Adverse yaw occurs when rolling of an aircraft in one direction, causing the yaw in the opposite direction. This unintended yaw can complicate flight maneuvers and can make coordinated turns difficult to execute. In this article, we will understand the effect of adverse yaw on coordinated flight and explore the ways to correct it using aileron design.

What is Adverse Yaw?

The ailerons on aircraft wings are usually placed at the outer portion of the wing span in order to maximize aileron control authority. The roll angle or bank angle produced by the aileron is caused by the rolling moment generated by the difference in lift generated by the opposite moving ailerons. The aileron control surface in the turn direction moves upward to slightly reduce the lift on the respective wing, while the opposite aileron trailing edge moves downward to generate a high lift.

The drag on aircraft has a lift dependent component due to profile drag and induced drag; it means high lift aileron sections of the wing also impose a higher drag and vice versa for opposite aileron. This difference in drag between the two aileron wing sections causes a yawing motion, which in turn causes side force on the airplane.

Effect on Coordinated Flight

Coordinated flight is essential for safe and efficient aircraft operation. In a coordinated turn, the aircraft rolls into the turn, the balance of lift is maintained, and no side force is generated. The sideslip and side-force in an uncoordinated turn can lead to situations such as a skid or a spin. Adverse yaw is prominent when entering into or leaving a turn because of maximum aileron deflection.  Therefore, understanding and correcting the adverse yaw is crucial for pilots. Some aircraft are more prone to the adverse yaw because of:

• High lifting wing section at aileron
• Low speed aircraft require high aileron deflections
• Airplanes with a large wing span and aileron placed at outward section of the wing

How to correct adverse yaw

Correcting adverse yaw requires a delicate balance between aileron inputs and rudder coordination to maintain a stable flight path. The pilot must apply the “rudder coordination” technique, which is to apply the rudder input in the direction of the turn. The amount of rudder is proportional to aileron and requires an instinctive action, which in terms of autopilot is called a “lead” or “feedforward” action. The effective rudder required for this correction can be calculated from the aerodynamic data:

Apart from the rudder input, the pilots can also “overbank” the aircraft so that the lift force vector into the turn plane can compensate for side force due to adverse yaw. This technique is not recommended due to the safety concern of baking beyond the aircraft limit.

Aileron Rudder Interconnect (ARI)

Many modern aircraft are equipped with an aileron-rudder interconnect system. This system connects the movement of the ailerons and rudder, providing direct (feed-forward) input to the net rudder surface. So, when the pilot moves and applies the aileron for the bank angle, the interconnect system automatically applies rudder input into the turn. The aileron-rudder interconnect can reduce the pilot’s workload and can also be beneficial during training of pilots.

Aileron Design 

The ailerons can be specifically designed to mitigate the adverse yaw tendency of an airplane. Two most common aileron designs are a differential aileron and a fries aileron configuration.

• Differential Ailerons: In differential aileron configuration, the upward moving aileron (one into the direction of turn) is deflected more as compared to the upward moving aileron (opposite of the direction of turn). This solution works on general aviation (GA) aircraft with camber airfoil on the aileron section. On such wing ross-sections, the change in airflow angle of incident on downward moving aileron produces much stronger lift/drag force as compared to upward moving aileron. A differential aileron is configured to equalize the effect of drag so that adverse yaw would be minimal.
• Frise Ailerons: In this aileron design, the hinge is placed slightly offset from the leading edge of the aileron. The leading edge of downward moving aileron in this case creates an extra drag that can balance out the drag on the other side of the wing.

Use of Spoiler in High Wing Span Aircrafts

The aircraft with a high wing span, such as airliners, have ailerons placed at the outboard portion of wings. Apart from aileron, such aircrafts have spoilers that can be used to spoil down the lift of the wing that can be beneficial in losing altitude during descent and reducing the airspeed. The use of spoilers for turns produces minimal adverse yaw compared to the ailerons.

The spoiler when deployed on a single wing can be an alternative to ailerons, especially during low-speed turns. Here the main benefit of spoiler is to reduce the workload on the ailerons that can additionally be used as flaps. When the spoiler is used as an aileron, it is only deployed on one side of the wing, specifically into the direction of turn. Compared to the ailerons, the spoilers have less drag and less adverse yaw while achieving similar bank angle performance.

Modern Flight Control System

Modern aircraft are equipped with advanced flight control technology that automates many aspects of flight handling. The advent of fly-by-wire (FBW) systems where the stick input is converted to the electronic signals and translated to electro-mechanical or electro-hydraulic actuators at the control surfaces. Therefore, the physical type of aileron-rudder interconnect (ARI) is seldom used. An example of ARI control implementation along with yaw damper is given below (Ref: Relook at Aileron to Rudder Interconnect)

In electronic FBW, ARI is implemented as a feed-forward input to the rudder servo from the feedback of aileron deflection. This implementation can also improve the dutch roll performance of the aircraft. Use of yaw damper and active rudder control with an advanced feedback control system (see the coordinated turns) can provide necessary lead in rudder action and may reduce the need for ARI owing to the added complexity of the system.