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Home » John E. McLain

Understanding the use of rudder: Its most important use is preventing yaw

By John E. McLain (June 2001)

I have been writing for The Southern Aviator and its predecessors for close to 15 years. Coming up with a fresh subject each month is often more difficult than the writing. I have now decided to resurrect some prior articles, but with a new slant.

Thirty-seven years as a flight instructor and 22 years as a pilot examiner have shown me time and time again that pilots do not understand how properly to use the rudder on their flying machine. The impression most pilots have is that it is used in some way to help turn the airplane. The most common expression used is that it coordinates the turn. While there is some validity to this, it is far from explaining the real purpose of the rudder.

The most important use of rudder is not to do anything positive. Rather, its primary purpose is to prevent yaw. There are some secondary uses of rudder, which I will discuss later in this article, but from a safety standpoint the control of yaw is its most important function. Obviously, then, I must include an explanation of yaw in this article.

Yaw, by definition, is the rotation of the airplane about its vertical axis, which is often — and appropriately — called the yaw axis. I will now make a statement that I hope every reader will take to heart: “Unless you are performing acrobatics, you should never allow the airplane to turn about the yaw axis.” Doing so can lead to two unpleasant consequences: The first is an airsick passenger in the back seat; the second is an unplanned roll, which could become uncontrollable.

Let’s start with the easy one, the airsick passenger. As the airplane rotates about the vertical axis, the passengers in the rear seat are forced from side to side, much like the passenger in the back seat of a car when it is turning on an unbanked road. This leads to inner ear unbalance, which leads to motion sickness. On the other hand, the pilot doesn’t have this problem, sitting on or close to the center of gravity, not feeling the effects of yaw. Nowhere is this more apparent then giving instruction in a tandem airplane, where the instructor sits in the rear seat, feeling all the effects of poor yaw control. The instructor, not wanting to suffer the discomfort or indignity of getting airsick, is quick to insist, in a not-too-calm manner, that the student correct the indiscretion.

Now to the serious effects of yaw control. I want you to imagine a specially equipped airplane. It has two airspeed indicators. One is connected to a pitot tube on the left wing, and the other to a pitot tube on the right wing. In a no-wind condition, with the airplane in coordinated, straight and level flight, both airspeed indicators will indicate the same. Now assume you apply hard left rudder. What happens? This creates a yaw to the left. The right wing is now advancing into the relative wind, and the left wing retreating. The result is the airspeed indicator connected to the right wing now indicates a higher airspeed than the one on the left wing. This means the right wing is generating more lift than the left wing, which results is a roll to the left, even though the ailerons are neutral. That is the simple explanation of why the pilot should not allow the airplane to turn about the yaw axis. Now lets examine the situations that induce yaw, then we will discuss the undesirable situations that result from poor yaw control.

The first thing you should have learned about yaw as a student has been variously refereed to as adverse yaw or aileron drag. This results from one of the basic principles of aeronautics. For years, aeronautical engineers have been fighting it, and while they are creeping up on it, the fact still remains: “When you increase lift, you increase drag.” Because of this, any time the pilot tries to bank or recover from a bank with the use of aileron, yaw will be induced. The yaw will be toward the wing with the increased lift.

The next inducer of yaw is what is commonly known as torque. This subject alone could cover an entire article. I hope most readers have a basic understanding of engine torque, “P” factor, and related items that tend to make an American airplane yaw to the left, and that this tendency increases with power. For convenience, we lump all of these factors into the word “torque.”

Other factors that induce yaw are, of course, poor use of the rudder and, for the multi-engine pilot, the loss of one engine. A seldom-considered cause of yaw is turbulence.

Now that the reader has a basic understanding of yaw and its causes, lets look at some situations where the proper use of rudder to prevent yaw will make for a safer and more comfortable flight.

I will start off with the use of rudder in initiating a turn. Every student pilot has had it pounded into him or her that rudder and aileron must be coordinated when entering a turn. However, and this is where I become a heretic even to myself, I believe that rudder should be applied slightly ahead of aileron when entering a turn. There is an old adage in flying that many pilots have either forgotten or never learned. It is simple. “Never let the degree of bank exceed the number of degrees you want to turn.” Thus, if you want to turn 15 degrees, use no more than 15 degrees of bank. This is a good rule to follow.

This is not much of a problem if you want to make a fairly large heading change. A 15 degree bank would be adequate for most turns of more than 15 degrees. It is when we begin to talk about small heading changes, such as on final approach or on an instrument approach, that the problem arises. Here, we are often facing heading changes of less than five degrees. I submit that the use of aileron to establish a bank will almost invariably result in more than five degrees of bank. This problem can be further complicated by the adverse yaw or aileron drag which, if not immediately countered by rudder, will delay the turn. The reaction of the pilot is the to apply more aileron and, suddenly, we have a 10 degree bank for a five degree turn. Believe me, you will then make more than a five degree turn and, in trying to correct this, the problem is repeated in the other direction. Pretty soon you are doing lazy eights on final.

My suggestion is that, for any turns of less than five degrees, use rudder only to establish the desired bank. A slight application of rudder in the direction of the turn will produce a very shallow bank and eliminate the adverse yaw. Please note that I did not say to keep the wings level with aileron, which is a common misconception. The rudder application will produce the desired small bank, and you should let it. See what I mean about being a heretic? I am advocating using a little rudder-induced yaw, with no aileron, to produce a slight bank. Whatever happened to aileron and rudder coordination, and not allowing the airplane to turn about the yaw axis? Well, there are exceptions to every rule.

The next time you are flying, try this. Trim the airplane for level flight, put your hands in your lap, and hold heading with rudder alone. Then, try turning five degrees in each direction using only rudder. Keep in mind that you should neutralize the rudder once the turn starts, and you will have to use opposite rudder to stop the turn. You might be surprised at how your heading control improves. If you want to take it one step farther, put your feet on the floor and try to hold heading and make five-degree turns using only aileron. I think you will get the point.

Now, how about entering a bank of more than five degrees? Aileron and rudder coordination is now required. I am a great believer that nobody alive today is perfect. We all make mistakes. Knowing this, I like to make my mistakes on the safe or best side. Perfect aileron and rudder coordination when entering a turn is virtually impossible. It is sort of like making a perfect landing every time. Therefore, I would suggest that you lead with the rudder in entering a turn. To do this, apply rudder to start the bank, and then apply aileron to make the bank steeper. I think you will find it improves your coordination in the turn.

Finally, remember, once a medium bank is established you must neutralize both the aileron and rudder. A common mistake I see with pilots is the failure to neutralize the rudder. This results in a continued yaw in the direction of the turn and a tendency for the bank to steepen. The pilot’s tendency is to apply opposite aileron, and we then have the dreaded cross-control. Don’t let this happen to you.

Next month, I will discuss several other scenarios where proper rudder technique can improve your flying proficiency and safety. This will include smoothing out turbulence, controlling torque, and avoiding stall/spin and spiral accidents.