Adverse yaw is the tendency of an airplane to yaw in the opposite direction of the turn. For example, as you roll to the right, your airplane may initially yaw to the left.

So why does this happen?

When you roll your airplane to the right, your right aileron goes up, and your left aileron goes down. The aileron in the upward position (the right aileron in this example) creates less lift and less drag than the aileron that is lowered. The aileron angled downward (the left aileron in this example) produces more drag and more lift, initially yawing the airplane in the opposite direction of your roll.

Check out the image below to see exactly what's happening.

But why does lowering an aileron increase drag? Just like flaps, when you lower the aileron, you change the chord line of the wing, creating a higher angle of attack (AOA). As AOA and lift increase, induced drag also increases, because the drag created as an aileron is lowered is induced drag.

Countering Adverse Yaw

In a coordinated turn, adverse yaw is countered by using the rudder (in almost all cases, stepping on the rudder into the turn). When you add rudder input, you're creating a side force on the vertical tail that opposes adverse yaw. By adding rudder, you create a yawing moment that helps turn the airplane in the correct direction and stay coordinated.

Remember to look at the slip indicator on your instrument panel to make sure you're flying a coordinated turn to best counteract adverse yaw.

The rudder doesn't balance out the drag by creating more form drag opposite to the wing. Instead, the rudder creates lift (like a wing) to yaw the nose directly into the relative wind and counter the downward aileron's drag moment.

Ailerons Designed To Counteract Adverse Yaw

To counter the effects of adverse yaw, you'll find a few different aileron designs on GA airplanes. Here are the two most common designs:

1) Differential Ailerons: One aileron is raised a greater distance than the other aileron is lowered. The extra upward aileron movement produces more drag change than an increase in AOA on the downward aileron. This produces an increase in drag on the descending wing, which reduces adverse yaw.

2) Frise Ailerons: The aileron being raised pivots on an offset hinge. The leading edge of the aileron is now pushed into the airflow, creating drag and reducing adverse yaw. In this case, frise ailerons are using form drag to counter induced drag.

Some airplanes have more adverse yaw than others, and in general, the slower the plane, the more aileron is needed to bank the airplane. You can typically expect to find more adverse yaw tendencies on slow-flying aircraft.

How much adverse yaw does your plane have? The next time you're flying, roll into some turns, both left and right, and see how much rudder you need to add to stay coordinated. Once you know how much rudder you need in both directions, it'll be easier to anticipate and react to the turning tendencies of your plane.