Vertical descent

 In vertical descent the nature of flow through the rotor undergoes significant changes. The stream velocity Vc is now negative while the induced velocity vi remains positive as the rotor continues to maintain lift. Initially small recirculating regions develop around the blade tips, as shown in Fig. 2.5. Becoming evident when Vc reaches a level about half vi, an interaction takes place between the upward flow around the disc and the downward flow through it, resulting in the formation of a vortex ring encircling the rim of the disc, doughnut fashion.The situation is illustrated in Fig.2.6.As this vortex-ring state develops the flow becomes very unsteady and the rotor exhibits high levels of vibration. It appears that the ring vortex builds up strength and periodically breaks away from the disc, spilling haphazardly into the flow and causing fluctuations in lift and also in helicopter pitch

and roll. Flight in the developed vortex-ring state, which reaches its worst condition when the descent rate is about three quarters of the hover induced velocity, is unpleasant and potentially dangerous. Because of the dissipation of energy in the unsteady flow, simple momentum theory cannot be applied. As the descent rate approaches the level of the induced velocity, a modified state is observed in which, corresponding to the near equality,there is little or no net flow through the disc.Now the flow is characterized by vortices shed into the wake in the manner of the flow around a solid bluff body. In this turbulent-wake state (Fig. 2.7) flight is still rough but less so than in the vortex-ring state. Simple momentum theory is again not applicable, since energy is dissipated in the eddies of the wake. At large descent rates, when Vc is numerically greater than about 2vi, the flow is everywhere upwards relative to the rotor, producing a windmill-brake state, in which power is transferred from the air to the rotor. With a flow pattern as in Fig. 2.8, simple momentum theory gives a reasonable approximation: thus with Vc negative and vi positive the thrust is: