Helicopter Adverse Yaw

Adverse yaw is an unfavorable yaw change that can occur while an aircraft rolls. The phenomenon is most significant for fixed wing aircraft, but can play a minor roll in helicopters. In this article, we’ll explain adverse yaw and how it can occur in a helicopter.

Phenomenon

Adverse yaw occurs while an aircraft has a roll rate. In forward flight, a roll rate initiates a coordinated turn. For example, a pilot may roll right (right wing down) to start a rightward turn to change heading from north to east.

In this right turn, the helicopter needs to turn the nose (yaw) right, in addition to rolling right. For the most part, helicopters tend to do this automatically—a right roll naturally induces a right yaw via directional stability. However, there can be a natural tendency to do the opposite: to yaw left as the helicopter rolls right. This undesirable yaw is called adverse yaw.

We limit the discussion here to right rolls: right side down / left side up. Realize that the situation is symmetric and applies to left roll as well. That is, when a helicopter rolls the left side down, a nose right yaw would be adverse yaw. The causes are analogous so we won’t repeat them.

Cause

A right roll rate causes the right side of the main rotor to move down and the left side of the rotor to move up. This induces upward airflow through the right side / downward flow through the left side (more accurately it decreases the downward flow through the right side in most scenarios). This, in turn, increases the angle of attack of blades on the right side, and decreases it on the left side. Depending on which direction the rotor spins (clockwise or counterclockwise) (CW or CCW), this may increase or decrease the torque load on the main rotor. (To investigate this you can analyze an airfoil on each side of the rotor and how the inflow changes AOA and lift/drag.)

Consider a rotor that spins CCW when viewed from above (i.e. an American helicopter). Torque will decreases with right roll rate. The engine will quickly reduce torque to prevent the rotor from speeding up. This torque reduction from the engine reduces the rightward yaw moment that the engine applies to the helicopter, and hence yaws the helicopter nose left.

Again, the phenomena described applies to a roll rate, not a roll angle. It will therefore only play a role as a turn is initiated—once the target roll angle is established the rate becomes 0 and directional stability will yaw the helicopter in the desired direction.