B l a d e r o t a t i o n B l a d e r o t a t i o n S t a l r e gi on 2 5 % $^{D}$r i v in g  re gio n 4 5 % D ri ve nr egio n  3 0 % Figure 2-47.  Blade regions during autorotational descent.  produces the forces needed 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 acceleration force. This inclination supplies thrust,  which tends to accelerate the rotation of the blade. Driving  region size varies with blade pitch setting, rate of descent,  and rotor rpm. By controlling the size of this region, a pilot can adjust  autorotative rpm. For example, if the collective pitch is raised,  the pitch angle increases in all regions. This causes the point  of equilibrium to move inboard along the blade’s span, thus  increasing the size of the driven region. The stall region also  becomes larger while the driving region becomes smaller.  Reducing the size of the driving region causes the acceleration  force of the driving region and rpm to decrease. 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 stall regions. The inner 25 percent of the rotor blade is referred to as the  stall region and operates above its maximum AOA (stall  angle), causing drag, which tends to slow rotation of the  blade. Part E of  Figure 2-48  depicts the stall region. driving region, and stall region.  Figure 2-48  shows three  blade sections that illustrate force vectors. Part A is the  driven region, B and D are points of equilibrium, part C is  the driving region, and part 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 AOA 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, part A of  Figure 2-48,  the  TAF acts behind 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 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, TAF 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, normally lies  between 25 to 70 percent of the blade radius. Part C of  Figure 2-48  shows the driving region of the blade, which  This chapter introduced the basics of aerodynamic  fundamentals and theory and how they relate to flying a  helicopter. This chapter also explained how aerodynamics  affect helicopter flight and how important it is for pilots to  understand aerodynamic principles and be prepared to react  to these effects. For additional information on aerodynamics,  refer to the aerodynamics of flight portion of the Pilot’s  Handbook of Aeronautical Knowledge. 2-26 Chapter Summary Prior to landing from an autorotative descent (or autorotation),  the pilot must flare the helicopter in order to decelerate. The  pilot initiates the flare by applying aft cyclic. As the helicopter  flares back, the airflow patterns change around the blades  causing the rpm to increase. Pilots must adjust the collective  as necessary to keep the rpm within operating limits. 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 disk, all three regions move  outboard along the blade span on the retreating side of the  disk where AOA is larger.  [Figure 2-49]  With lower AOA  on the advancing side blade, more of the blade falls in the  driven region. On the retreating side, more of the blade is in  the stall region. A small section near the root experiences a  reversed flow; therefore, the size of the driven region on the  retreating side is reduced. Autorotation (Forward Flight)