of us have ignorantly followed sim- The Role and the Use plistic rules. When the is in equilibrium flight, it is not accelerating in any di- of the rection or about any axis. It is only then that the attitude indicators are reli- able. You know how long it takes for Daniel L. Johnson, Different parts of the aircraft may the to catch up with reporter be in quite different air. This causes a change in angle of attack. Such lags uncommanded pitch, yaw, and roll are present in all indicators, reflecting changes, to which we respond swiftly time needed for the aircraft to come "It is interesting that we, as student with control movements. The thermal into equilibrium after any change in a pilots, are given several hard and fast tries to spit you out. force or moment. rules, which, as we venture out on our There are two situations in which The primary purpose of the vertical own, arent so hard and fast after all." this is especially important: circling in is to achieve yaw, or weath- — RBick a thermal and landing in turbulence. ercock, stability. When the is We also learn that we cannot correct yawed, the vertical stabilizer creates a - Author caveat: I have done my best to every wrinkle in the air, and we must restoring moment that realigns it with be faithful to the science while clarifying learn to "fly attitude," knowing when the airflow, in the same way a weather- control use. But like all inexpert writing, to let the little wrinkles average them- vane works. A larger vertical stabilizer this is a precis. For detail, take a degree in selves out, and which big wrinkles de- drives spiral instability, a small one aerodynamics. mand quick response. facilitates Dutch roll. The goal of the Two summertime letters to the Soar- designer is to balance these two issues. ing editor about the rudder seemed to "Maughmer's Rule" To be exact, it's side force that turns be having a mild argument about the Rus Howard's letter in "Soaring the aircraft. Yawing motion is initiated same truth. This caused me to inquire Mail" (July 2020, p 4) quoted the di- or stopped and modified by rudder. Tilt- and study a bit. The basics: dactic Maughmer's Rudder Rule:"... the ing the lift vector with bank is best, and We can say that 3-axis control, first thermal is trying to spit you out .... backstick is necessary to stay level, or understood by the Wright brothers, is While circling in a thermal the nose a spiral descent will occur. Dirigibles the foundation of aviation. should be traveling along the horizon and submarines don't turn by banking, : roll control at a constant rate. If the rate slows they generate side force by yawing the Rudder: yaw control down, you use the corresponding rud- craft. A forward slip involves a large Elevators: pitch control der to speed it back up. If it speeds up, bank angle, but does not create a turn In a level turn, all three are neces- you use the appropriate rudder to slow because the yawed fuselage and ver- sary; none can be neglected. it down." (Top rudder slows the rate of tical tail create a side force opposing Ailerons tip the lift vector to the turn, bottom rudder speeds it.) the tilted lift vector of the bank. This inside of the turn by inducing an ac- Walt Cannon responded (Soaring, is not aerodynamically coordinated, so celeration around the roll axis. October 2020, p 5): "A turn in any it's draggy, but that's the point. Rudder initiates the rate of yaw that aircraft is initiated and maintained by Coordinated flight corresponds to a is necessary to turn. the ailerons. The rate of turn is deter- steady turn rate, and Maughmer's Rule increases pitch to create an mined by the angle of bank. The rud- is simply a way of restoring coordinat- increased angle of attack to increase der is used primarily to keep the air- ed flight when the steady state is dis- lift so that both level flight and turn- craft in coordinated flight. Turbulence turbed by turbulence. You don't wait ing occur. may require independent use of the for the yaw string to find a new equi- "Coordinated flight" simply refers ailerons or the rudder, but in smooth librium; you respond instantaneously to using the controls together. Aero- flight the turn is initiated and main- to the movement of the nose against dynamically, its goal is symmetrical tained by the ailerons and coordinated the horizon. We are not flying the yaw attached airflow across the lifting sur- by the rudder." string, we are flying the aircraft. faces. Well, this is often taught by those Our challenge as pilots operating in of us without knowledge of aerody- What Is Optimal Rudder Use? the turbulent air that we seek for soar- namics. It's like all inexpert writing: I've been piloting aircraft, off and on, ing is that this air is full of swirls that It reflects truth and misses nuance. for 60 years. During most of this time, instantaneously change angle of attack Maughmer is an aerodynamicist, Can- I was mystified as to the optimal use and wind velocity on lifting surfaces. non is a surgeon. I write because many of the rudder. I found myself a slave Soaring • April 2021 to the ball and needle in and In my early life as a soaring pilot, the yaw string in gliders. For example, I was a slave to the idea that a cen- almost 25 years ago, while flying a 1-26 tered yaw string is mandatory, believ- & Specialty Co. on the downwind leg of the pattern, ing this ensured "coordinated" flight. the cumulus that had been growing In steep turns, it merely indicates the SOARING all afternoon over the gliderport sud- flow of air past the string, not its flow SUPERSTORE denly dumped its water, and my yaw over any part of the lifting surfaces or string stuck firmly to the wet canopy. control surfaces. I suddenly realized how dependent I Spanwiseflow always occurs. Airflow was on keeping the string centered, across a wing is not parallel with the and that I had no feel for how to fly wing chord, but bends away from this, TIRES the without the yaw string. I especially at the outboard end, where decided to fly fast, in case I failed to the ailerons tend to be located. The GOODYEAR - MICHELIN - SPECIALTY coordinate the turn. A couple of de- magnitude depends on wing charac- cades later, the elderly and frayed yaw teristics and direction of incident flow string departed my Ventus one April and lifting load. afternoon, and I did not bother to re- The banked turn inevitably induces place it that summer. I was now flying asymmetrical spanwise flow, as the mostly by feel, always using a little top two wings are in different airflows OXYGEN SYSTEMS | rudder to decrease adverse yaw and to and doing different work. The direc- keep the stick off the top stop, which tion of the gravity vector and the di- AEROX - MOUNTAIN HIGH also helps avoid skidding turns. rection of airflow across the canopy Helmut Reichmann, in Cross-Coun- cannot be thought to portray airflow try Soaring (p 11, top left): across wings. Now add turbulence that puts the wings in airflows of dif- "IMPORTANCE OF fering velocity. CLEAN FLYING WHILE Before the winglet was invented, in

THERMALING a steep turn, approximately the out- - BOSE "Naturally, clean and coordinated board 1/3 of the flow atop the wing flying is a prerequisite for decent ther- is detached, and only a couple of feet maling. The yaw string is an irreplace- of the inboard have attached able and ultrasensitive 'instrument' flow. Winglets cause this flow to be that immediately indicates even the attached, decreasing adverse yaw and slightest skid or slip. Nevertheless, it is improving aileron authority. PARACHUTES more important to center the thermal Without winglets, slipping the turn rapidly; the most gorgeous textbook points the up wing aft and increases NATIONAL - STRONG circle is of little practical use if only the area of attached flow, increasing half of it is in the updraft. Therefore: aileron authority and decreasing ad- first, center the thermal, then be sure verse yaw. This is the key to efficiently you are flying in a clean and coordi- thermaling an older, long-winged nated manner." sailplane. INSTRUMENTS

What Is Coordinated Flight? What Does the Rudder Do? MID-CONTINENT Most of my piloting life, I thought The two key achievements of the that the ball defined coordinated flight. Wright brothers, that created avia- Well, no. The ball describes which way tion, were a lightweight engine with AOPA gravity + lift is pointing. That is use- a highly efficient (IVe read ful, but it's a proxy for symmetrical air- that it's difficult to exceed the 85% ef- flow over the wings and is insensitive ficiency theirs achieved) and effective to gusts. Similarly, the yaw string only 3-axis control. They were the parents shows the airflow past the yarn. It is of the rudder. 1-877-4SPRUCE a proxy for aerodynamic coordination The vertical stabilizer on the tail FREECATAIOG! that is most accurate when it is least hinders it from swishing back and needed: straight flight. forth (Dutch roll, in which the wing- www.aircraftspruce.com tips make circles). explanations say that overbanking oc- began practicing this. The stick came Movement of the rudder essentially curs because the lift is greater on the off the top stop and I no longer fell warps the vertical tail to produce lift outside wing than on the inside due to into an incipient with thermal on one side or the other. speed. But there is also a difference in turbulence. Steep turns became as This lift induces acceleration around spanwise airflow that exacerbates this, effortless as level flight. the vertical (yaw) axis to create the which is made worse by maintaining When I've related this to pilots or slow rotation needed for a turn. If a the bank with opposite aileron. instructors, there's often a vigorous de- slip is wanted, the rudder is held over fense: "You must realize that the yaw until the design forces that give an Effect of Winglets string is forward of the center of grav- aircraft longitudinal stability - that Whatever other good things wing- ity, and in a turn the air flow across the cause it to want to fly straight forward lets have done, they decrease spanwise canopy is at an angle." — balance the lateral lift of the rud- flow and make airflow more symmet- Do the math, kids! The yaw string der. (A skid is dangerous when close rical in turns. This creates aerodynami- is 3 to 6 ft forward of the center of to stall.) cally coordinated turns and makes gravity (CG) or yaw axis. The angle A turn in an aircraft involves a slow, handling in steep turns safer and easi- it would make with the glider cen- constant yawing that is best initiated er. Those of us without winglets should terline is congruent with the angle and regulated by the rudder. After this read on. between the CG and the yaw string is established, the rudder is held con- attachment. The yaw string angle is stant until it's used to stop rotation Standard Teaching therefore the arctangent of [the dis- when rolling out of the turn or to cor- One of the finest of soaring instruc- tance from yaw axis to yaw string] di- rect the effects of turbulence. tors, Dale Masters, in the first edition vided by {the radius of the turn}. At The bank itself predisposes the air- of his excellent Soaring Beyond the Ba- an extreme, if the yaw string is 6 ft frame to yaw, so that continuous rud- sics (not in print) said about thermal- ahead of the yaw axis in a turn 200 der displacement is not needed to ing technique on p 71: ft in diameter, the angle is 3.4°. If maintain the rate of turn in smooth "Making the most of strong, nar- the yaw string is 3 ft forward and the air. Aerodynamic coordination of a row thermals requires walking the circle 300 ft in diameter, the angle is turn requires bank, initiated by aileron edge of a stall or spin in turbulent 1.14°. This is trivial. movement. air. Some sailplanes can be turned There is another effect that means Altitude is maintained by up eleva- so tightly that full rudder into the that the yaw string angle will not fol- tor - backstick pressure - so that the turn, full opposite aileron to coun- low this simple math - there will be centripetal force that creates turning ter the over-banking tendency upwash along the side of the cockpit, is added to the lift that holds the air- (with the yaw string centered!), the exact amount unknowable, de- craft up. and full back stick, will produce pending on cockpit shape and turn the fastest climb. Remember that dynamics, that may exaggerate the de- Adverse Yaw and Overbanking when you roll out of a very tight viation. Thus, the yaw string is a flut- Initiating a bank with aileron in- turn your high angle of attack will tering approximation. duces adverse yaw (the tendency of the be too great for straight flight, and The point is that some off-center nose of the aircraft to deflect toward the cure for that is neutral elevator drift toward the top wing - a slip - is the outside of the turn when bank is as the wings come level." the best way to reduce overbanking initiated), which is countered with I believed this and practiced it for and allow full 3-axis control in the rudder movement that accelerates the years, slavishly centering the yaw glider without winglets. A centered tail in the correct direction for the in- string; in steep turns frequently put yaw string in a steep turn is not nec- tended turn. into incipient spins by turbulence. essary or desirable; but it should not In a coordinated banked turn, there Steep, slow turns had the stick hard point toward the low wing! is also an overbanking tendency that in- against its limit; I had to learn to creases with bank angle and wingspan. manage the nose attitude and yaw rate Adverse Yaw Is Related to Wingspan Throughout my career, I received the with rudder and elevator alone. This This effect is smaller in airplanes standard instruction, "You may need was awkward and tiring. with short wings and greater in a long- opposite aileron to maintain your bank Then one day, watching video of winged glider. Regardless, overbank- angle." a seminar, a famous racing pilot re- ing eventually puts the stick to the Well, yes, this maintains a bank marked that his open class ship stop and then only rudder and eleva- angle, but if you think about it, this climbed much better when he slipped tor are available to keep the nose level does not create aerodynamically co- turns 10° or 15°. I drew 15° angles on and yaw at a proper rate. Responding ordinated flight. The lay-instruction my canopy from the yaw string and to thermal turbulence is complicated, www.ssa.on and the risk of a turbulence-induced Thermal Turbulence Airspeed excursions due to thermal incipient spin is high. In a strong thermal, the air is never turbulence can often be more than 10 The better way to correct overbank- calm or smooth. This requires con- kt. Sink rate increases rapidly when ing is to use a little top rudder to slip tinual responses with ailerons, eleva- airspeed is on the slow side of best the turn. This neutralizes the stick or tor, and rudder to keep one's airspeed glide for your chosen bank angle. In very nicely. Aerodynamically, this within a desired range, to maintain a turbulence, a little bit fast is safer and makes the spanwise flow more sym- bank that will ensure staying near the more efficient than a little bit slow. metrical (better aerodynamic coordina- thermal center, and to manage yaw Slip the glider a bit in turns so that tion) and greatly reduces pilot work- when the turbulence has stopped it or pilot workload is less, because fatigue load. This has been a secret technique accelerated it. impairs climb performance more than among racing pilots for decades. The Meanwhile, it's useful to remember anything, by delaying and slowing re- advent of winglets has greatly reduced where the safe boundaries lie: sponses, and impairing decisions. adverse yaw and removed the need to Slipping turns is safe, skidding turns slip turns. is not safe. Err toward the slip (top Directional Slips rudder) rather than a skid (bottom The rudder is useful in straight Slipping Turns Provides rudder). flight, to fly crosswise through the 3-Axis Control My own experience is that, in air- planes, when the ailerons have been neutralized by top rudder in a steep turn, the ball is about 1/3 to 1/2 of its width off-center — and the can be comfortably kept in the turn indefinitely. For example, I read that greater than 3g causes mild hypoxia by collapsing the lower third of the lung and remov- ing circulation from the upper third. I wanted to find out whether 2g could be problematic. So I filed IFR for a local flight, and with a recording ox- imeter, maintained a 60° bank for five minutes at each of 3 altitudes up to 12,000 ft. With a little top rudder, this was easy to do. (The oximeter record declared that no hypoxia occurred at 2g in one elderly male pilot, a very limited experiment.) In the Ventus, enough top rudder to bring the stick well off the stop restores normal 3-axis control and makes steep turns relaxing and comfortable. Yes, the yaw string drifts toward the out- side of the turn. But the spanwise flow is more symmetrical. When I tell instructor pilots this Scan Me to take technique, they get emotional, as if to let the yaw string go off center creates the Wing Runner a near-death experience. No, only if it Course goes to toward the low wing, honey! To skid a turn at low speed - risking a Complete the Wing Runner cross-control stall — should be experi- Course, let us know and we enced and understood. The break hap- will send you one of these badges. pens in a flash. air, to adjust altitude or position, es- For instance, in the 1980s, the Coast Nuances: pecially when landing. Coordinated Guard's HU-25, a derivative of the The typical pilot does not under- flight is deliberately abandoned in Dassault Falcon business jet, was be- stand that pitch attitude is not the straight-ahead slipping: The function ing tested at Edwards Air Force Base. same as angle of attack, that heading of the rudder is then to produce unco- The test pilots were doing sideslip angle is not the same as yaw angle, or ordinated flight. The slip is described tests at about 38,000 ft. When the that bank angle is not the same as roll. as & forward slip when increased rate pilot put in the last input to reach full Attack angle, yaw angle, and roll of descent is needed, as a side slip rudder pedal deflection, the vertical angle reference the aircraft and free when a change of direction is needed. tail stalled and the HU-25 swapped stream wind direction. As always, reality is often a combina- ends. The pilot, who was an A-7 Pitch angle, heading angle, and tion. Drag is increased, so descent rate departure pilot, said it was a 3g lat- bank angle reference the aircraft and increases versus straight flight. In a eral departure, the worst that he had the earth', the horizon and north. glider, carrying a steep slip to landing ever experienced. Fortunately, when For example, in a loop, an aircraft may touch the wingtip first, a danger they released the inputs, the airplane could have a constant angle of attack, to ship and crew. stopped yawing and recovered. perhaps 10°, while the pitch angle ro- A sideslip is the angle about the ver- Increasing pedal deflection should tates through 360°. Similarly, the yaw tical axis between the nose of a sail- also require increasing force. This is angle is the crosswind component of plane or airplane and the flight path not a problem on sailplanes because apparent wind velocity, and is unre- vector. A left rudder input yaws the the forces are low. On a big airplane lated to heading. nose left and causes the relative wind such as a KC-135 (Boeing 707), the to come from the right, aka, in the force to reach full rudder deflection What's the Point? flight test world, "wind in the right can be up to 250 Ib. If the force gradi- "Keep the yaw string centered" is a ear." When a sailplane has a sideslip, ent reverses, a pilot might have trou- good rule and a useful didactic guide. it experiences rolling and yawing mo- ble moving the pedals back to neu- Like all rules, it is based on a principle. ments. A stable sailplane will have a tral. Some sailplanes, such as a Grob Like all rules, it is a simplification that left rolling moment with "wind in the 103, can have rudder force reversals lacks nuance and has limits. There's an right ear" and a right yawing moment. at full deflection (which the pilot can old aphorism, the exception that proves easily overcome). the rule, which means that the rule is Rudder First? Aileron First? In a "steady heading sideslip," the tested or revealed by understanding its I've read arguments about whether pilot moves the ailerons opposite exceptions. In soaring, the steep turn is a slip should be established with aile- the rudder input to balance the roll- that exception, where the pilot should ron or rudder first. My experience in ing moment due to the sideslip. In a not rigidly follow the rule — and by a small number of aircraft is that if a sailplane with lots of side force due understanding the aerodynamics that steep slip is desired, the rudder should to sideslip, such as a Schweizer SGS underly it, deviate intelligently. lead before it gets blanketed by dis- 2-33, the bank angle required to bal- turbed airflow, and that this depends ance the side force is significant. In a Acknowledgments on aircraft design. The aircraft slides flying wing, which has a low side force My sources want to remain anony- toward the down wing. Bank affects due to sideslip, the required bank an- mous, so please do not send an email sideways movement and rate of de- gle is low. about this, especially if you feel like scent, and the rudder is modulated, debating. John Updike once wrote, as always, to adroitly manage the for- Ground Handling "Language is intrinsically approxi- ward-motion vector. Some gliders have a powerful rud- mate, since words mean different To achieve a straight-ahead forward der that works well even at low air- things to different people, and there slip, it seems most effective to first use speeds. If the glider has a swiveling is no material retaining ground for rudder to turn away from any cross- tailwheel, or if rollout speed is enough the imagery that words conjure in one wind and then promptly lower the up- to lift a fixed tailwheel, turns can be brain or another." Failure to hold this wind wing to slip into the crosswind made during the ground roll by using in mind is a cause of futile debate. while increasing descent to the rate opposite aileron (to prevent a ground The reporter. Dan Johnson is a retired needed, while using rudder to keep loop by touching a wingtip) and us- internist and AME who likes writing the aircraft on the rhumb line to the ing rudder in the direction of the term papers. runway. turn. This skill should be practiced on The desired response to increasing a wide runway so that you have it in rudder pedal input is an increasing your kit of tricks when a runway in- sideslip. If not, it can be a wild ride. cursion looms.