One of the more maddening aspects of instructing pilots who have already earned their certificate is continuous exposure to a rather curious dance in which their feet realize they are supposed to be doing something--but they neither hear the music, nor feel the rhythm. In other words, their feet are confused and don't know exactly what they are supposed be doing or when. Maybe it's time to examine the rudder's purpose in the greater scheme of flying.
The rudder is a widely misunderstood control. When used properly, it does so many different things in so many different situations. The ailerons and elevators are fairly one-dimensional in their application--they do only one thing. But the rudder is involved in so many different aspects of flight that it's the renaissance man of control surfaces. This is curious, considering that all it does is move the nose left and right.
No one has any doubt about what stepping on the rudder pedal does. Step on the left pedal and the nose moves left, and vice versa. So, what's the big deal? The big deal is that the rudder is asked not only to be part of the normal control mix in making turns, but it is also the control that has to compensate for lots of other interacting forces that occur, in varying amounts, in different flight regimes. It's more than just the control that helps us to make turns, although it often is being ignored in that--its primary duty--as well.
The rudder isn't really a primary control. It's a "fixer" control. It's there to "fix" things that would go wrong because of the way various forces affect the airplane. In the air, it doesn't make the airplane do anything (a gross generality, I know, but bear with me). It doesn't make the airplane turn: The aileron's job is to set up the bank that causes the turn, but the rudder does make the airplane turn correctly. It doesn't make the airplane climb--that's what the throttle and elevator are for--but the rudder does make the airplane climb more efficiently. It doesn't cancel out a crosswind, another aileron job, but it does make the results more pleasant. And on and on.
So what exactly does the rudder do?
The rudder's primary purpose is to "purify" flight situations by eliminating unwanted yaw, thereby keeping the airplane aerodynamically clean (intentional slips notwithstanding). There is a whole list of forces against which the rudder has to fight, and no place is this more clearly demonstrated than in the simple turn--where, as in most flight regimes--there is an aerodynamic villain just itching to reduce efficiency. In this case, it's adverse yaw.
The common turn: rudder versus adverse yaw
Nothing in life is free, and this is especially true of aerodynamic lift. Increase lift and you increase drag. So, when we bank the airplane by lowering the outside aileron and raising the inside one, we've asked for more lift on the outside wing and less on the inside wing. This is another way of saying we've increased drag on the outside wing and decreased it on the inside one. So, what happens when there is more drag on the outside wing than on the inside one? That extra drag pulls the outside wing back, so the nose yaws to the outside of the turn. Enter the rudder to save the day. A little rudder is applied in the direction of the turn, along with the ailerons, to offset that yaw and keep the nose turning.
A rule not to be violated: Whenever an aileron is deflected, there is unbalanced lift/drag on the wings and rudder is needed to offset the unwanted yaw. However, as soon as the bank angle is established, the ailerons should be neutralized, which means the rudder input is no longer needed. No aileron, no rudder. Period.
Takeoffs and gyroscopic precession
Gyroscopic precession is a curious phenomenon in which you try to move a spinning object--for example, a gyroscope--and the spinning motion changes the direction in which the force is acting; the result is a move 90 degrees to the original line of force. The effect is greater for large, heavy objects like propellers. So, if you forcibly move a spinning propeller, such as when the tail is lifted on a tailwheel airplane, even though the force was applied in a downward direction (tail goes up, nose goes down), the resultant motion includes an urge to move left.
On smaller, lighter airplanes, like an Aeronca Champ or Piper Cub, whose wooden props may weigh less than 15 pounds, the effect is barely noticeable. On something like a two-place Pitts Special where the prop weighs more than 60 pounds, the movement is more obvious. That same prop on a larger airplane, however, won't have as much effect because the airplane will be heavier, and the precession forces will be resisted by inertia.
You'll experience gyroscopic precession on tricycle-gear airplanes as well, although the effect usually is more noticeable on tailwheel aircraft. The yaw caused by precession is easily handled with just a little right rudder pressure.
Climbs and that dastardly torque
Torque is a radial action that tries to twist the airframe opposite to the direction in which the propeller is turning. If the airplane is on the ground with the gear firmly planted, it can't twist the airplane because the landing gear is stopping it. Torque is often blamed for the sudden turn to the left when the tail is picked up on a tailwheel airplane, but precession is actually the villain there.
Torque effect is felt most strongly right at the moment of liftoff. Here the gear is leaving the ground, and the airplane is slow and in its most vulnerable moment of flight. On most general aviation airplanes, the effect is slight because the prop is light, the power relatively low (even at full throttle), and the airplane fairly heavy. However, as the power-to-weight ratio improves (bigger engine, lighter airplane), the airplane will try to drift left (assuming clockwise prop rotation as seen from the cockpit) as it comes off the ground, and some right rudder will be needed to keep the nose straight and prevent any drift.
On all airplanes, repeat all airplanes, during the climb--when the airplane is slow and the power high--the ball in the inclinometer is going to try to slide to the right, and rudder will be needed. On some training aircraft the ball will barely nudge out of center, and it may seem easy to argue that rudder isn't needed. That's absolutely not the case. If the ball is even slightly off-center, the airplane is trying to climb with the nose yawed to one side, which means it's less efficient than it should be--not to mention that the effect may be uncomfortable to any passengers. A lot of horsepower is being wasted trying to drag the airplane through the sky sideways, and that just doesn't make sense when just a touch of rudder is all that's needed.
Climbing turns--the plot thickens
When the airplane is climbing there's more at stake than simply rate of climb. Sooner or later, you're going to have to turn the airplane when still in the climb, and this is where an understanding of torque and the rudder is necessary.
In a climb the torque--assisted by spiraling slipstream--is constantly trying to yaw the airplane left, and in that situation how you use the rudder in a turn will change depending on whether you're turning left or right. In a left turn, you don't need left rudder; you just need less right rudder. In a right turn, you need more right rudder. In fact, in a climbing, full-power left turn in most airplanes you'll probably be holding a little right rudder through the turn to keep the ball centered and the airplane properly coordinated.
Approaches, P-factor, and an erratic skid ball
If you'd like to see a clear demonstration of the interplay between power, torque, and P-factor--the asymmetric thrust/drag caused by the prop--try this little exercise the next time you're flying. Set up a best-rate climb at full power and leave your feet off the rudder pedals. See where the ball settles: It'll be off center to the right. Then, holding the same speed--which is probably the same as, or close to, best glide speed in the pilot's operating handbook--reduce the power, lower the nose to a glide attitude, and see what the ball does. It'll be off to the right in a climb and will gracefully slide to the left in the glide as torque is exchanged for P-factor.
When you pull the power to idle on downwind for a power-off approach (you do practice power-off approaches, don't you?), you'll see the ball slide to the left. It'll be more obvious in some airplanes than in others. That means the nose is to the right of your flight path, the airplane is aerodynamically "dirty," and you're losing altitude faster than is necessary. If you turn left onto the base leg in that condition, while you're in the turn not only is the airplane dirty, but the nose is to the right and slowing down the turn so it takes more time to get through the turn, and you spend much more time with a wing down than you would if the turn were coordinated. Because of that combination, you lose much more altitude than necessary. All in all, not a very efficient way to fly an airplane. And if the airplane slows so much that the wings lose lift and stall, the lack of coordination could result in a spin--so this isn't very safe, either.
A little touch of left rudder in the approach will keep the maneuver coordinated and greatly increase your airplane's ability to glide. Most airplanes need only a hint of rudder, but if you watch closely, you'll see that it is needed.
The forward slip and unwanted altitude
Although we should be constantly trying to keep our flying coordinated, there are times when that's not what we want to do--and the rudder is the primary player in these instances.
If the airplane is put into a bank and held there with displaced aileron, and enough rudder is applied in the opposite direction to offset the turning tendency and keep the airplane flying in a straight line, you are performing a forward slip.
The airplane may be moving straight ahead in a very high-drag configuration. It's actually flying somewhat sideways, and unless you apply a lot of power, the airplane is going down--fast! And that's the purpose of the forward slip: to lose altitude.
By using opposing rudder and aileron on final approach, with the power off, the resulting super-high-drag configuration causes the airplane to descend faster than it ordinarily would. Because the control inputs for a forward slip can be applied to any degree, the drag can be used to increase the rate of descent just a little or a lot, allowing you to fine-tune the glide path to put you exactly where you want to be on the runway.
The sideslip and the villainous crosswind
If the rudder does not exactly balance the bank angle in a forward slip--e.g. there isn't enough rudder authority to keep the airplane on the intended course--then the airplane will move in the direction of the down wing. The exception, of course, is if there's a crosswind from that side to hold the airplane in place. This proves handy when it's time to make a crosswind landing.
Here, just enough rudder is applied to keep the nose pointing straight down the centerline, and enough bank angle is held to balance the crosswind. This is called a sideslip. The result is that the airplane flies straight ahead, but with the upwind wing lowered.
The rudder is the magic tool that cleans up what might otherwise be some pretty ugly--and uncomfortable--flying. Can you fly without understanding the rudder? Of course, but you won't be flying correctly, and your airplane will be both inefficient and less likely to go exactly where you want it to go. So, it's your choice. Fly right, or fly wrong. Not much of a choice, is it?
Budd Davisson is an aviation writer/photographer and magazine editor who has written approximately 2,200 articles and has flown more than 300 different types of aircraft. A CFI since 1967, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his Web site.
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Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
