Tail rotor stall
It has already been seen that the vortex-ring state can cause a lifting rotor to lose effectiveness in descending flight. Likewise vortices can lead to tail rotor stall and a consequent loss of yaw control. Any situation that causes the tail rotor to pump air into an oncoming airstream has the potential to generate a vortex ring state. Sideways flight, rapid spot turns or out-of-wind hovering are situations when the tail rotor is operating in a state equivalent to either a climbing or descending rotor. If flow into the rotor opposes the inflow (the descending case) then a vortex ring may form which reduces the ability of the tail rotor to generate an anti-torque thrust. Depending on the size and loading of the tail rotor it may be possible for the pilot to detect the incipient stages of this phenomena by observing unusual activity in yaw or by feeling irregular buffeting of the tail cone. Failing either of these cues the pilot may only become aware that something is amiss when full pedal deflection has no effect in arresting an unwanted yaw rate. In this situation the tail rotor is effectively power settling and the only course of action left is to reduce the anti-torque requirement by either lowering the collective lever or closing down the engine(s).
1.8 POWERED DESCENTS
Prior to analysing the performance characteristics of a helicopter in an autorotation it is necessary to consider the limits on stable descent. Two basic conditions exist: descent under power characterized by the rotor imparting a downwards induced velocity vector on the upward moving flow, and autorotation when smooth momentum flow takes place with the rotor blades generating thrust via a transfer of energy from the airstream to the rotor. Descent under power from the hover or at low forward speed can lead to the potentially hazardous flight regime of vortex ring therefore determining the combination of airspeed and RoD that leads to the vortex-ring state is important.
Earlier the vortex-ring state was mentioned in association with purely vertical flight when it was concluded that for rates of descent between zero and 2vih, a vortex would exist somewhere in the streamtube passing through the rotor. This broad definition was further refined by the realization that stable rotor behaviour was only in question when a vortex existed close to the disk. Consequently the danger area was reduced to vertical rates of descent between 0.7vih and 1.5vih. It is now necessary to determine the effect of a forward speed component on the rates of descent that can generate this potentially hazardous situation.