Rotor Blade Angles Angle of Incidence Out of Ground Effect (OGE) In Ground Effect (IGE)  that the helicopter and locally disturbed air will sink at  alarming rates. This effect is called vortex ring state (formerly  referenced as settling-with-power) and is discussed at length  in Chapter 11, Helicopter Emergencies and Hazards. A higher  blade pitch angle is required to maintain the same AOA as in  IGE hover. The increased pitch angle also creates more drag.  This increased pitch angle and drag requires more power to  hover OGE than IGE.  [Figure 2-23] Ground effect is the increased efficiency of the rotor  disk caused by interference of the airflow when near the  ground. The air pressure or density is increased, which  acts to decrease the downward velocity of air. Ground  effect permits relative wind to be more horizontal, lift  vector to be more vertical, and induced drag to be reduced.  These conditions allow the rotor disk to be more efficient.  Maximum ground effect is achieved when hovering over  smooth hard surfaces. When hovering over surfaces as tall  grass, trees, bushes, rough terrain, and water, maximum  ground effect is reduced. Rotor efficiency is increased  by ground effect to a height of about one rotor diameter  (measured from the ground to the rotor disk) for most  helicopters. Since the induced flow velocities are decreased,  the AOA is increased, which requires a reduced blade pitch  angle and a reduction in induced drag. This reduces the  power required to hover IGE.  [Figure 2-22] There are two angles that enable a rotor disk to produce the  lift required for a helicopter to fly: angle of incidence and  angle of attack.  Angle of incidence is the angle between the chord line of a  main or tail rotor blade and its rotor disk. It is a mechanical  angle rather than an aerodynamic angle and is sometimes  referred to as blade pitch angle.  [Figure 2-24]  In the absence  of induced flow, AOA and angle of incidence are the same.  Whenever induced flow, up flow (inflow), or airspeed modifies  the relative wind, the AOA is different from the angle of  incidence. Collective input and cyclic feathering (see page  2-12) change the angle of incidence. A change in the angle of  incidence changes the AOA, which changes the coefficient of  lift, thereby changing the lift produced by the airfoil. The benefit of placing the helicopter near the ground is lost  above IGE altitude. Above this altitude, the power required  to hover remains nearly constant, given similar conditions  (such as wind). Induced flow velocity is increased, resulting  in a decrease in AOA and a decrease in lift. Under the correct  circumstances, this downward flow can become so localized  Figure 2-22.  In ground effect (IGE).  2-12 Lift vector inclined well to rear Lift Induced flow velocity= 60 ft/sec Axis of rotation α = 10° Large blade pitch angle Pitch angle = 18° Induced flow velocity = 60 ft/sec Large blade-tip vortexes V+ P− MAX VELOCITY = 120 ft/sec