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The Role and the Use of the Rudder

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The Role and the Use of the Rudder of us have ignorantly followed sim- The Role and the Use plistic rules. When the aircraft is in equilibrium flight, it is not accelerating in any di- of the Rudder 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 airspeed indicator 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 stabilizer is to achieve yaw, or weath- — RBick a thermal and landing in turbulence. ercock, stability. When the fuselage 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, Ailerons: 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 Elevator 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 airplanes 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 glider 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 AVIONICS 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 aileron 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 propeller (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.
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