Figure 5-1

CHAPTER 5

HELICOPTER AERODYNAMICS WORKBOOK

RETREATING BLADE STALL

A tendency for the retreating blade to stall in forward flight is inherent in all present-day

helicopters, and a major factor in limiting their forward speed. Just as the stall of an airplane

wing limits the low speed possibilities of an airplane, the stall of a rotor blade limits the high

speed potential of a helicopter (figure 5-1).

Figure 5-1

The airspeed of the retreating blade (the blade moving away from the direction of flight)

slows down as forward speed increases. The retreating blade must still produce an amount of lift

equal to that of the advancing blade. Therefore, as the airspeed of the retreating blade decreases

with forward speed, the blade AOA must be increased to equalize lift throughout the rotor disk.

As this AOA increase continues, the retreating blade will stall before the advancing blade at

some high forward speed.

As forward airspeed increases, the "no lift" area moves further left of center of the disk,

covering more of the retreating blade sector. This places a demand for greater lift from the outer

area of the retreating side. In the area of reversed flow, the rotational velocity of the airfoil is

slower than the aircraft airspeed; therefore, the air flows from the trailing edge to the leading

edge of the airfoil. In the negative stall area, the rotational velocity of the airfoil is faster than

the aircraft airspeed, allowing air to flow from the leading to the trailing edge. However, due to

the relative arm and induced flow, blade flapping is not sufficient to produce a positive AOA.

Blade flapping and rotational velocity in the negative lift area are sufficient to produce a positive

AOA, but not to a degree which produces appreciable lift.

5-2 FLIGHT PHENOMENA