Unless they remember their ground school training, new pilots may be surprised at how their aircraft reacts as they engage their ailerons to make a turn. They may be caught wondering why if they were making a turn to the right, the aircraft suddenly seemed to have a mind of its own and pivoted to the left first. Sounds like it is time for a refresher on the topic of adverse yaw.
What is adverse yaw?
Adverse yaw is the tendency of a fixed-wing aircraft to yaw in the opposite direction as you roll into a banked turn. If you roll the plane to the right, it will have an automatic tendency to make an unintended yawing motion to the left and vice versa.
This adverse yaw tendency exists to varying degrees on all fixed-wing aircraft although it is less pronounced on planes with shorter wings and those flying at higher speeds.
What causes adverse yaw?
Adverse yaw is caused by a lift and drag differential between your two wings. To initiate a right banking turn, you need to roll the plane to the right. This is done by raising the right aileron and lowering the left. Raising an aileron decreases both the lift and drag it generates. In our example, raising your right aileron will cause the right-wing to dip, initiating your right roll.
In contrast, lowering an aileron generates more lift and correspondingly more induced drag. The reason a lowered aileron generates more lift is that it alters the chord line and increases the angle of attack. At a higher angle of attack, more lift is generated. When we lower our left aileron, it will result in the left-wing lifting.
If the drag decrease on one wing was perfectly balanced by the drag increase on the other, the plane would simply roll without yawing, however, in real life, there is a drag differential between the two wings. A drag differential occurs because the raised aileron is deflecting into lower pressure airflow and the lowered aileron is deflecting into higher pressure airflow. If the amount of deflection is the same, the drag on the lowered aileron will be greater than it is on the higher aileron. This causes the plane to yaw in the direction of the lowered aileron which is the opposite direction of your roll.
How does aileron design impact adverse yaw?
The two main types of ailerons in use today were both purposefully designed with countering and minimizing adverse yaw in mind. Frise and differential aileron designs can be used solo or combined so their benefits work in a tandem hybrid configuration.
Since the adverse yaw is caused by a drag differential between the up and the down aileron, a frise aileron is designed to create more drag on the up aileron. This helps balance the higher degree of drag experienced by the lowered aileron. The increased drag is achieved by setting the aileron hinge point backward. When the aileron is raised, part of it protrudes lower than the bottom of the wing. The air hits this protrusion and generates extra drag.
On a differential aileron design, adverse yaw is decreased through limiting the aileron’s downward range of motion compared to its upward range. When one aileron is fully raised and the other is fully lowered, the raised aileron will move up further than the lowered aileron goes down. By decreasing the range of motion of the lowered aileron, the amount of generated drag is also reduced so it more closely matches the drag of the raised aileron. This decreases the amount of drag differential and thus the severity of the adverse yaw.