This flight condition is similar to an airplane gliding if its engine fails while in flight. As long as the helicopter maintains forward airspeed, while decreasing altitude, and the pilot lowers the blade angle on the blades with the collective pitch, the rotor blades will continue to rotate. The altitude of the helicopter, which equals potential energy, is given up in order to have enough energy kinetic energy to keep the rotor blades turning.
As the helicopter nears the ground, the cyclic pitch control is used to slow the forward speed and to flare the helicopter for landing. With the airspeed bled off, and the helicopter now close to the ground, the final step is to use the collective pitch control to cushion the landing. The airflow through the rotor blades in normal forward flight and in an autorotation flight condition are shown in Figure In Figure , a Bell Jet Ranger is shown approaching the ground in the final stage of an autorotation.
If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor allowing the main rotor to rotate freely. The manufacturer specifies a minimum and maximum RRPM for each helicopter type.
The normal RRPM range is marked on the RPM gauge as a green arc, with permitted cautionary ranges marked in yellow or amber and enclosed by a red mark indicating the minimum and maximum permitted.
At the instant of engine failure, the main rotor blades are producing lift and thrust by a combination of their angle of attack and velocity. When engine power fails, the drag component will rapidly reduce the rotor speed. The Flight Manual will stipulate a minimum RRPM, below which, if the rotor speed reduces, it may be impossible to recover RRPM to a flight value: the rotor will stall entirely and cease to rotate.
The pilot must, therefore, if engine power fails, immediately reduce collective pitch and thus decrease both lift-induced and blade profile drag , as a result of which 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 changes the lift and drag vectors along the span of the blades to produce an inboard section where the drag acts in the plane of rotation of the blades: and thus keeps them turning.
This provides sufficient thrust to maintain rotor RPM throughout the descent, whilst also producing some lift. Nevertheless, rates of descent in autorotation are typically fpm in many helicopters, and may be higher in some. Since the tail rotor is driven by the main rotor transmission during autorotation, balance is maintained as in normal flight. The primary control of the rate of descent is airspeed. Higher or lower airspeeds are obtained with cyclic stick control of pitch attitude, just as in normal flight.
In theory, the pilot has a choice of airspeeds to vary the angle of descent, 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 to 70 knots for most light and medium helicopters, depending upon the particular helicopter type and the factors just mentioned. As the airspeed increases beyond that which gives minimum rate of descent, the rate of descent increases again. Such conditions may give rise to additional range in autorotation, and, as RRPM rises with increasing airspeed, the RRPM may be controlled at a reduced value within the Flight Manual limits by using additional collective pitch: this will normally maximise range.
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. Additionally, there will be a speed in autorotation above which the aft-dragging sections of the rotor blades extend along the blade span to the extent that the rotor will now begin to slow markedly.
This airspeed will normally be expressed as a Flight Manual airspeed limitation for autorotation. Since RRPM is at its lowest value in a zero airspeed or low airspeed autorotation, and there is no effective airspeed for a flare manoeuvre prior to touchdown discussed below , there may be insufficient inertia in the rotor system to dissipate the rate of descent before touchdown.
This is particularly the case in helicopters with low-inertia rotor systems, such as, commonly, the R22, Rotorway models, Enstrom models and some others.
Secondly, the range speed autorotation puts the helicopter into a configuration that will need to be modified in a timely manner in order for the pilot to execute an autorotative or Engine-Off landing. The helicopter will need to be decelerated to a suitable ground speed for touch down and ground-run along the landing area, either on its skid or wheeled undercarriage. Just as in the aeroplane case, the helicopter pilot may simply re-apply power to initiate a go-around at any height, and except in cases where the throttle must be manipulated by the pilot in co-ordination with an application of collective pitch in most cases this will simply be matter of raising the collective lever and setting climbing power, with an appropriate adjustment of the pitch attitude.
Autorotations to touch-down, or Engine-Off Landings EOL are practiced routinely in almost all single-engine helicopters and are a required manoeuvre for Skills Tests. When conducting EOL training the instructor or examiner will retard the throttle or inhibit the engine governing system, so that the engine remains at idle power when the collective lever is raised.
Where turns are carried out in order to make good the chosen landing area, the aim should always be for the helicopter to be lined up with the landing area by no later than ft agl in steady autorotation, at the recommended IAS which will normally be a few knots higher than the minimum Rate of Descent IAS in order to maximize the benefits of the flare, discussed below.
At a suitable height typically between 40 and ft depending on helicopter type airspeed is reduced to a comfortable speed for a run-on landing using a decelerative, nose-up flare attitude. The flare has the benefit both of reducing forward speed and increasing RRPM during the flare, which will increase the stored energy in the rotor: necessary to cushion the touchdown.
The additional lift created during the flare reduces the rate of descent. The speed at touchdown and the resulting ground run depends on the rate and amount of flare.
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