How exactly does flapping change a rotor blade’s angle of attack? That was a great question with many good explanations provided by readers. I think to fully understand it is important to know the difference between pitch angle and angle-of-attack. Pitch angle is the angle between the rotor blade’s chord line (a straight line intersecting the leading and trailing edges of an airfoil) and a reference plane of rotation. Angle-of-attack is the angle between the rotor blade’s chord line and the relative wind (the airflow that results from, and is opposite of, the velocity of an airfoil. Velocity is used here as a vector to mean speed and direction.)
When the rotor blades stay in the reference plane of rotation the pitch angle and angle-of-attack are the same. The pilot controls the pitch angle with the collective control and thus the angle-of-attack as well. However, when a rotor blade leaves the plane of rotation (flapping causes this to happen) the direction component of its velocity changes. Since relative wind is a function of velocity, it changes as well. In the case of a blade that flaps up the relative wind moves opposite the blade’s new direction. This change in relative wind direction reduces the blade’s angle-of-attack. The opposite is true for the blade that flaps down on the retreating side.
As the helicopter’s forward speed continues to increase, the retreating, or down flapping, side encounters higher angles of attack. Eventually, the rotor system encounters retreating blade stall.
From the pilot’s perspective, when this happens an abnormal vibration will be felt, the nose can pitch up, and the helicopter can have a tendency to roll in the direction of the stalled side. The amount and severity of pitch and roll will vary depending on the rotor system design.
The tendency for the nose to pitch up is because the spinning rotor system acts like a gyroscope and therefore experiences gyroscopic precession (a physical property that states when an external force is applied to a rotating body the effect will happen approximately 90 degrees later in the direction of rotation). As such, when the retreating blade stalls and stops producing lift, the effect of this happens toward the rear of the rotor disc. This causes the disc to tilt back, and the nose to pitch up.
Conditions like high density altitude, steep or abrupt turns, high blade loading (caused by high gross weight), turbulent air and low rotor rpm will increase the likelihood of encountering retreating blade stall when operating close to a helicopter’s Vne (never exceed speed). Helicopter flight manuals contain a chart or textual description in the limitations section that reduce the helicopter’s Vne at higher altitudes and temperatures. This is the airspeed limitation chart from a Bell 407.
Should a pilot encounter retreating blade stall, lower the collective and reduce airspeed. Other actions that will help are increasing rotor rpm and decreasing the severity of any roll or pitch maneuvers. Taking immediate action at the first sign will normally result in a quick recovery. However, if a pilot attempts to increase speed a severe stall would develop with possible loss of control.
Tags: Tim McAdams