Adverse yaw is an aerodynamic characteristic in aircraft flight dynamics. Understanding this phenomenon is important for comprehending aircraft control and the techniques pilots use to achieve smooth, coordinated flight. It is an inherent aerodynamic characteristic that aircraft designers and pilots must account for to ensure stable operation.
Understanding Adverse Yaw
Adverse yaw describes an aircraft’s tendency to yaw, or rotate around its vertical axis, in the direction opposite to an intended roll. When a pilot initiates a turn by deflecting the ailerons to bank, the nose initially swings away from the desired turn. For example, if an aircraft begins to roll to the right, its nose will momentarily yaw to the left. This effect is adverse because it counteracts a coordinated turn.
This phenomenon is a direct consequence of how ailerons generate roll. Ailerons are movable control surfaces on the wing’s trailing edge. When a pilot commands a roll, one aileron deflects downward while the other deflects upward. This differential movement creates a lift imbalance, causing the aircraft to bank. However, this lift differential also creates unequal drag, leading to the adverse yawing motion.
The Aerodynamic Cause
The primary cause of adverse yaw is the differential drag produced by ailerons during a roll. When an aileron is deflected downward, it increases its angle of attack. This increase in lift is accompanied by a significant increase in induced drag. Conversely, the aileron on the opposite wing deflects upward, which decreases its angle of attack, reducing both lift and drag.
As the wing with the downward-deflected aileron generates more lift, it also experiences substantially more induced drag. This increased drag on the “down” wing pulls that wing backward, causing the aircraft’s nose to yaw towards that wing, away from the direction of the roll. For instance, in a right roll, the left aileron moves down, creating more lift and significantly more induced drag on the left wing. This higher drag on the left wing pulls the nose to the left, which is adverse yaw.
Mitigating Adverse Yaw
Pilots primarily counteract adverse yaw through coordinated use of the rudder. When initiating a turn, pilots simultaneously apply rudder input in the same direction as the desired roll. This rudder input generates a yawing force that directly opposes adverse yaw, aligning the aircraft’s nose with its flight path and ensuring a coordinated turn.
Aircraft designers also incorporate features to minimize adverse yaw. Differential ailerons are one common design where the upward-moving aileron deflects a greater distance than the downward-moving one. This design increases the drag on the ascending wing, balancing the drag from the descending wing. Another design is the Frise aileron, which, when the aileron moves up, its leading edge protrudes into the airflow, creating additional drag. This added drag counteract the increased induced drag from the downward-deflected aileron, reducing adverse yaw.
Impact on Aircraft Control
If adverse yaw is not properly managed, it leads to uncoordinated flight, a condition where the aircraft’s nose is not aligned with its flight path. In an uncoordinated turn, the aircraft experiences a sideslip. This sideslip increases the overall drag on the aircraft, reducing flight efficiency and increasing fuel consumption.
Uncorrected adverse yaw also increases the pilot’s workload, requiring constant compensation. Uncoordinated flight can be uncomfortable for passengers, causing discomfort. At slower speeds, uncorrected adverse yaw can also increase the risk of a wing stall, which can be dangerous.