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Feature Article

Matt Debsky, Aircraft Owner WVFC tdebber@alum.mit.edu

Adverse Yaw(n) 

One of the exercises many beginning pilots go through during the first few flights is a coordination drill.  The pilot banks the airplane left to right using the ailerons and keeps the nose pointed at the same spot on the horizon using the rudder pedals.  The drill helps to demonstrate the different effects of the ailerons and rudder.  While explaining to a beginning pilot how the drill would work, I explained that when banking right he would initially need to apply right rudder to counteract the adverse yaw, but would then need to gradually shift to apply left rudder in order to hold the spot on the horizon.  The beginning pilot asked why he would need to apply left rudder, because if adverse yaw persisted shouldn't he need to constantly hold right rudder to counteract it.

After the flight, I reread the section on adverse yaw in the Pilot's Handbook of Aeronautical Knowledge (PHAK) as well as in Aerodynamics for Naval Aviators (ANA) and a little bit of Wikipedia.  The answer to the pilot's question, of course, has to do with the dynamic behavior of the roll.  The explanation in PHAK emphasizes that the wing producing greater lift experiences greater drag, implying that induced drag is the main contributor to adverse yaw.  However, both ANA and the sources cited on Wikipedia discount this.  Both explicitly discuss the increased profile drag of the deflected aileron, an element of parasitic drag. The other element at play, not mentioned by PHAK, is the effect of the rolling movement of the plane on the angle of attack of each wing.  In the case of a right bank, as the left wing moves up, its angle of attack decreases causing its lift vector to move backward while the opposite effect on the right wing causes its lift vector to  move forward.  The result is a yawing force to the left, counteracted by right rudder. 

As long as the airplane is rolling, this situation remains and the pilot must apply right rudder to compensate for this adverse yaw.  However, when the bank is established and the yoke is centered, the ailerons are no longer producing different amounts of lift and depending on the degree of bank and the stability of the airplane, the airplane remains in a constant bank.  Additionally, since the plane is no longer rolling, the angles of attack are no longer different and the adverse yaw caused by the different lift vectors no longer exists.  At this point, the plane is entering a turn to the right and left rudder is necessary to keep the nose pointed at the same point on the horizon.  See the references at the end of the article; both have diagrams helpful for visualizing the vectors and relative wind.

I found it interesting to go back and review some of the forces that cause adverse yaw.  Airplane designers have several tricks up their sleeves to minimize adverse yaw.  As with most airplane design decisions, they all have their tradeoffs.  The upside for pilots is that when rolling into or out of a turn, use of rudder to counter adverse yaw will allow the airplane to enter and leave a turn in a coordinated fashion. 

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The Wikipedia article is at http://en.wikipedia.org/wiki/Adverse_yaw

Aerodynamics for Naval Aviators is available at http://www.faa.gov/library/manuals/aviation/media/00-80T-80.pdf

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