Autorotation In a helicopter, an autorotative descent is a power-off  maneuver in which the engine is disengaged from the  main rotor disk and the rotor blades are driven solely by  the upward flow of air through the rotor.  [Figure 11-1]  In  other words, the engine is no longer supplying power to  the main rotor.  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  cases, 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 it 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 (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 disk.  This upward flow of air through the rotor disk 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:  bank angle, density altitude, gross weight, rotor rpm, trim  condition, and airspeed. The primary ways to control the rate  of descent are with airspeed and rotor rpm. 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 straight vertical to maximum  horizontal 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.  Refer to the height/velocity diagram discussion in Chapter  7, Helicopter Performance. 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 (see  Figure 5-3 ) gives more distance in an autorotation.  Heavier helicopter weights may require more collective to  control rotor rpm. Some helicopters need slight adjustments  to minimum rotor rpm settings for winter versus summer  Normal Powered Flight Autorotation Direction of flight Direction of flight Figure 11-1.  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.  11-2