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).
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