Features  / 12.11 /

Yaw Killer

Understanding the complex world of the rudder


What is the purpose of the rudder? The majority of pilots will quickly answer, "To help turn the airplane." And that's not an incorrect answer. It is, however, an incomplete answer, because the rudder comes into play in a lot of different situations in a lot of different ways. For that reason, it is the least understood control surface.

Rudder

How can a rudder possibly be misunderstood? You step on the left pedal and the nose moves left. Step on the right pedal and it goes right. What’s so complicated about that? Nothing, really, except that the flight regimes in which the rudder is required to make, or control, that right/left movement can be complex and are almost always subtle.

Controlling the right/left movement, otherwise known as yaw, is what the rudder is all about. So, the basis of understanding the rudder’s role in life is understanding when unwanted yaw is present and must be eliminated and when needed yaw is not present and must be added.

Takeoff forces

RudderFirst, let’s talk about the difference between torque and P-factor, and how they relate to yaw. Both come from the reaction of the prop to the engine, but for different reasons and with different results.

Torque is often mistakenly believed to be the left-turning culprit on takeoff, when it isn’t. Torque is the action/reaction concept that Isaac Newton first put into words. In this situation, it means that as the propeller rotates in one direction, there is an equal and opposite force that rotates the airplane in the opposite direction. The bigger and heavier the propeller in relation to the airplane, and the more horsepower there is involved (high-power setting), the harder the propeller tries to rotate the airplane.

Note, I said rotate the airplane, not turn: Torque is a roll reaction, not a yaw reaction. It tries to rotate the airplane about its longitudinal axis. There is no yaw involved. So the control to be used is the aileron. Not the rudder. And, as long as the landing gear is on the ground, the airplane can’t roll, so the effect won’t be noticed. Most general aviation airplanes don’t have big enough propellers or enough horsepower for this to be a factor.

This is not true when it comes to P-factor. P-factor is caused by unequal thrust generated by the different propeller blades. In a nose-up situation (higher angle of attack) on takeoff, when the power is maximized but speed is still building, the up-moving blades on the left (as seen by the pilot) have a lower angle of attack than the downward-moving blades on the right. Therefore, the blades on the right side of the airplane have more lift (known as thrust here) so they try their best to “pull” the nose to the left. It is almost pure yaw, so the rudder is the tool most needed to fight it. The effect is seen as the ball sliding out to the right while the nose is moving to the left.

If the airplane has enough horsepower, the effect of P-factor can be seen the instant the airplane starts to get light on the gear and becomes pronounced the second it leaves the ground. This is the case with lots of high-horsepower general aviation airplanes and aerobatic birds. For pilots of aircraft such as these, it becomes second nature for their right foot to build pressure on the right rudder as it starts to leave the ground.

P-factor

P-factor is a silly name for a relatively easy concept. The blades of a propeller have an angle of attack just like a wing. When the nose of the airplane is pointing at an upward angle, the blades on the left side moving up have a smaller angle of attack than the descending blades on the right side. Because they are moving at the same speed, the higher angle of attack produces more lift (or thrust), and thus moves the nose of the airplane to the left.

There is an additional force here: spiraling slipstream. This is the circular motion of the air from the propeller spiraling around the fuselage and striking the vertical tail at an angle. This, too, is pure yaw and corrected by right rudder. The actual effect of spiraling slipstream is difficult to quantify because it works in conjunction with P-factor, yawing the tail to the right, so the effects of the two forces can’t be separated. But it isn’t necessary to separate them because the corrective action for one also corrects the other. The bottom line is, if the nose is trying to yaw left, use enough right rudder to keep it straight: If the ball is to the right, step on it and keep it centered.

Gyroscopic precession is another wild card. The propeller is acting as a gyroscope. When a gyroscope is pushed, it will react by pushing back, except the reaction force will be 90 degrees to the input force. In the case of a tailwheel airplane, precession is put into play when the tail is raised, which forcibly pivots the plane of the propeller disk downward to a nearly vertical position. The gyroscope (propeller) reacts by trying to move the nose to the left. Again, it is pure yaw, so right rudder is required.

Gyroscopic precession is affected by the angular rate that the prop disk is pivoted and the speed of the airplane. The faster the tail is raised and the lower the airspeed, the more powerful the effect. If you want a really exciting ride, slam the stick forward on a high-horsepower taildragger just after the throttle hits the stop: As the tail comes up, the airplane will make a potentially violent turn to the left, testing the speed of your reactions. Even worse, the low speed guarantees an ineffective rudder, so a lot of rudder is needed. Sometimes a whole lot. And maybe even some right brake.

The key to controlling precession on a taildragger is to let the airplane accelerate a little before slowly (repeat, slowly) bringing the tail up so the rudder is much more effective when it’s needed. Don’t rush anything. And, if the nose starts moving, don’t slam the rudder down. Just steadily increase the right rudder pressure until it balances the left yaw.

Forces in climb: the asymmetric rudder thing

OK, so here we are off the ground, climbing at pilot’s operating handbook best rate of climb speed. Our goal is to climb straight ahead and turn left at 500 feet agl. As we maintain a nose attitude that gives us the proper speed, we note that, as the speed on takeoff increased, we needed less right rudder to keep the nose straight. That’s because the tail is much more effective and the effect of P-factor, et cetera, is less noticeable. So we relax. Then we get that “feeling” again and glance at the ball: It’s out to the right, but not as much as at takeoff. And the airplane is trying to turn left. So, we pressure the right rudder and all is right with the world. But, then we have to turn left and this is where the rudder is often misunderstood and misused.

Do helicopters have rudders?

The foot pedals located on the floor of a helicopter resemble the rudder pedals found in a fixed-wing airplane, but they don't operate a rudder. They're called anti-torque pedals, and they control the pitch of the tail rotor blades to vary the amount of thrust produced. Tim McAdams, author of AOPA's Hover Power blog, explains that the anti-torque pedals do control yaw, but they function differently. Assuming the helicopter's rotor system turns counterclockwise, when a pilot increases power by raising the collective control, the torque applied to the fuselage increases, and the pilot must add left pedal to increase the tail rotor's pitch—and therefore thrust—to keep the nose straight. Decreasing power requires right pedal input, McAdams says: "Right pedal reduces the pitch and thrust, allowing excess engine torque to turn the fuselage." —Jill W. Tallman

In climbing turns two forces are at work: While in the act of rolling into the bank, adverse yaw is trying to drag the nose slightly right but P-factor and spiraling slipstream are trying really hard to drag it left. Because the P-factor is doing its best to yaw us left, we have to keep that force in mind throughout the turn.

Normally, when entering a left turn, it would be left aileron and rudder applied at the same time. But in a climbing left turn, the power-induced P-factor is already trying to yaw it left. So, when rolling into the bank, no left rudder is needed. All that needs to be done is to slightly relax the right rudder we’re using to counteract the P-factor and let the existing left yaw balance any adverse yaw that comes from the outside aileron being down. Then, as the desired bank angle is reached, the ailerons are neutralized and the right rudder reapplied while turning as necessary to keep the ball in the middle.

If making a climbing right turn, just the opposite is true: The right rudder that is already depressed has to be depressed even farther, while in the act of rolling in (while the ailerons are deflected). Then, when the ailerons are neutralized to hold the bank, the amount of right rudder is reduced, but part of it stays in place throughout the turn, and is again reduced, while in the act of rolling (left) out of the bank.

In other words, whether making climbing turns right or left at full power, a slight amount of right rudder will be held almost all the time except when rolling left.

Level flight forces

RudderFortunately, all the forces we’ve discussed almost disappear in level flight because the engineers have compensated for them in the airframe design, mainly by offsetting vertical surfaces. However, every single time the ailerons are deflected (normally for turns), there will be unbalanced lift on the wings, left and right. This happens even when just fighting turbulence, and especially when rolling into turns. If there is unbalanced lift, there is unbalanced drag, so the wing with the most lift (outside wing in a turn, which has down aileron), will have the most drag, and it will try to lag behind the other one. When it does that, it yaws the nose toward the high wing (outside of turn). So, rudder into the turn is needed, but only when the ailerons are deflected. As soon as the ailerons are neutralized, while in the turn, lift on the wings is balanced (if it wasn’t balanced it would continue to roll), so drag is balanced and no more rudder is needed. When the turn is completed, outside aileron and rudder go in together and the turn is completed.

Rudder during the approach

Remember that P-factor on takeoff? Well, it’s still there on landing approach, but in reverse because the nose is down and power reduced. The efficiency of the airplane on approach will be greatly determined by how well the ball is kept centered. That, in turn, is determined by how good of a job we do in canceling out the yaw caused by P-factor at all times during the descent.

If it’s a power-on approach, the P-factor’s turning effects will be minimal. However, as soon as the power is brought all the way back, the nose is going to go right and the ball left. The degree of both of these actions will be determined by the size of the propeller in relation to the airplane and whether it is a constant-speed prop. The cure is a little left rudder.

Now, however, think about the turns.The nose is trying to go right, so in a gliding left turn, while rolling into the turn, a lot of left rudder will be needed to keep the ball in the center, and a little will be needed the rest of the time. If turning right, without power, the airplane already wants to yaw right from P-factor, so, while the ailerons are deflected, no right rudder is likely to be needed. Then, while in the gliding right turn, a little left rudder may be needed to keep the ball in center.

Rudder changes with angle of attack

Rudder

Rudder must be applied on the ground to keep the nose going straight down the runway. This is because the pedals control the steering, just like the wheel in a car. But soon after takeoff, compensating for wind should be done with a crab angle. Applying only rudder to keep the nose straight would result in a sideslip.

Here’s an important factor that almost no one talks about: Adverse yaw increases with angle of attack. This is a simple fact of geometry: As the airplane is slowed down, a higher angle of attack is required to maintain level flight. This tilts the lift vector back while the vertical vector that opposes gravity has to be at least long enough to keep the airplane in the air. The result is the drag vector that completes the triangle becomes longer.

As the airplane is slowed for approach, and then slowed more for touchdown, the adverse yaw skyrockets.

So, if you’re trying to turn slightly to realign an airplane on the centerline during flare, much more rudder than normal is needed to keep the ball in the center and the nose ahead of the airplane. This is one of the foremost causes of touching down crooked. In most flight regimes, the rudder is there to control yaw, which is another way of saying it is there to keep the ball in the middle (slips notwithstanding). This sounds simple but, in reality, it is one of the most complex, overarching statements in aviation. However, if you master rudder usage, you’ll find that you have flying an airplane licked and you’ll be a smoother, more efficient pilot.

Budd Davisson is an aviation writer/photographer and editor. A CFI since 1967, he teaches about 30 hours a month in is Pitts S2-A Special. Visit his website.


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