Helicopters do not just drop like a stone if the engine fails; they can be put into autorotation and landed safely. However, there is one aerodynamic state which can cause an uncontrolled and irrecoverable descent – ie a crash – in a rotary aircraft. This is ‘vortex ring.’
What is Vortex Ring
Sometimes considered as the equivalent of the fixed wing stall, and called ‘settling with power’ in some countries, vortex ring state is a condition of powered flight in which the helicopter descends into its own downwash, or the air being forced down by the blades. Under such conditions, the helicopter will fall at an extremely high rate, eventually 2,500 feet per minute or more. It will randomly yaw, pitch, and roll; there will be buffeting and juddering, or shaking, of the airframe, with eventually total loss of control and possible structural damage… if the aircraft hasn’t hit the ground first!
What Causes Vortex Ring
To get into vortex ring, three factors must all be present:
- There must be little or no airspeed.
- There must be a rate of descent.
- There must be power applied.
Note that all three of these must be going on at the same time.
How to Get out of Vortex Ring
It is actually very easy to get out of vortex ring… at least in the incipient stage when the juddering and yawing starts. Some say it is impossible to get out of the fully developed state, but when you start to perceive signs of vortex ring, all you need to do is remove one of the three factors noted above. So, you push the cyclic forward to increase airspeed, or lower the collective to reduce power. It is not possible to reduce the rate of descent to stop vortex ring, as that would involve increasing power. In practice, pilots usually increase the airspeed, as unless the helicopter is very high, you don’t want to lower the collective and risk hitting the ground!