[FF-4] Power

Settling with Power. Settling with power can occur at any airspeed or altitude combination
whenever power required exceeds power available. This condition is not to be confused with power settling or vortex ring state.

Power Settling or Vortex Ring State. Power settling or vortex ring state is a condition of powered flight were the helicopter settles in its own down wash. Conditions conducive to power settling are a vertical or near-vertical descent of at least 300 feet-per-minute and low forward speed. The rotor system must also be using some of the available engine power (from 20 to 100%) with insufficient power available to retard the sink rate. These conditions typically occur during approaches with a tail wind or during formation approaches when some aircraft are flying in turbulence from other aircraft. 

The helicopter may descend at a high rate which exceeds the normal downward induced flow rate of the inner blade sections. As a result, the airflow of the inner blade section is upward relative to the disk. This produces a secondary vortex ring in addition to the normal tip vortex system. The secondary vortex ring is generated about the point on the blade where airflow changes from up to down. This results is an unsteady turbulent flow over a large area of the disk that causes loss of rotor efficiency even though power is still supplied from the engine. During vortex ring state, roughness and loss of control is experienced because of the turbulent rotational flow on the blades and the unsteady shift of the flow along the blade span. Power settling is an unstable condition. If allowed to continue, the sink rate will reach sufficient proportions for the flow to be entirely up through the rotor system. If continued, descent will reach extremely high rates.

The vortex ring state can be completely avoided by descending on a flight path shallower than about 30 degrees (at any speed). At very shallow angles of descent, the vortex ring wake is dispersed behind the helicopter. For steeper approaches, the vortex ring stage can be avoided by using a speed either faster or slower than the area of severe turbulence and thrust variation. At steep angles, the vortex ring wake is below the helicopter at slow rates of descent and above the helicopter at high rates of descent. Recovery may be initiated during the early stages of power settling by applying a large amount of excess power, which may be sufficient to overcome the up flow near the center of the rotor. If the sink rate reaches a higher rate, power will not be available to break this up flow and thus alter the vortex-ring state of flow.

The normal tendency is for pilots to recover from a descent by application of collective pitch and power. If sufficient power is not available for recovery, application of pitch may aggravate power settling. This results in more turbulence and a higher rate of descent. Recovery can be accomplished by lowering collective pitch and increasing airspeed. Increasing airspeed normally is the preferred method of recovery, since less altitude loss usually results than by lowering collective pitch method. Both of these methods require sufficient altitude in order to be successful.

WARNING: A considerable loss of altitude may occur before the power settling or vortex ring state condition is recognized and recovery is completed.

 

Power Required to Hover. Gross weight has the single biggest impact on power required to hover. As hover height increases, power required to hover also increases. This is due to the diminished influence of ground effect. The power required to hover increases slightly as the pressure altitude increases. There is a misconception that as the temperature decreases, the power required to hover decreases significantly. In certain cases, at extremely high PA and low temperatures, the power required to hover increases at a given gross weight. Winds also have an effect on the power required to hover. As the wind velocity increases to transverse flow and ETL speeds, there will be a decrease in the power required to hover because of increased efficiency of the rotor system. When executing an approach with a narrow power margin, the approach must be smooth and controlled. An approach airspeed below ETL reduces rotor efficiency and could lead to power settling or settling with power. An airspeed moderately above ETL requires the application of power to control closure rate. At the bottom of an approach (unlike a stabilized hover) the lift vector has a rearward component (to stop forward motion) which must be compensated for. Flying the approach at the airspeed where ETL just begins (using the transverse flow burble as a target) usually requires the least power. During the approach, power applied should slowly increase (to compensate for increasing induced drag and control the rate of descent), but should be below hover power. Remember, at the bottom of the approach, if you desire to terminate at 10 feet, you must have the power available to hover at 10 feet, stop your descent, and compensate for a rearward lift vector.

Power Available. Power available is a function of engine design/condition and atmospheric
conditions, such as temperature and PA. If the outside air temperature or the PA increases, maximum power available decreases.