From a piloting perspective, entry into autorotative flight may require transition through the VRS, especially during flight at low airspeeds (see Section 5.7). Because of the flow unsteadiness at the rotor in VRS, it can lead

to significant blade flapping and a loss of rotor control. If the VRS occurs on the tail rotor, such as during sideways flight or hovering in a crosswind, then directional (yaw) control may be seriously impaired. This is a safety of flight issue – see Section 6.9.