As a flight instructor and card-carrying aerodynamics geek, adverse yaw is something I thought I knew all about. I can smoothly deliver the checkride-ready explanation for what it is and how airplanes are designed to correct for it. So, I was a little shocked when I recently discovered some of my ideas about adverse yaw were wrong, and I didn’t understand the concept as well as I thought. Somehow this eluded me for almost two decades, but I’m always excited to learn something new.
As you may know, adverse yaw is an effect of aileron movement. When an airplane rolls one direction, the deflected ailerons create a yaw in the opposite direction. The down-turned aileron increases the drag on the rising wing while the opposite up-turned aileron reduces drag on the lowering wing. This imbalance in drag pulls the rising wing back. As a result, when an airplane is rolling in one direction it begins to yaw in the opposite direction. This behavior is called “adverse aileron yaw” or simply “adverse yaw.”
Rolls from the Netherlands
My big epiphany came during a flight. I had recently watched a webinar put on by the Society of Aviation and Flight Educators (SAFE) on stall and spin awareness. One of the topics discussed by the distinguished panel of pilots and flight instructors was a coordination exercise known as Dutch Rolls. The purpose of performing Dutch Rolls is to sharpen pilots’ coordination skills when rolling into and out of turns – specifically effectively countering adverse aileron yaw. Note that “Dutch Roll” also refers to an instability characteristic common in swept wing aircraft, so some argue that the coordination exercise is more correctly referred to as “Rolls on a Heading.” Also note that neither type of Dutch Roll involves the Netherlands or bread and butter.
Dutch Rolls are performed by smoothly banking the airplane left and right in a repeating pattern. With each deflection of the ailerons the airplane will have a tendency to yaw. The objective is to prevent any yaw through coordinated application of rudder pressure. The Dutch Roll exercise presented a great opportunity to better understand adverse yaw.
It wasn’t all that subtle. In fact it was plain as day once I was looking for it.
A Little Experiment
When I decided to try out Dutch Rolls myself, I thought I was going to have difficulty seeing the effect of adverse yaw in the training airplanes I flew. Most aircraft incorporate design features intended to minimize adverse yaw, and apparently I had come to believe that the effect virtually was imperceptible in most light training airplanes. As I quickly discovered, this is untrue.
To begin I picked a reference point just ahead of the nose and compared it to a cowl screw near the nose. I started by rolling my wings back and forth about 5 degrees either side of level with my feet flat on the floor I watched my reference point carefully. As I turned the control wheel the nose yawed away from the bank. When I turned back the nose slid the other way. It wasn’t all that subtle. In fact it was plain as day once I was looking for it.
Oh, That Adverse Yaw
How have I missed this for so long? Well, the little nose waggle upon rolling into a turn was, in fact, familiar. I’ve noticed it before, but somehow I never connected what I was seeing with my book knowledge of adverse yaw. In my mind adverse yaw “looked” different. I thought it was subtle, invisible, or had been completely engineered out of airplanes. I thought that because I didn’t realized I have actually seen adverse yaw before. The missing puzzle piece was literally right in front of my nose all this time. This misunderstanding probably hindered development of my own coordination skills, not to mention my students’. I’m a well trained pilot and conscientious instructor. If I’ve missed this, I’m certain I’m not the only one.
After having demonstrated what adverse yaw looks like, I repeated the maneuver. This time I attempted to cancel the adverse yaw with rudder input. I wanted to hold the nose still without permitting any yaw. In my first attempt I over-corrected with rudder, skidding the airplane into each roll. I was countering the slight yaw from minimally deflected ailerons with a huge rudder, so I lightened my touch and found my rhythm. With a few minutes practice, I was able to prevent around 80% of the yaw about 80% of the time.
Next I just did some “normal” rolls into and out of turns – I flew like I normally do without any special effort to do anything new. Overall I did a decent job of coordinating my rolls and turns with skills I had developed in spite of my adverse yaw ignorance. Convinced I wasn’t a complete buffoon after all, I continued practicing Dutch Rolls trying to perfect them. Dutch Rolls are easy to practice yet very difficult to master. I imagine most pilots, like me, do a good approximation of canceling the adverse yaw during rolls which has always been “close enough.” Now I have a new target to shoot for.
This misunderstanding probably hindered development of my own coordination skills, not to mention my students’. I’m a well trained pilot and conscientious instructor. If I’ve missed this, I’m certain I’m not the only one.
The Dutch Roll exercise taught me a few things that surprised me:
- It’s Pretty Obvious. I actually have seen this effect all along but never knew what it was nor understood its significance. I’ve seen the little nose waggle upon rolling into a turn now and then. For some reason I never made the connection between that observed motion and adverse yaw. I actually thought that common training airplanes didn’t exhibit much adverse yaw thanks to the design of their aileron systems. Undoubtedly it helps, but the yaw is still so obviously
- It’s Immediate. The yaw occurs instantly – responding perfectly to any change in aileron position. The more aileron deflection, the more yaw. This surprised me because I had envisioned the adverse yaw lagging behind the roll. I imagined this was similar to how the airplane responds to rudder-only inputs. Applying rudder produces an immediate yaw followed by a roll moments later. In that case the rudder causes the yaw which then creates the condition that produces the roll. One happens and then the other. This is not so with adverse yaw. Instead, aileron deflection causes both the roll and yaw directly – the effects are both immediate and simultaneous.
- It’s sensitive. Adverse yaw is pronounced and subtle at the same time. By that I mean that the effect is easily seen (when you’re looking for it) but correcting for it requires fine rudder inputs. Dutch rolls are a precision maneuver. To really nail them requires a degree of precision and finesse that exceeds most other maneuvers. Rudder inputs should be both quick and small – this is a recurring theme in good rudder use.
As mentioned above, adverse yaw is a byproduct of aileron movement. Each aileron moves in opposite directions – when the left one goes up, the right one goes down and vice versa. Ailerons work by increasing lift on one wing and decreasing it on the other wing causing the airplane to roll around it’s longitudinal (nose to tail) axis.
When an aileron is deflected downward, it effectively increases the angle of attack for that section of the wing by moving the trailing edge downward and changing the angle of the chord line. This increases lift but also produces a corresponding increase in induced drag. On the opposite wing, the up-turned aileron reduces the angle of attack for its section of the wing. This reduces lift with a corresponding reduction in induced drag. The wing with the downturned aileron now has higher lift and drag than the wing with the upturned aileron. This causes the airplane to roll toward the wing with the up-turned aileron and yaw in the opposite direction.
I imagine most pilots, like me, do a good approximation of canceling the adverse yaw during rolls which has always been “close enough.” Now I have a new target to shoot for.
Aileron system design mitigates some Adverse Yaw. Designers engineered two main approaches to reducing adverse yaw:
- Differential ailerons are rigged so the upturned aileron deflects further than the down-turned aileron. This projects more of the up-turned aileron into the airflow which increases its profile drag. By deflecting the down-turned aileron less, you reduce both the induced and profile drag it experiences.
- Frise-type ailerons, part of the up-turned aileron extends down into the airstream below the wing. As before, this increases its profile drag and helps to balance the drag produced by the aileron on the other wing.
During my A&P training I actually rigged the differential ailerons on a Cessna 152 which was pretty cool and gave me a good picture of what was happening. My memory is fuzzy, but I seem to recall the ailerons deflected around 20 degrees upward and only 14 degrees downward – it’s a significant difference. Despite these efforts to minimize the effects of adverse yaw through design, the pilot must still learn to coordinate their aileron and rudder inputs. As you move the wheel or stick, the rudder pedals should move as well.
My first flight instructor had coached me to “lead” my turns with rudder. This common advice actually isn’t the correct technique (rudder and aileron inputs should be simultaneous). In retrospect I think his advice was intended to address a specific habit of mine. He wasn’t actually wanting me to lead with the rudder, but to stop lagging with it. As the stick moves, so do the feet. The reason Dutch Rolls are such a beneficial exercise is because they require a degree of precision and finesse that is hard to master. Practicing them helps to train you on the precise coordination of aileron and rudder inputs.
Don’t Play Ball
One of the tricks behind Dutch Roll and any coordination exercise is to perform the maneuver by using outside visual references and the feel in the seat of your pants. You can detect much smaller changes in roll and yaw by looking at a point on the horizon than you can by looking at any of your instruments. Like most instruments, the ball inclinometer exhibits a noticeable lag behind what the airplane is doing. The ball might help you keep a steady turn coordinated, but the roll into and out of the turn typically occurs too quickly for the ball to give you meaningful feedback on your coordination. Learn to develop a feel for the airplane and improve your perception of outside visual cues to aid in coordination.
It’s funny how some topics can seem to elude complete understanding even after many years. If nothing else, this is a good reminder that we are always learning and can always improve. It’s also a good argument for going beyond the minimums and the textbook explanations. Dutch Rolls (i.e. Rolls on a Heading) are not a required maneuver for any checkride, but they can make a valuable learning tool that only takes a few minutes here and there. You can even practice them on the climb to the practice area and not add any time to the lesson at all! I hope you find it as useful as I did.