Autorotation (helicopter) - Descent and Landing

Descent and Landing

For a helicopter, "autorotation" refers to the descending maneuver where 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 anytime the engine rpm is less than the rotor rpm. 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 an autorotation is an engine malfunction or failure, but autorotations 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 settling with power. In all cases, a successful landing depends on the helicopter's height and velocity at the commencement of autorotation (see height-velocity diagram).

At the instant of engine failure, the main rotor blades are producing lift and thrust from their angle of attack and velocity. By immediately lowering collective pitch, which must be done in case of an engine failure, the pilot reduces lift and drag and the helicopter begins an immediate descent, producing an upward flow of air through the rotor system. This upward flow of air through the rotor provides sufficient thrust to maintain rotor rpm throughout the descent. Since the tail rotor is driven by the main rotor transmission during autorotation, heading control is maintained as in normal flight. However, as noted above, there is virtually no torque generated during autorotation, so to maintain flight in a straight line the pilot must keep one anti-torque pedal pressed to eliminate the tail rotor's anti-torque thrust.

Several factors affect the rate of descent in autorotation: density altitude, gross weight, rotor rpm, and forward airspeed. The pilot's primary control of the rate of descent is airspeed. Higher or lower airspeeds are obtained with the cyclic pitch control just as in normal flight. Rate of descent is high at zero airspeed and decreases to a minimum at approximately 50 to 60 knots, depending upon the particular helicopter and the factors previously mentioned. As the airspeed increases beyond that which gives minimum rate of descent, the rate of descent increases again. Even at zero airspeed, the rotor is quite effective as it has nearly the drag coefficient of a parachute despite having much lower solidity.

When landing from an autorotation, the energy stored in the rotating blades is 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.

Each type of helicopter has a specific airspeed at which a power-off glide is most efficient. The best airspeed is the one which combines the greatest glide range with the slowest rate of descent. The specific airspeed is somewhat different for each type of helicopter, yet certain factors 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 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.