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ALC-104: Helicopter - General and Flight Aerodynamics
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Aerodynamics - Autorotations

Autorotation is the state of flight where the main rotor system is being turned by the force of the relative wind rather than engine power. It is the means by which a helicopter can be landed safely in the event of an engine failure. In this case, the potential energy of altitude is converted to kinetic energy during the descent and touchdown. All helicopters must have this capability in order to be certified. Autorotation is possible owing to a freewheeling unit, which allows the main rotor to continue turning even if the engine is not running. In normal powered flight, air is drawn into the main rotor system from above and exhausted downward. During autorotation, airflow enters the rotor disc from below as the helicopter descends. See figure 6-1.



Most autorotations are performed with some amount of forward speed. For simplicity, the following aerodynamic explanation is based on a purely vertical autorotative descent 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. Therefore, dissymmetry of lift resulting from helicopter airspeed is not a factor.

Figure 6-2 illustrates the blade sections and the force vectors acting on them. Region A is the driven region, points B and D are points of equilibrium, region C is the driving region, and region E is the stall region. Force vectors are different in each region because rotational relative wind is slower near the blade root and increases continually toward the blade tip. Also, blade twist gives a more positive angle of attack in the driving region than in the driven region. The combination of the inflow up through the rotor with rotational relative wind produces different combinations of aerodynamic force at every point along the blade.


The driven region, also called the propeller region, is nearest the blade tips. Normally, it consists of about 30 percent of the radius. In the driven region, the total aerodynamic force vector points aft of the axis of rotation, resulting in an overall drag force. The driven region produces some lift, but that lift is offset by drag. The overall result is a deceleration in the rotation of the blade. The size of this region varies with the blade pitch, rate of descent, and rotor RPM. When changing autorotative RPM, blade pitch, or rate of descent, the size of the driven region in relation to the other regions also changes.

There are two points of equilibrium, B and D, on the blade - one between the driven region and the driving region, and one between the driving region and the stall region. At points of equilibrium, total aerodynamic force is aligned with the axis of rotation. Lift and drag are produced, but the total effect produces neither acceleration nor deceleration.

The driving region or autorotative region illustrated in part C, normally lies between 25 to 70 percent of the blade radius, and produces the forces required to turn the blades during autorotation. Total aerodynamic force in the driving region is inclined slightly forward of the axis of rotation, producing a continual force, which tends to accelerate the rotation of the blade. Driving region size varies with blade pitch setting, rate of descent, and rotor RPM. Controlling the size of this region controls the autorotative RPM. If collective pitch is raised, the pitch angle increases in all regions. This causes the point of equilibrium B to move inboard along the blade’s span, thus increasing the size of the driven region. Equilibrium point D moves outboard, increasing the stall region while further decreasing the driving region. Reducing the size of the driving region causes the acceleration force of the driving region and RPM to decrease.

The inner 25 percent of the rotor blade is referred to as the stall region, illustrated by section E, and operates above its maximum angle of attack (stall angle), causing drag which tends to slow rotation of the blade. A constant rotor RPM is achieved by adjusting the collective pitch so blade acceleration forces from the driving region are balanced with the deceleration forces from the driven and the stall regions.



Autorotative force in forward flight is produced in exactly the same manner as when the helicopter is descending vertically in still air. However, because forward speed changes the inflow of air up through the rotor disc, all three regions move outboard along the blade span on the retreating side of the disc where angle of attack is larger, as shown in figure 6-3. With lower angles of attack on the advancing side blade, more of that blade falls in the driven region. On the retreating side, more of the blade is in the stall region and a small section near the root experiences reverse flow.