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3.9.  RETREATING BLADE STALL

Figure 3.8 illustrates the tendency for the helicopter's retreating blade to stall in forward flight. It is a major factor in limiting their forward speed. Just as the stall of an airplane wing limits the low-airspeed possibilities of the airplane, the stall of a rotor blade limits the high-speed potential of a helicopter. The airspeed of the retreating blade slows down as forward airspeed is increased. The retreating blade must produce an amount of lift equal to that of the advancing blade, as shown in figure 3.8B. As the airspeed of the retreating blade is decreased with forward airspeed, the blade angle of attack must be increased to equalize lift throughout the rotor disc area. As this angle increase is continued, the blade will stall at some high forward airspeed as shown in figure 3.8C.

Figure 3.8. Retreating Blade Stall.

Upon entry into blade stall, the first effect is generally a noticeable vibration of the helicopter. This vibration is followed by the helicopter's nose lifting and a rolling tendency. If the cyclic stick is held forward and the collective pitch is not reduced, or is raised, the stall becomes aggravated, and the vibration increases greatly. Control of the helicopter may then be lost.

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