Why Should You Care About Adverse Yaw?



When an airplane is flying quickly, the ailerons behave pretty nicely.  You roll the aircraft left, and it rotates pretty nicely, with the ball in the center.  Engineers have used all sorts of tricks to make that happen – differential ailerons, etc.


But when you slow down, it all falls apart.  As a very rough rule of thumb on newer aircraft, on the front side of the power curve, the ailerons behave pretty well.  But when the aircraft is slowed down to the bottom and on the back side of the power curve, they can be expected to produce measurable adverse yaw.


Adverse yaw is easy to understand.  When you roll the aircraft left, the right aileron goes down into the high pressure air underneath, and pulls the nose right.  That’s not good, when you are trying to turn left.  Older aircraft have this problem in spades.


This is compensated for with the rudder pedals, which on a modern nosewheel aircraft can be forgotten about most of the time.  This isn’t the case when you are flying slowly.  When you displace the ailerons for a roll to the left, you must depress the left rudder.  When you neutralize the ailerons in the turn, after the desired bank is obtained, you can generally get off the rudders, unless the prop is doing something rude to you.


This is actually pretty simple to understand; just use the ailerons and rudders together, to keep the ball in the center.  If in doubt, “step on the ball” to center it.


This lesson is taught in slow flight, but most people fear and hate slow flight.  They don’t like how the aircraft feels in slow flight, and they fear stalling, which is too bad.


I like to slow the airplane down until the stall warning is continuously on, and I love spending time with the student doing turns, climbs and descents in slow flight at various power settings, right on the edge of the stall.  And sometimes a wing will stall and drop, but that’s no big deal.  Just don’t try to “pick it up” with aileron, because the adverse yaw will make you enter a spin.  Instead, use the rudder to pick up the downgoing wing, and perhaps decrease the angle of attack with a little forward elevator, as required.


After a while, the student is doing 30 degree banked turns in both directions with the stall warning on.  A great confidence and skill-building maneuver!


It’s a pity most people hurry through slow flight, and spend the absolute minimum amount of time on it, because they are going to be in slow flight during every takeoff and landing very close to the ground, so they’d better know how to precisely control the aircraft in slow flight, otherwise bad things can happen.


A good example is a vanilla takeoff – no crosswind, lots of runway.  The nose in a prop single will typically yaw left for about a millions reasons, none of which matter very much.  What matters is that people learn the lessons of slow flight and use right rudder to bring the nose right.


Unfortunately for many students, who have spent decades driving a car, the learning factor of primary has taught them that when they want the nose of a vehicle to go right, they spin the steering wheel to the right.


And, that’s what they do on takeoff, instead of using right rudder.  They crank the control column to the right, which lowers the left aileron into the high pressure beneath the wing.  The adverse yaw produced pulls the nose MORE to the left, so they crank the control column to the right even more, which produces even more adverse yaw, which pulls the nose left even more.  Sigh.


At this point, I am gibbering like a monkey and fearing for the continued health of the runway lights on the left edge of the runway.  This produces a litany from me which sounds something like “rightrudderRightRudderRIGHTRUDDER!”.


Hopefully you can see the importance of mastering slow flight and use of the rudder during the takeoff. 


It’s just as important during the landing, too!  For example, if there is any crosswind at all, during the landing rollout you should continuously crank the control column (or stick) over into the crosswind, as you slow down.  This not only raises the aileron to spoil lift on the upwind wing, but more importantly lowers the aileron on the downwind wing, and the adverse yaw pulls the nose against the weathervaning tendency of the aircraft as it slows down.  Groundloops are not cool.  This is a great example of how you can use adverse yaw to your advantage, instead of fighting it.



acboyd@gmail.com   Dec 2012