AutorotationThe ability to maintain and control rotor RPM in theevent of an engine malfunction so controlled flightmay be continued to the ground.Airflow during helicopter descent provides thenecessary energy to overcome blade drag and to turnthe rotor.The aviator gives up altitude at a controlled rate inreturn for the needed energy to turn the rotor at anRPM that provides aircraft control. Stated anotherway, the helicopter has potential energy by virtue ofits altitude.
UH OH! POOF!In powered flight, rotor drag is overcome with enginepower. When the engine fails, or is otherwisedisengaged from the rotor system, some other forcemust sustain rotor RPM so controlled flight can becontinued to the ground.
If a loss of power should occur with the helicopter in this condition, RPM decay is rapid.To prevent RPM decay, thecollective must be loweredimmediately to reduce the drag andincline the TAF vector forwardtoward the axis of rotation
Entry and DescentSpecific entry technique may varyand will be determined by suchfactors as airspeed, gross weight,density altitude and altitude abovethe landing surface.
Entry and Descent cont..From cruise altitudes and airspeeds, thecollective must be reduced and the cyclicadjusted to achieve an airspeed thatmaintains RRPM while affording areasonable glide distance and rate ofdescent.
Entry and Descent cont..Once a steady state autorotation has beenachieved, any movement of the cyclic willaffect Rotor RPM.Aft cyclic will initially increase R-RPM andforward cyclic will reduce RRPM. R-RPM willstabilize at some other value once cyclic inputsare stopped.
Maximum Glide Distance•Best Glide Distance is determined through flighttests•The specific speed at which a power-off glide willcover the maximum distance•Typically 4 to 1 (4 feet forward for every 1 foot ofdescent) Or One NM per 1,500’AGL•Rotor RPM Approximately 90%•Airspeed Approximately 75 KIAS
Minimum rate of descent•For each aircraft, there is an airspeed thatwill result in the minimum rate of descent.•The values for minimum rate of descent aredetermined through flight tests.•For the R-22 - 53KIAS•Values are very close to the airspeed forminimum drag.
Driven Region Driving Region30% of radius 45% of radius Blade regions in a vertical autorotationStall Region25% of radius
Stall Region•That area inboard of the 25% radius•Operates above the critical angle of attack•Contributes little vertical lift but somerotational drag
Driving Region•That blade region between approximately 25% and70% radius•Operates at comparatively high angles of attack•Resultant aerodynamic force is inclined slightlyforward of axis of rotation in the direction of rotation•Inclination of the total aerodynamic force provideshorizontal thrust in the direction of rotation and tendsto increase RRPM
Driven Region•The blade region outboard of the 70% radius•Operates at slightly less angle of attack than Drivingregion•Because of higher relative wind speed, providesmost of the vertical lift opposing weight•Inclination provides horizontal drag, opposite thedirection of rotation, which tends to decrease RRPM
Forward Driven Driving Autorotative regionsStall in forward flight. Regions incline towards the retreating side
Driven A Region A B Point of EquilibriumB, D Driving C Region C D Point of Equilibrium E Stall E Region
The rotor disk TAF is tiltedwell forward providing thenecessary thrust to propelthe helicopter at the desiredairspeed However, the individual blade segment TAF is inclined well aft of the axis of rotation. The engine is needed to overcome the drag forces generated by this situation.
RequirementsThe rotor system must be decoupled fromthe engine(s)This occurs if an engine malfunctions, or ifthe pilot retards the throttle, as in a simulatedengine failure.The collective must be lowered so the angleof attack will not become so excessive thatRPM will be lost.