Vortex ring state redirects here. For other meanings see Vortex ring

Vortex ring

A vortex ring, also called a toroidal vortex, is a region of rotating fluid moving through the same or different fluid where the flow pattern takes on a toroidal shape. The movement of the fluid is about the poloidal or circular axis of the doughnut, in a twisting vortex motion...

.

In helicopter

Helicopter

A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

 flight, it is possible for the rotors

Helicopter rotor

A helicopter main rotor or rotor system is a type of fan that is used to generate both the aerodynamic lift force that supports the weight of the helicopter, and thrust which counteracts aerodynamic drag in forward flight...

 to descend into their own downwash

Downwash

In aeronautics downwash is the air forced down by the aerodynamic action of a wing or helicopter rotor blade in motion, as part of the process of producing lift....

, a cone of turbulent air previously forced downward in the generation of lift

Lift

Lift may mean:* Lift , a mechanical force generated by an object moving through a fluid* Lift , rising air used by soaring birds and glider, hang glider and paraglider pilots for soaring flight...

. As turbulent air does not have the same physical properties as still or clean air, the rotors produce less lift and the aircraft may descend further into the turbulent air. Settling with power describes such a helicopter's descent, or settling, even with adequate engine power to continue flight.

The hazard condition is also known as a vortex ring state because the flow around the rotor

Helicopter rotor

A helicopter main rotor or rotor system is a type of fan that is used to generate both the aerodynamic lift force that supports the weight of the helicopter, and thrust which counteracts aerodynamic drag in forward flight...

 is a vortex ring

Vortex ring

A vortex ring, also called a toroidal vortex, is a region of rotating fluid moving through the same or different fluid where the flow pattern takes on a toroidal shape. The movement of the fluid is about the poloidal or circular axis of the doughnut, in a twisting vortex motion...

. Vortex ring state, the aerodynamic condition that causes the settling, occurs when a helicopter develops excessive descent rates at low speeds and high power settings, depending also on gross weight, winds, etc. This issue also affects tiltrotors

Tiltrotor

A tiltrotor is an aircraft which uses a pair or more of powered rotors mounted on rotating shafts or nacelles at the end of a fixed wing for lift and propulsion, and combines the vertical lift capability of a helicopter with the speed and range of a conventional fixed-wing aircraft...

, and was responsible for an accident involving a V-22 Osprey.

Description

A helicopter normally encounters settling with power when attempting to hover out of ground effect above the hovering ceiling for the aircraft, hovering out of ground effect without maintaining precise altitude control, and while making downwind or steep, powered approaches when the airspeed

Airspeed

Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....

 drops to nearly zero. The signs of settling with power are a vibration in the main rotor system followed by an increasing sink rate and possibly a decrease of cyclic

Helicopter flight controls

A helicopter pilot manipulates the helicopter flight controls in order to achieve controlled aerodynamic flight. The changes made to the flight controls are transmitted mechanically to the rotor, producing aerodynamic effects on the helicopter's rotor blades which allow the helicopter to be...

 authority. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and impact with terrain, a frequently fatal event.

In forward flight, there is no upward flow (upflow) of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins because there are no airfoil surfaces in the mast and blade grip area. As volume of upflow increases, the induced flow (air pulled or "induced" down through the rotor system) of the inner blade sections is overcome and the blades begin to stall

Stall (flight)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

 near the hub. As the inner blade sections stall, a second set of vortices, similar to the rotor tip vortices, form in the center of the rotor system. The inner set of vortices decreases the amount of lift being produced and causes an increase in sink rate. In an accelerated condition, the inner and outer vortices begin to feed each other to the point where any increase in rotor blade pitch angle increases the interaction between the vortices and increases the rate of descent.

Pilot reaction

Helicopter pilots are most commonly taught to avoid settling with power by monitoring their rates of descent at lower airspeeds. When encountering settling with power, pilots are taught to apply forward cyclic to fly out of the condition or lowering collective pitch. While transitioning to forward or lateral flight will alleviate the condition by itself, lowering the collective to reduce the power demand decreases the size of the vortices and reduces the amount of time required to be free of the condition. However, since the condition often occurs near the ground, lowering the collective may not be an option; a loss of altitude will occur proportional to the rate of descent developed before beginning the recovery. In some cases, vortex ring state is encountered and allowed to advance to the point that the pilot may lose cyclic authority due to the disrupted airflow. In these cases, the pilot's only recourse may be to enter an autorotation

Autorotation (helicopter)

Autorotation is the state of flight where the main rotor system of a helicopter is being turned by the action of air moving up through the rotor rather than engine power driving the rotor...

 to break the rotor system free of its vortex ring state.

Tandem rotor helicopters

In a tandem rotor

Tandem rotor

Tandem rotor helicopters have two large horizontal rotor assemblies mounted one in front of the other. Currently this configuration is mainly used for large cargo helicopters....

helicopter, forward cyclic will not arrest the rate of descent caused by settling with power. In such a helicopter, which utilizes differential collective pitch in order to gain airspeed, lateral cyclic inputs must be made accompanied by pedal inputs in order to slide horizontally out of the vortex ring state's disturbed air.