For a helicopter, "autorotation" refers to the descending maneuver in which the engine is disengaged from the main rotor system and the rotor blades are driven solely by the upward flow of air through the rotor. The
freewheeling unit is a special clutch mechanism that disengages any time the engine rotational speed is less than the rotor rotational speed. If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor, allowing the main rotor to rotate freely. The most common reason for autorotation is an engine malfunction or failure, but autorotation can also be performed in the event of a complete
tail rotor failure, or following
loss of tail-rotor effectiveness, since there is virtually no
torque produced in an autorotation. If altitude permits, autorotations may also be used to recover from a
vortex ring state, also known as
settling with power. despite consisting of blades. When landing from an autorotation, the kinetic energy stored in the rotating blades and the forward movement of the aircraft are used to decrease the rate of descent and make a soft landing. A greater amount of rotor energy is required to stop a helicopter with a high rate of descent than is required to stop a helicopter that is descending more slowly. Therefore, autorotative descents at very low or very high airspeeds are more critical than those performed at the minimum rate of descent airspeed. An autorotation landing consists of multiple stages: glide, flare, landing. A stable glide at an appropriate speed for the helicopter is maintained during descent. At some altitude above the ground, typically 50–100 feet, the pilot raises the nose and applies up collective. Raising the nose leads to more airflow through the rotor and raising the collective converts this airflow into more rotor lift which will slow the helicopter's forward speed. Raising the collective also keeps the rotor RPM from exceeding redline. Depending on the helicopter, before airspeed has completely decayed, the helicopter is leveled. Most helicopters at this point will still have forward velocity and will perform a run-on landing using the remaining rotor momentum. Some helicopters have enough momentum in the rotor system that the helicopter can be brought to a standstill and lowered to the ground with no forward velocity. Each type of helicopter has a specific airspeed at which a power-off glide is most efficient. The best airspeed is the one that combines the greatest glide range with the slowest rate of descent. The specific airspeed is different for each type of helicopter, yet certain factors (density altitude, wind) affect all configurations in the same manner. The specific airspeed for autorotations is established for each type of helicopter on the basis of average weather and wind conditions and normal loading. A helicopter in autorotation can be flown backwards, sideways or vertically with no lateral velocity. A
slip can be used to decrease the glide angle. A helicopter operated with heavy loads in high density altitude or gusty wind conditions can achieve best performance from a slightly increased airspeed in the descent. At low density altitude and light loading, best performance is achieved from a slight decrease in normal airspeed. Following this general procedure of fitting airspeed to existing conditions, the pilot can achieve approximately the same glide angle in any set of circumstances and estimate the touchdown point. This optimum
glide angle is usually 17–20 degrees.
Autorotational regions During vertical autorotation, the rotor disc is divided into three regions—the driven region, the driving region, and the stall region. The sizes of these regions vary with the blade pitch, rate of descent, and rotor rotational speed. When changing autorotative rotational speed, blade pitch, or rate of descent, the sizes of the regions change in relation to each other. The driven region, also called the propeller region, is the region at the end of the blades. Normally, it consists of about 30 percent of the radius. It is the driven region that produces the most drag. The overall result is a deceleration in the rotation of the blade. The driving region, or autorotative region, normally lies between 25 and 70 percent of the blade radius, which produces the forces needed to turn the blades during autorotation. Total aerodynamic force in the driving region is inclined slightly forward of the axis of rotation, producing a continual acceleration force. This inclination supplies thrust, which tends to accelerate the rotation of the blade. Driving region size varies with blade pitch setting, rate of descent, and rotor rotational speed. The inner 25 percent of the rotor blade is referred to as the stall region and operates above its maximum angle of attack (stall angle) causing drag, which slows rotation of the blade. A constant rotor rotational speed is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the deceleration forces from the driven and stall regions. By controlling the size of the driving region, the pilot can adjust autorotative rotational speed. For example, if the collective pitch is raised, the pitch angle increases in all regions. This causes the point of equilibrium to move inboard along the blade's span, thereby increasing the size of the driven region. The stall region also becomes larger while the driving region becomes smaller. Reducing the size of the driving region causes the acceleration force of the driving region and rotational speed to decrease. == Broken Wing Award ==