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Adverse yaw

In early aviation, pilots found that getting airborne was not a problem. Level flight was not a problem. So what was the problem? Turning! Flying in a straight line was fairly simple, but trying to turn these early planes was something totally different! Initial heading control designs were fairly crude, and improper use of these claimed the lives of many a fledgling aviator. One early method of turning was to change the aircraft heading by using the rudder alone. The technique was similar to the way a boat is turned, and this method became known as "boat turning" or "flat turning" since the airplane remained wings level during the turn. Flat turning, however, was slow, created a lot of drag, and was a poor solution to the problem of heading control. Another way was needed, and that way was to use the wings to roll the aircraft into a bank. The result was a turn.

This raised the question of how to cause the aircraft to roll about the longitudinal axis. One of the first ways of creating a bank was a concept known as "wing warping". The Wright brothers, in fact, used wing warping on their first airplanes. The pilot’s controls were used to pull cables that literally bent the outer portions of the wing up or down to create a roll. By the advent of WW1, most aircraft had adopted the aileron as the means of controlling a bank. Pilots found that a combination of bank angle and elevator determined the quickness of their turns. They also found something else. If they rolled into a bank quickly, the nose of their plane seemed to respond in a funny way. If they rolled in one direction, the nose seemed to move the other way momentarily before reversing its direction. They quickly learned that they could control this instability by using a little rudder to accompany their aileron input. If the pilot added rudder in the same direction as the aileron, the nose did not move at all, and the aircraft seemed to roll right around its longitudinal axis. In fact, the rudder seemed to have a pronounced effect on turn stability. The pilot was keenly aware that there was a fine line between using just the right amount of rudder to begin a bank...and using too much or too little.

Clearly, the idea that if a little was good, a little more would be even better was not going to be the case in performing a turn! Just the right amount was known as a "coordinated turn". Too much rudder in the direction of bank was termed a "skid", and too little (or rudder opposite the bank) was called a "slip". This is how it looked to them - Note that the point of emphasis is on how the fuselage (longitudinal axis) is aligned with the flight path:

 

When asked what was happening to the nose when a bank was initiated, these pilots determined that they needed to call this instability something...so they named the sideways movement "yaw"...and since the movement was opposite the direction that they intended, they named it "adverse yaw". Next question...what was causing this "adverse yaw"? Those aileron things that the pilots were using to bank the aircraft! They were the problem! Simply speaking, the aileron opposite the direction of bank was pulling the nose "back" or opposite the roll. If the pilot added a little rudder during the roll, this "back" motion or "yaw" was eliminated. And so it remains today. Our flight control systems are a jillion times more sophisticated, but we still have to control adverse yaw in any airplane. Sometimes the pilot does it, sometimes a computer does it, and other times the basic aircraft design corrects the problem...but, regardless, adverse yaw has to be accounted for.

Our simulations are no different. In some, the AI (artificial intelligence) handles the rudder input when rolling and no action is needed from the pilot. In others, the pilot may have to use traditional rudder skills to offset adverse yaw. And in other sims, neither case applies since the sim programmers simply did not include adverse yaw in the flight model! Rage’s Typhoon is a good example of the first, and Il-2 of the second. The third type covers most of the older generation sims. Finally, you will probably find that adverse yaw is not a big deal in your sim. No matter. Yaw control is what rudder use is all about, and a good understanding of the subject will make your flying more immersive and, consequently, more enjoyable. So there you have the background on the rudder. Use it for directional control during takeoffs and landings, maintaining coordinated flight during banking maneuvers, and keeping the fuselage aligned with the flight path in straight flight.

Having said that, let’s move on to the actual rudder controls themselves.

Rudder Implementation In Flight Sims

There are four ways you may make rudder inputs in our sims...one good one, one so-so one, and two not-so-good ones!

The best way to input rudder commands in a sim is to use a set of rudder pedals. Two of the most common on the market are the Thrustmaster Elite pedals and the CH Products Pro Pedals. SimPed also offers a high quality rudder pedal set.

  

The advantage of the pedal method is that the system is a "hands off" direct simulation of the real world pilot action. The pedals are fairly easy to set up on your computer. The range of motion of the pedals allows the pilot to easily vary the amount of rudder input.

Second on the list of rudder set ups is the stick-based rudder control. Various brands of flight sticks have a grip that can be rotated or "twisted" either clockwise or counter-clockwise. This twisting simulates the rudder pedal input. The typical stick grip does allow the pilot to vary the amount of rudder input, but I have found the twist grip too easy to inadvertently rotate in the "heat of battle" and thus make an input when none was needed or wanted.

Next on the list is using the default keyboard keys to make rudder inputs. As a rule, I have found this to be unsatisfactory in that the action requires the pilot to take his hand off the stick or throttle at a time when that may be very impractical. In addition, the use of the keys does not give the pilot much of a "feel" for what is a relatively delicate flight control input. Rudder often requires a "fine touch", and it’s hard to achieve that sometimes when using a keyboard.

Lastly, those of you with a programmable HOTAS may choose to map the rudder keys to your throttle or stick. My feeling is that this is not any better than using the keyboard other than it does save the pilot from having to move his hands off the controls. The replication of using buttons for rudder input is a poor simulation of rudder feel.

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