“Building Good Habits for a Better Future” -Rudder Mixing Explained By Dave Scott. Instructor, 1st U.S. R/C Flight School Illustrations by Dave Scott

Adverse Yaw

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Dave Scott is a champion full-scale S ..RCFIH SC FLIGHT R/C U.S. 1ST

1ST U.S. R/C FLIGHT SC aerobatic competitor, founder of 1st U.S. R/C Flight School, and author of numerous training manuals. His books and articles feature the accelerated training techniques that he developed instructing over 1500 Drag R/C pilots during his school's 4 & 5- day courses. More information about his books and flight school can be found at www.rcflightschool.com Introduction The following article details the better flyer. Thus, rarely does adverse practice of using Aileron-Rudder (A/R) yaw or the advantages of A/R mixing potential for over-controlling, as well as transmitter mixing to eliminate when learning to fly ever come up. deal with a lack of consistency caused adverse yaw, i.e., the inherent opposite However, if you where to objectively by the out-of-sync relationship yaw or skid that is especially compare the results achieved training between their control inputs and the pronounced during aileron deflections with A/R mixing versus without, you response of the plane. on flat-bottomed wing , such as would discover an immediate those used for primary flight training. improvement in consistency and As a rule, adverse yaw is most therefore the rate of learning. pronounced on high flat-bottom As the name implies, adverse yaw is an wing aircraft and gets worse at slower adverse or unfavorable condition that, 1st U.S. R/C Flight School’s mission airspeeds and/or when making larger among other things, delays achieving mandates taking advantage of all the aileron inputs -- thus explaining why solo abilities. Traditionally, struggling tools available, including the use of some people can fly a flat-bottom wing and committing to many hours of A/R mixing, to ensure every primary trainer around ok, but then struggle to practice before soloing has been the student can safely solo in all kinds of control the plane during slower speed assumed burden of the student pilot conditions in less than a weekC with landings. (Adverse yaw is so severe on until his or her skills improve. the knowledge that they will then use a scale Piper Cub for example, that Unknowingly and unnecessarily, that foundation and confidence to when flown near speed it will novices have been fighting the continue learning and advancing on actually turn left when right aileron is additional challenge of flying with their own. In fact, learning to fly with applied and vice-versa.) Also, since the adverse yaw. Indeed, novice pilots A/R mixing can be credited for helping principle effect of wind is exaggerating have always assumed that the lack of to instill proper control habits that deviations that would otherwise be correlation between their control actually accelerate future progress. minor on calmer days, adverse yaw inputs/intentions and the response of creates a whole slew of problems when the plane to be strictly the need for AdverseYaw trying to fly a trainer in windy Space does not permit going into all more practice, or wind, when in fact conditions. adverse yaw has been a big reason! the aerodynamics involved during aileron deflections, so put simply; Some common approaches to reduce It’s probably safe to say that most of the adverse yaw is caused by the wing with the effects of adverse yaw in R/C people reading this learned to fly at the the down aileron generating more lift have been: Flying at higher speeds; side of a recreational flyer/instructor and therefore more drag than the wing making the trainer less stable and with little pre-flight preparation. As a with the raised aileron. Consequently, more maneuverable by lessening wing result, most pilots are conditioned to the drag differential causes the dihedral; differential aileron travel “reacting" to what the airplane does, airplane to yaw/skid in the opposite (more up aileron travel than down); as opposed to having a plan and pro- direction that the are applied avoiding wind; accepting it as how actively controlling the plane. while banking into turns, making trainers fly; and continued Consequently, most pilots naturally course corrections, exiting turns, etc. reassurance from club members that think that getting better at making (see figure 1). Pilots therefore have to the student will eventually get it with corrections, good reflexes, and more hold in the aileron longer to overcome more practice -- all of which only help to stick-time are the keys to becoming a the adverse skid, thus increasing the small and varying degrees. Aileron-Rudder (A/R) Mixing The logical solution to counter course corrections, rolling, etc. (figure adverse yaw in order to achieve a adverse yaw is with the surface that 2). Most importantly, with adverse yaw precise “axial” bank and roll response. controls yaw, i.e., the rudder. eliminated, the response of airplane 1st U.S. R/C Flight School trains its Coordinated rudder deflections along conforms more closely to the inputs primary students on planes setup to with and in the same direction as the and intentions of the pilot! ailerons prevent the plane from automatically coordinate the rudder Be clear, the function of the rudder here skidding in the opposite direction while with the aileron through the A/R is not to turn the airplane. Rather, the banking into and out of turns, making mixing function in the radio. Radio purpose of the rudder is only to prevent manufacturers have in fact been

Adverse yaw: An inherentopposite yaw or skid that occurs during aileron deflections (esp. on a flat-bottom wing).

Ê A flat-bottom with a curved top surface produces low-pressure lift as air flows over it (ideal for upright flight at slower speeds, i.e., primary training).

Right Aileron Bank Example:

Ë The lowered aileron increases the curvature of the left wing, thus increasing low pressure lift, causing the wing to lift up and also greater drag.

ath Ì The raised aileron reduces the effective curvature of the right wing, causing a reduction in lift and less drag.

Í Increased drag on the rising left wing drags it rearward as the plane is

banking right, a.k.a., adverse yaw. Original flight p

Ê Right Bank r curved r ove surface Ai = lift & dra g Drag

Basic: A flat-bottom airfoil with a curved top surface produces low-pressure lift as air flows over it.

Ë ater curvature = greater Í re lift & Increased drag on the G Left wing d Drag lifting wing drags it ra rearward as the plane The left wing with the lowered aileron g is banking right, thus generates more low pressure lift as the causing the nose to air flows over the increased curved yaw left. surface, causing the wing to lift up and also an increase in drag. Lift Lift Ì Drag uced curvatu ed re = les R s lift & drag Right wing

The raised aileron reduces the effective curvature and lift of the right wing, causing the wing to lower and a reduction in drag.

Note: The summary information above might upset some career aerodynamicists because it does not address angle-of- attack, camber,induced and profile drag , airspeed, etc.. The objective here is not to teach aerodynamics, but to condense adverse yaw theory into a simple explanation that the average pilot can wrap his head around and thereby improve his odds of success. Anyone wishing to delve into the subject further can easily do so online, e.g., http://en.wikipedia.org providing A/R mixing for the purpose of countering adverse yaw since the 1980’s, but since most instructors are

inclined to keep passing down the way Axial that they were taught, it is still not widely used or even understood in R/C.

Understand, 1st U.S. R/C Flight School initially operated in the 80’s without A/R mixing because it wasn’t the norm in R/C at the time, although, full-scale aviation has widely applied the use of yaw damper (A/R mixing) since the 1950’s so pilots are less encumbered w/o mixing while performing more important objectives. When the school started Rudder deflecting mixing aileron-rudder on all its with the ailerons, in primary trainers, the students’ weekly the same direction, average number of landings leaped prevents the nose from 10-15, to 30-40 -- an increase of from skidding to the over 200%! The reasons for the left. increase are simple: 1. Whether a Adverse yaw is thus beginning R/C flyer applies his inputs prevented, banks correctly or not, he always has honest and corrections, results to learn from. 2. The airplane’s even rolls, will be smoother and more truer representation of the control axial, and you will inputs made leads to an improved feel more connected Note that the rudder does not turn understanding of proper control. to the plane. the plane, but simply keeps the 3. This enhances a pilot’s ability to rising wing from sliding back during teach himself with greater retention, the brief period that the aileron is applied to set the bank for the turn. and therefore his overall practice is more consistent, predictable, and extra productive.

Clearly, one who learns to fly a trainer set up to more precisely reflect the control inputs made is going to learn proper control earlier. However, as a bonus, A/R mixing expands the aerobatic capabilities of primary trainer airplanes as well, e.g., thanks to A/R mixing, aileron rolls remain perfectly axial and on heading throughout. Furthermore, the improved control achieved with A/R mixing permits flying in winds that Aileron-Rudder Mixing would normally ground most trainers. Note: If you’re new to A/R mixing, know Setup Rules-of-Thumb I.e., since the main challenge of flying that pilots still have independent Upon activating A/R mixing, you in wind is that it tends to exaggerate rudder control on the left stick for left- need to confirm that the rudder moves deviations, the positive control hand ground steering and maneuvers in the same direction as the aileron, achieved with A/R mixing makes it requiring independent rudder. In fact, i.e., the rudder should move toward the possible to more precisely and learning to use independent rudder up aileron (figure 3). On a flat-bottom promptly correct deviations before the proves easier after learning to fly with wing airplane, the rule-of-thumb is to wind has a chance to exploit them. A/R mixing because much of the right set up the A/R mixing so that the degree Thus, even sport flyers have good stick control will have become routine or angle of rudder throw matches the reasons to utilize this setup on their or automatic thanks to the mix. aileron throw 1-to-1. flat-bottom wing planes. Full deflection Right wing

Rear view Top view Activate A/R mixing and confirm that the rudder deflects Vertical Fin toward the up aileron. On a flat-bottom wing airplane, 1-to-1 adjust the A/R mixing percentage so that thedegree of rudder deflection matches the aileron deflection 1-to-1.

As a result, adverse yaw will be negligible. Banks, corrections, and rolls will be smooth and axial, and you will feel more connected to the plane when you fly.

At the School, we simply gauge the to help further reduce the chances of (un-mixed) will have to re-train their degree (angle) of the aileron deflection adverse yaw occurring, particularly at habits when flying a sport model visually, and visually match an equal slower airspeeds. without adverse yaw. degree of rudder travel. If for any While differential aileron travel is a Sadly, since so many people learn to fly reason we are unable to set a 1-to-1 common practice used to reduce with adverse yaw and thus exhibit the relationship, we’ll get it as close as we adverse yaw, its effect is slight, and the same natural-progression of bad can -- knowing from experience that a only way to fully eliminate adverse yaw habits, (e.g., climbing into turns with few degrees more or less is not going to with a flat-bottom wing airplane is with descending finishes), a lot of veteran make any appreciable difference. We simultaneous rudder. If you did pilots fail to recognize that their then check the setup by flying the attempt to reduce adverse exclusively difficulties in wind and transitioning to airplane directly at or away from us with differential, you would end up with different aircraft types, inconsistent while banking left and right to confirm so much up aileron travel that the landings, etc., are because the that the banks are axial and the airplane will push downward entering techniques they learned to fly with are fuselage stays pointed in the same turns, at the start of rolls, etc.. Thus, a no longer appropriate. direction throughout. little differential is good, just don’t get Thus, the aim of A/R mixing a primary You can be confident that after carried away. trainer is not only to facilitate learning applying the 1-to-1 rule-of-thumb to a to fly, but also to compliment the flat-bottom wing airplane adverse yaw A/R Mixing for the Future Many new flyers eventually go on to transition into higher performance will be undetectable; banks, enjoying the “flying on rails” handling sport airplanes that ultimately don’t corrections, and rolls will be smooth and increased capabilities of require A/R mixing. and axial, and you will feel more symmetrical wing sport models. connected to the plane when you fly. While on the subject, if you’re inclined Note that symmetrical wing airplanes at some point to switch off the A/R Note: Due to the improved control do not require A/R mixing because mixing on a flat-bottom wing trainer, with mixing, a rank beginner might be adverse yaw is negligible with this type expect to make a lot more control wise to initially set up the plane with (see figure 4). inputs to overcome the sloppier slightly less aileron travel than what Actually, those who learn to fly a flat- responses (something that you do not the manufacturer recommends to tame bottom wing trainer with A/R mixing want to make a habit of if you also plan the response a bit, and then match the will find the transition into to fly less encumbered sport models). rudder to that 1-to-1. Furthermore, if symmetrical wing sport models easier Of course, you could physically you’re hesitant to use the A/R mix, you than most. That’s because the control coordinate the rudder and aileron could always start with less, and then habits learned flying an A/R mixed together using 1-to-1 movements of keep increasing it until the bank and trainer are the same techniques used to both control sticks to eliminate adverse roll response is finally axial. optimally fly sport models, since in yaw, but understand that this DifferentialAileron both instances pilots are flying without technique only applies to flat-bottom If your airplane utilizes 2 aileron adverse yaw and maintaining a direct wing airplanes, and would not be servos, you can program a small correlation between their inputs and appropriate when flying sport amount ofdifferential aileron travel the response of the plane. Conversely, airplanes. (more up aileron deflection than down) those who learn to fly with adverse yaw Conclusion Symmetrical wing As stated, maintaining a direct correlation between control inputs and Right wing = Drag the response of the plane is instrumental to developing optimum control habits. Consider that when Equal curvature = equal amounts the initial control inputs are applied of low pressure and drag correctly, the need for additional Result: No adverse yaw corrections may not even exist. That’s when a pilot becomes free to think Left wing Drag ahead of the airplane and more = efficiently take on new challenges. Hence, by removing the obstacle of adverse yaw, A/R mixing proves to be one of the most effective tools to ensure that pilots learn proper control from the Axial start and therefore continue to enjoy Symmetrical wing airplanes exhibit minimal adverse yaw and thus inherently remain axial steady advancement and a more when banking and rolling. Thus, there’s not much successful future. Until next time. Symmetrical need forA/R mixing with this type. Flat-bottom wing airplanes exhibit significant adverse yaw and thus benefit from A/R rudder mixing to achieve an axial bank/roll response.

Axial When it’s all said and done, learning to fly a flat- bottom wing trainer utilizing A/R mixing to eliminate adverse yaw leads to learning the same optimal control habits used to fly a symmetrical Flat-bottom wing aircraft, thus making the transition easier.