Most autorotations are performed with forward airspeed. For simplicity, the following aerodynamic explanation is based on a vertical autorotative descent (no forward airspeed) in still air. Under these conditions, the forces that cause the blades to turn are similar for all blades regardless of their position in the plane of rotation. Dissymmetry of lift resulting from helicopter airspeed is therefore not a factor, but will be discussed later.
During vertical autorotation, the rotor disk is divided into three regions:
The force vectors are different in each region, because the rotational relative wind is slower near the blade root and increases continually toward the blade tip. When the inflow up through the rotor combines with rotational relative wind, it produces different combinations of aerodynamic force at every point along the blade.
In the driven region, the total aerodynamic force acts behind the axis of rotation, resulting in an overall dragging force. This area produces lift but it also opposes rotation and continually tends to decelerate the blade. The size of this region varies with blade pitch setting, rate of descent, and rotor RPM. When the pilot takes action to change autorotative RPM, blade pitch, or rate of descent, he is in effect changing the size of the driven region in relation to the other regions.
Between the driven region and the driving region is a point of equilibrium. At this point on the blade, total aerodynamic force is aligned with the axis of rotation. Lift and drag are produced, but the total effect produces neither acceleration nor deceleration of the rotor RPM. Point "D" is also an area of equilibrium in regard to thrust and drag.
Area "C" is the driving region of the blade and produces the forces needed to turn the blades during autorotation. Total aerodynamic force in the driving region is inclined forward of the axis of rotation and produces a continual acceleration force. Driving region size varies with blade pitch setting, rate of descent and rotor RPM. The pilot controls the size of this region in relation to the driven and stall regions in order to adjust autorotative RPM. For example, if the collective pitch stick is raised, the pitch angle will increase in all regions. This causes the point of equilibrium "B" to move toward the blade tip, decreasing the size of the driven region. The entire driving region also moves toward the blade tip. The stall region becomes larger and the total blade drag is increased, causing RPM decrease.
A constant rotor RPM is achieved by adjusting the collective pitch control so blade acceleration forces from the driving region are balanced with the deceleration forces from the driven and stall regions.
Because of lower angles of attack on the advancing side blade, more of that blade falls into the driven region. On the retreating side blade, more of the blade is in the stall region, and a small section near the root experiences a reversed flow. The size of the driven region on the retreating side is reduced.
Autorotations may be divided into three distinct phases; the entry, the steady state descent, and the deceleration and touchdown. Each of these phases is aerodynamically different than the others. The following discussion describes forces pertinent to each phase.
Entry into autorotation is performed following loss of engine power. Immediate indications of power loss are rotor RPM decay and an out-of-trim condition. Rate of RPM decay is most rapid when the helicopter is at high collective pitch settings. In most helicopters it takes only seconds for the RPM decay to reach a minimum safe range. Pilots must react quickly and initiate a reduction in collective pitch that will prevent excessive RPM decay. A cyclic flare will help prevent excessive decay if the failure occurs at thigh speed. This technique varies with the model helicopter. Pilots should consult and follow the appropriate aircraft Operator's Manual.
The following figure shows the airflow and force vectors for a blade in powered flight at high speed:
Note that the lift and drag vectors are large and the total aerodynamic force is inclined well to the rear of the axis of rotation. If the engine stops when the helicopter is in this condition, rotor RPM decay is rapid. To prevent RPM decay, the pilot must promptly lower the collective pitch control to reduce drag and incline the total aerodynamic force vector forward so it is near the axis of rotation.
The following figure shows the airflow and force vectors for a helicopter just after power loss:
The collective pitch has been reduced, but the helicopter has not started to descend. Note that lift and drag are reduced and the total aerodynamic force vector is inclined further forward than it was in powered flight. As the helicopter begins to descend, the airflow changes. This causes the total aerodynamic force to incline further forward. It will reach an equilibrium that maintains a safe operating RPM. The pilot establishes a glide at the proper airspeed which is 50 to 75 knots, depending on the helicopter and its gross weight. Rotor RPM should be stabilized at autorotative RPM which is normally a few turns higher than normal operating RPM.
The following figure shows the helicopter in a steady state descent:
Airflow is now upward through the rotor disk due the descent. Changed airflow creates a larger angle of attack although blade pitch angle is the same as it was in the previous picture, before the descent began. Total aerodynamic force is increased and inclined forward so equilibrium is established. Rate of descent and RPM are stabilized, and the helicopter is descending at a constant angle. Angle of descent is normally 17 degrees to 20 degrees, depending on airspeed, density altitude, wind, the particular helicopter design, and other variables.
The following figure illustrates the aerodynamics of autorotative deceleration:
To successfully perform an autorotative landing, the pilot must reduce airspeed and rate of descent just before touchdown. Both of these actions can be partially accomplished by moving the cyclic control to the rear and changing the attitude of the rotor disk with relation to the relative wind. The attitude change inclines the total force of the rotor disk to the rear and slows forward speed. It also increases angle of attack on all blades by changing the inflow of air. As a result, total rotor lifting force is increased and rate of descent is reduced. RPM also increases when the total aerodynamic force vector is lengthened, thereby increasing blade kinetic energy available to cushion the touchdown. After forward speed is reduced to a safe landing speed, the helicopter is placed in a landing attitude as collective pitch is applied to cushion the touchdown.