One of the more maddening aspects of instructing pilots who already have their license is to be continually exposed 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. So, maybe it’s time to examine the rudder’s purpose in the greater scheme of things and see what it can do to make our lives better.
The rudder is a widely misunderstood control because, when used properly it does so many different things in so many different situations. The ailerons and elevators are probably jealous of the rudder because they are fairly uni-dimensional in their application—they only do one thing. But the rudder? That guy is flapping around back there and 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 what stepping on the rudder 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 ask not only to be part of the normal control mix in making turns, but it is the control that has to cancel out, or control, lots of other interacting forces that occur in differing amounts at different times in different flight regimes. It’s more than just the guy who helps us make turns, although he often is being ignored in that, his primary duty, as well.
The rudder isn’t really a primary control. It’s a “fixer” control.
The rudder’s primary purpose in life is to “purify” flight situations by eliminating unwanted yaw, thereby keeping the airplane aerodynamically clean (intentional slips not withstanding). There is a whole list of forces the rudder has to fight against and no place is this more clearly demonstrated than in the simple turn, where, as with most regimes of flight, there is an aerodynamic villain just itching to cause efficiency problems. In this case, it’s adverse yaw.
The Common Turn: Rudder Versus Adverse Yaw
A rule not to be violated: as long as an aileron is deflected, there is unbalanced lift/drag on the wings and rudder is needed to kill the unwanted yaw. However, as soon as the bank angle is established, the ailerons should be neutralized, which means the rudder is no longer needed. No aileron, no rudder. Period.
Takeoffs and Gyroscopic Precession
On smaller, lighter airplanes, like a Champ or Cub, where the wooden prop may weight less than 15 pounds, the effect is barely noticeable. On something like a two-place Pitts Special where the prop weights over 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 than the little Pitts so the precession forces will be resisted by inertia.
The yaw caused by precession is easily handled with just a little right rudder pressure, although on really healthy airplanes, like a P-51 Mustang, part of the required pressure is applied by preset right rudder trim but you’ll still need to put a fair amount of right leg in it (been there, done that).
Climbs and that Dastardly Torque (supporting role by spiraling
Where torque effect is most strongly felt is right at the moment of lift off. Here the gear is leaving the ground and the airplane is slow and in its most vulnerable moment of flight. On most GA airplanes, the effect is slight because the prop is light, the power low and the airplane fairly heavy. However, as the power to weight ratio improves (bigger motor, lighter airplane), the airplane will try to drift left (assuming clock wise prop rotation as see from the cockpit) as it comes off the ground and some right rudder will be needed to keep the nose straight and kill the drift.
On higher performance airplanes, the Pitts being one, if left to it’s own devices, the airplane will almost instantly start drifting to the left at about a 15 degree angle.
On all airplanes, repeat all airplanes, during the climb, when the airplane is slow and the power high, the ball is going to want to slide to the right and rudder will be needed. Okay, granted, on common GA airplanes of the Cessna/Piper type, the ball will barely be nudging out of center and it’s easy to argue that rudder isn’t needed. But, 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 dirty and less efficient than it should be. A lot of horsepower is being wasted trying to drag the airplane through the sky sideways and that just doesn’t make good sense, when just a touch of rudder will clean it up.
Climbing Turns—the plot thickens.
In a climb the torque (assisted by spiraling slip stream) is constantly trying to yaw the airplane left and what you do with the rudder in a turn in that situation changes 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 carrying a little right rudder through the turn to keep the ball in the middle and the airplane properly in trim.
The good news in this situation is that your butt will be moving back and forth in the seat and, if you listen to it, it’ll tell you when need to be paying more attention to the ball.
Approaches, P-factor and an Erratic Skid Ball
When you kill the power on downwind for a power-off approach (you do do power-off approaches right?), you’ll see the ball slide to the left. It’ll be more obvious in some airplanes than others. That means the nose is to the right, the airplane is dirty, and you’re losing altitude faster than is necessary. If you turn left onto base in that condition, while you’re in the turn the airplane is not only dirty, but now 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 in a dirty condition than you would if coordinated. Because of that combination, you loose much more altitude than necessary. Plus, you’re skidding away from the runway in the turn, which is the same thing as loosing altitude. All in all, not a very efficient way to fly an airplane.
A little touch of left rudder in the approach will keep the ball in the middle 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
The airplane may be flying straight ahead, but it is in an unbelievably high-drag configuration. It’s flying sideways and unless a lot of power is fed into the equation, the airplane is going down. Fast! And that’s the actual reason for the forward slip in the first place, losing altitude in an accelerated manner.
By using opposing rudder and aileron on final, with the power off, the resulting super high drag configuration causes the airplane to come down faster than it would normally. Because slipping controls can be applied to any degree, the drag can be used to increase the rate of descent just a little or a whole lot, as it is needed. It can be varied constantly allowing you to fine tune the glide slope to put you exactly where you want to be on the runway.
The Side Slip and the Villainous Crosswind
So, What is the Rudder Really For?Getting back to the original question: the rudder is the magic tool that reaches in and cleans up what would otherwise be some pretty ugly regimes of flight. Can you fly without understanding it? 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? BD
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