In a helicopter, an autorotative descent is a power-off 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. [Figure 1-1] In other words, the engine is no longer supplying power to the main rotor.
|Figure 1-1. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal speed. In effect, the blades are “gliding” in their rotational plane|
The most common reason for an autorotation is failure of the engine or drive line, but autorotation may also be performed in the event of a complete tail rotor failure, since there is virtually no torque produced in an autorotation. In both areas, maintenance has often been a contributing factor to the failure. Engine failures are also caused by fuel contamination or exhaustion as well resulting in a forced autorotation.
If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor allowing the main rotor to rotate freely. Essentially, the freewheeling unit disengages anytime the engine revolutions per minute (rpm) is less than the rotor rpm.
At the instant of engine failure, the main rotor blades are producing lift and thrust from their angle of attack (AOA) and velocity. By lowering the collective pitch, which must be done immediately in case of an engine failure, lift and drag are reduced, and the helicopter begins an immediate descent, thus 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 with the antitorque pedals as in normal flight.
Several factors affect the rate of descent in autorotation: density altitude, gross weight, rotor rpm, and airspeed. The primary way to control the rate of descent is with airspeed. Higher or lower airspeed is obtained with the cyclic pitch control just as in normal powered flight. In theory, a pilot has a choice in the angle of descent varying from a vertical descent to maximum range, which is the minimum angle of descent. Rate of descent is high at zero airspeed and decreases to a minimum at approximately 50–60 knots, depending upon the particular helicopter and the factors just mentioned. As the airspeed increases beyond that which gives minimum rate of descent, the rate of descent increases again.
When landing from an autorotation, the only energy available to arrest the descent rate and ensure a soft landing is the kinetic energy stored in the rotor blades. Tip weights can greatly increase this stored energy. 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 and rotor rpm at which a power-off glide is most efficient. The specific airspeed is somewhat different for each type of helicopter, but certain factors affect all configurations in the same manner. In general, rotor rpm maintained in the low green area gives more distance in an autorotation. Higher weights may require more collective pitch to control rotor rpm. Some helicopters need slight adjustments to minimum rotor rpm settings for winter versus summer conditions, and high altitude versus sea level flights. For specific autorotation airspeeds and rotor rpm combinations for a particular helicopter, refer to the Federal Aviation Administration (FAA)-approved rotorcraft flight manual (RFM).
The specific airspeed and rotor rpm for autorotation is established for each type of helicopter on the basis of average weather, wind conditions, and normal loading. When the helicopter is operated with heavy loads in high density altitude or gusty wind conditions, best performance is achieved from a slightly increased airspeed in the descent. For autorotation 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 and rotor rpm to existing conditions, a pilot can achieve approximately the same glide angle in any set of circumstances and estimate the touchdown point.
Pilots should practice autorotations with varying airspeeds between the minimum rate of descent to the maximum glide angle airspeed. The decision to use the appropriate airspeed for the conditions and availability of landing area must be instinctive. The helicopter glide ratio is much less than that of a fixed wing aircraft and takes some getting used to. The flare to land at 55 KIAS will be significantly different than the flare from 80 KIAS. Rotor rpm control is critical at these points to ensure adequate rotor energy for cushioning the landing.
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