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3.15.  AUTOROTATION

If engine power fails, or certain other emergencies occur, autorotation is a means of safely landing a helicopter. The transmission in a helicopter is designed to allow the main rotor to turn freely in its original direction when the engine stops. Figure 3.12 illustrates how the helicopter is allowed to glide to earth and by using the main rotor rpm, make a soft landing.

Figure 3.12. Approach to Landing, Power Off.

The rotor blade autorotative driving region is the portion of the blade between 25 to 70 percent radius, as shown in figure 3.13, blade element A. Because this region operates at a comparatively high angle of attack, the result is a slight but important forward inclination of aerodynamic force. This inclination supplies thrust slightly ahead of the rotating axis and tends to speed up this portion of the blade during autorotation.

Figure 3.13. Autorotation Blade Forces.

The blade area outboard of the 70 percent circle is known as the propeller or driven region. Analysis of blade element B in figure 3.13 shows that the aerodynamic force inclines slightly behind the rotating axis. This inclination causes a small drag force that tends to slow the tip portion of the blade. Rotor rpm stabilizes, or achieves equilibrium, when autorotative force and antiautorotative force are equal.

The blade area inboard of the 25 percent circle is known as the stall region because it operates above its maximum angle of attack. This region contributes considerable drag that tends to slow the blade.

All helicopters carry an operators manual (-10) that has an air speed versus altitude chart similar to the one shown in figure 3.14. The shaded areas on this chart must be avoided. The proper maneuvers for a safe landing during engine failure cannot be accomplished in these areas.

Figure 3.14. High Velocity Diagram.

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