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On the skids? How to fix adverse yaw

On the skids? How to fix adverse yaw

Adverse yaw is an inherent opposite yaw or skid that occurs with aileron deflections. A positive angle of attack is generally required to produce the wing lift needed to keep an airplane in the air. When the ailerons are deflected at a positive angle of attack, the down aileron presents a wider frontal cross section, thus creating more drag and causing the airplane to yaw in the opposite direction that the ailerons are applied. When two aileron servos and the flaperon function are used, adverse yaw can be lessened by programming a small amount of differential aileron travel, e.g., approximately 5 degrees less down aileron than up, improving control and producing cleaner axial rolls. The exception is when the airplane has a flat bottom wing. Drag on the side of the down aileron and adverse yaw is so much more pronounced with a flat bottom wing that differential aileron travel has little effect. To eliminate adverse yaw, rudder must be coordinated or mixed in the same direction with the aileron. As a rule, a symmetrical wing plane may require only a 3-5% rudder mix with the aileron to eliminate adverse yaw, whereas a flat bottom wing plane requires nearly as much rudder deflection (in degrees) as aileron. Not only does eliminating adverse yaw improve control, pilots who initially learn to fly with aileron/rudder mixing are also able to more easily transition into higher performance symmetrical wing airplanes, since they are already accustomed to flying with minimal adverse yaw. Contrast that to those who learn to fly with adverse yaw, and then have to retrain their flying habits when they switch to a symmetrical wing plane with very little adverse yaw.

Updated: February 25, 2016 — 11:31 AM
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  1. Dave, you’re on the right track but you are carrying a football around the base line.
    If you think Adverse Yaw is caused by how much “area” is presented by the down aileron, your airplane is way out of trim and you are not doing an axial roll. You are doing a barrel roll. Adverse Yaw is caused by two simple forces. Lift, which according to Rule #1 of flight causes Drag. And Moment, which enhances the effect of a force. Anytime a control surface is presented to the wind, it causes DRAG. It is not much of a concern to elevators or rudders, but then you apply the associated drag out on a wing tip, you have a problem. Adverse yaw is most noticeable on planes like Piper Cubs and Citabrias and even Cessnas and Piper Cherokees. All of these airplanes have what is called “Barn Door” Ailerons. As opposed to “Strip” ailerons which run the length of the trailing edge. Imagine a Cub flying straight and level and it’s time to roll into a left turn. Left aileron goes up, right aileron goes down. Which aileron has the most work to do? The right aileron has to lift the right wing – Lift = drag. And when you concentrate all of that force into a small section of the outer length of the wing, you have a lot of drag with a lot of moment out on the wing. This imbalance of forces compared to what the left aileron is doing is what causes Adverse Yaw.
    When Adverse Yaw occurs, the nose of the airplane points in the wrong direction for the turn and causes a “slip” situation. Rudder must be used at the same time as the ailerons it order to counter the adverse direction of the nose. This is a very simple phenomenon to demonstrate by just rocking your wings back and forth about 30 degrees each way and watch the direction the nose points. Now, add rudder at the same time you are adding aileron and adjust your amount of rudder until you can rock your wings and the spinner stays in one place the entire time. I use to love doing this in a full scale Cessna 150 when I was learning to fly.
    So, your suggestion that a flat bottom wing is more of a problem is misplaced because a lot of flat bottomed winged airplanes happen to have Barn Door ailerons. Not to mention, they are also high wing aircraft. Think of the pendulum effect of a high wing aircraft like a Cub or Cessna. How about the Hall Bulldog with the Gull Wing. In order to bank those aircraft, the lifting wing has a lot more work to do to overcome the pendulum weight of the fuselage. Lift means more drag and all that drag pulling on the wing all those inch moments out there is a lot to overcome. Have you ever noticed the size of the tail on the Hall Bulldog?
    So I take issue with you on the aileron cross section (due to angle of attack) being the cause. Although many change the aileron travel to create “differential” in throws. But, if you are doing a roll, and get inverted, and the -now increased aileron- is lifting the airplane’s wing on the other side, dynamics change and you still have to FLY the airplane. By this time the adverse yaw caused by the inverted wing is minimized due to the momentum of the roll but you will need a lot of down elevator if you are flying a Cub.
    There are not many people that learn to fly with Adverse Yaw unless they are learning to fly a full scale airplane. Most R/C trainers have strip ailerons which have little or no adverse yaw due to their low profile fuse and center of balance. So students learn to fly with their right hand and taxi with their left hand. It’s when they get their hands on a scale airplane with Barn Door Ailerons that makes them have to use their rudder as a flight control.

  2. Dan and David, you’re both right. David is talking about parasitic drag and Dan is talking about induced drag. Dan, the imbalanced induced drag your are talking about is momentary. It only occurs while the roll rate is accelerating. Once the roll rate stabilizes, both wings are generating the same lift (same angle of relative wind) and therefore produce the same induced drag. We don’t notice it in or planes because it happens so quickly. In a full scale plane however, you can feel it. The parasitic drag of the ailerons is persistent with deflection.

  3. Dihedral also causes adverse yaw in the same way an upright V tail causes adverse roll. The ailerons are behind the C.G. and therefore have a moment arm to the yaw axis. Dihedral angle increases the phasing of the ailerons from the roll axis to the yaw axis. The more dihedral the worse the yaw component. This can only be cured with coordinated rudder. Differential aileron has no effect. Ask someone who has flown a Morsley Bravo. They’ll know.

  4. Better to just learn how to use the rudder…

  5. David,
    I have been a CFI (Certified Flight Instructor) for forty years and I have never heard adverse yaw explained quite like you have. It’s really very basic….the downward moving aileron creates lift and lift creates drag. To keep it simple for students in basic training we taught them that rudder is required whenever the ailerons are deployed. yes, that is an “oversimplification” but it speeds up the training process. I am also a longtime RC Scale enthusiast…this is my 60th year in the hobby.
    Vince Veltri
    Sarasota, FL

  6. Hi Dan, regarding your response about Barn Door ailerons. Have you ever seen a mid-wing plane with a symmetrical wing? These planes have ailerons that are much larger than any Cub and perform perfectly axial rolls with 45 degree aileron deflection without any rudder adjustment.

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