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|Category:||Theory of Flight|
What is a Vortex Ring?
Vortex Ring, also called ‘settling with power’ or ‘power settling’ is a flight condition in which helicopter main rotor lift is suddenly lost even though it is powered and loss of control results. Recognition that this condition is taking effect can be difficult and its onset quick. All helicopter types are susceptible to it and it can occur at any height once out of ground effect. An understanding of how it develops is the best protection from an encounter.
How it happens
The cause of what might be termed secondary vortex ring is the recycling of the normal main rotor tip vortices into the induced airflow of inner blade sections of the rotor. This condition is possible when the rate of descent of a helicopter is greater than half the air speed induced by the rotor and results in airflow of the inner blade sections changing from downward relative to the rotor disk to upwards. The boundary along the rotor disc at which the upward motion of the inner blade sections meets the normal downward motion of the outer blade sections produces the severe airflow instability of the ‘vortex ring’ and the resultant aerodynamic inefficiencies. The vortex ring condition is an unstable one. If it is allowed to continue, loss of control in rough air conditions will occur as the ring moves outwards along the blade span until there is no lift at all.
Although vortex ring induced descent becomes rapid very quickly, the circumstances in which the condition can occur in are quite limited. A descent at greater than 30 degrees but less than 90 degrees to the horizontal must be being made and the forward speed must be positive and below 10 kts18.52 km/h
5.14 m/s. However, since indication of forward airspeed is unreliable below about 30 KIAS, this figure must be taken as the practical limit.
In a descent shallower than 30 degrees, the vortex ring wake is shed behind the helicopter. In a vertical descent, the vortex ring wake will be offset and below the helicopter at rates of descent up to about 500fpm and offset and above it for rates of descent above about 1200fpm
- Incipient vortex ring conditions are typically increased vibration and buffet, the onset of small amplitude ‘twitches’ in both pitch and evidence of longitudinal, lateral and directional instability.
- Established vortex ring conditions are characterised by a very rapid increase in rate of descent towards and beyond 3000fpm, reduced effectiveness of cyclic inputs in roll or pitch and the application of collective pitch failing to arrest the rate of descent and possibly even increasing it.
- Incipient Stage. Once recognised, an immediate response is required. Keep the Collective position unchanged and apply forward cyclic to achieve a nose down attitude so as to increase forward airspeed quickly. As soon as a steady increase in airspeed is indicated, more power can be applied if necessary without waiting until the best rate of climb speed is reached. If this action does not resolve the situation rapidly then it is best to treat the condition as established and take the actions below.
- Established Condition. Recovery can only be effected by changing the airflow around the rotor and will inevitable lead to significant loss of height which makes recovery from a low level occurrence impossible. There are two possible actions and combining them is likely to produce the quickest recovery with the least height loss. Application of forward cyclic should increase airspeed but large input held for several seconds may be required before significant pitch attitude and consequent speed change is achieved, with a significant nose down attitude resulting. Lowering the collective to reduce power towards auto-rotation will also be effective but forward airspeed must be gained before power is re-applied during recovery.
- B412, vicinity Karlsborg Sweden, 2003 (HF LOC) describes an accident in which the crew of a helicopter lost control of the aircraft as a result of pilot mishandling associated with the development of a Vortex Ring State.