Upon entry into blade stall, the first effect is generally a noticeable vibration of the helicopter. This
vibration is followed by the helicopter's nose lifting and a rolling tendency. If the cyclic stick is held
forward and the collective pitch is not reduced, or is raised, the stall becomes aggravated, and the
vibration increases greatly. Control of the helicopter may then be lost.
SETTLING WITH POWER
An aviator may experience settling with power accidentally. Causes of settling with power are
typified by a helicopter in a vertical or nearly vertical powered descent of at least 300 feet per minute
with a relatively low airspeed. The rate depends upon the load, rotor rpm, and density altitude. The
rotor system must be using from 20 to 100 percent of the available engine power and the horizontal
velocity must not exceed 10 knots. The helicopter descends in turbulent air that has just been
accelerated downward by the rotor. This turbulence reacts upon the main rotor blades at increased
angles of attack and stalls the blades at the main rotor hub. The stall then progresses outward along the
blades as the rate of descent increases. To recover from settling with power, reduce collective pitch,
lower the helicopter's nose into the wind, and, as airspeed increases, start a slow right turn.
In general, the fuselage of the helicopter is suspended from a single point and has considerable mass.
Because of this it has some freedom to move laterally or longitudinally, a pendular action. However,
correct aviator cyclic control movements, free of overcontrol, cause the rotor tip path and the fuselage to
move in unison.
For semirigid helicopters, the normal corrective device is the addition of a synchronized elevator
attached to the tail boom and operated by the cyclic stick.
A helicopter hovers when it maintains a constant position at a selected point in the air, usually a few
feet above the ground. To hover, a helicopter main rotor must supply lift to equal the total weight of the
helicopter, including crew; fuel; passengers and cargo, if carried; and armament, if armed. The
necessary lift is created by rotating the blades at high velocity and increasing the blade angle of attack.