How to Fly Helicopters

What is Vortex Ring – Settling With Power?

Vortex ring is commonly called settling with power in the USA. Settling with power is a better descriptive name but vortex ring is technically more correct.

When the helicopter is descending into its own induced flow, there is a risk of vortex ring developing. If the rate of descent is high enough, raising the collective to reduce the rate of descent will make matters worse and the rate of descent will actually increase.

Vortices are always present at the tips of all rotor blades. These vortices can be reduced by good design but they can never be completely eliminated.

Vortices decrease the efficiency of the blades. The outer one third of the main rotor blades produce most of the lift. Losing any lift at the tips due to vortices has a large effect on the total lift generated by the blades.

During vertical descents, the opposing airflow from below changes the relative airflow and the angle of attack of the main rotor blades is increased. Due to washout (blade twist) on the main rotor blades and also the low rotational velocity of the blades close to the hub, the angle of attack is increased so much that this part of the blade starts to stall causing a vibration in the helicopter.

Raising the collective to reduce the rate of descent while the helicopter is in vortex ring has two effects:

1. The vortices get bigger and move inboard from the tips of the blades thus reducing lift further and causing the helicopter to descend faster.
2. The stalled area of the blades close to the hub starts to move outboard and lift is further reduced causing the helicopter to descend even faster.

Conditions For Vortex Ring – Settling With Power