Step on it!

Step on it!

November, 2002

Purpose of this tutorial

To discuss and explain why the pilot needs to use rudder in a turn.

A short lead-in

A pilot and an aeronautical engineer were driving through the countryside when they came along a group of sheep, grazing in a pasture. The pilot remarked "Look, those sheep have just been shorn." The aeronautical engineer observed "Well, at least the side facing towards us has".

Why do I tell this remarkably non-funny story? Because I'm a pilot, not an aeronautical engineer. The following is based on what I know as a practicing aviator.

OK, let's cut to the chase. Just why do you need to use rudder in a turn?

First off, let's establish some guidelines so that we're all on the same page.

Control surfaces on an airplane

Take a look at the diagram below.

I apologize in advance if this gets a little mind-glazing to the reader, but in order to understand this "rudder in the turn thing", we have to understand why an airplane does what it does, and have a common understanding of what we're talking about.

Some definitions

Pitch:

Pitch is rotation around the "Y" axis of the airplane- the horizontal axis. Consider the "Y" axis as running through the center of the wing, from one end to the other. The airplane "pivots" around this axis in the "pitch mode." Pitch is controlled by the Elevator. Note that in the diagram, and on many airplanes, the elevator is actually in two pieces. In actuality, these two "halves" are married together internally and act as one unit. Collectively they are just called the elevator.

Here the pilot has pulled the wheel back, raising the elevator.

The airflow striking the raised elevator pushes the tail down. The aircraft will pitch up, rotating around the horizontal axis, the "Y" axis.

In this case, the pilot has pushed the wheel forward, lowering the elevator.

The airflow striking the lowered elevator pushes the tail up, and the nose down.

Roll:

Roll is rotation about the longitudinal axis of the airplane, the "X" axis. Roll is controlled by the ailerons.

Roll,and the yaw associated with, it is why we are here. We need to take a look at "yaw" and "drag" before we come back to "roll". (Sorry, there is no "rock".)

Yaw:

Yaw is rotation around the "Z" axis of the airplane- the vertical axis. Think of the "Z" axis running vertically through the airplane, up and down. The aircraft will pivot about this point.

When it does, it is called "yaw".

The pilot has pushed in the left rudder pedal.

The airflow striking the displaced rudder will push the tail to the right. The nose of the airplane will yaw to the left around the vertical "Z" axis of the airplane.

The pilot has pushed in the right rudder pedal.

The airflow striking the displaced rudder will push the tail to the left. The nose of the airplane will yaw to the right around the vertical "Z" axis of the airplane.

OK, so far so good. But......

Notice that we have been moving only one control surface so far- either the elevator or the rudder. Now we get into the ailerons, and we'll be moving two control surfaces. And, to really muck up the waters, one will be moving up, and the other one down. So, finally, we get down to the question of why rudder is necessary in a turn.

A word about drag

Sorry. Before we get into using rudder in a turn it is necessary to understand one other concept- the relationship of lift and drag on an airplane.

Perhaps you always thought that drag was drag, plain and simple. Ah Grasshopper, you just knew that nothing in aviation is simple. In a nutshell there are two forms of drag.

Whoa! "Two" forms of drag?

Yep, there's "form" drag and "induced" drag. Sometimes you may hear different terms used, but these are the two we will use here. To understand why you need to use rudder in a turn you need to understand the concept of drag on an airplane.

Form drag

Form drag is the drag of the air going around the airplane, and all of the stuff that hangs out from and sticks out from the airplane. The air that passes over the airframe constitutes most of this drag, but the antennas sticking out, and the gear are other contributers. Outside of retracting the landing gear, there is little that the pilot can do about form drag. However, form drag increases by the square of the airspeed. That is, when you increase airspeed from 100 knots to 200 knots the Form drag has gone up by a factor of 4.

In a nutshell, that's why doubling the horsepower of an airplane will not double its airspeed.

Induced drag

This is the drag that is created by lift. It's the old physics thing- you can't get something for nothing. When you get lift, you get drag. The more lift you have, the more drag you have- plain and simple. The good news is that for all intents and purposes induced drag doesn't increase with airspeed- you just need to counter gravity to get the airplane in the air. However when you are doing (for example) a steep turn, you are effectively increasing the weight of the airplane because of "g" loading- that's why you have to pull back on the wheel and increase the angle of attack when in a steep turn.

Let's make a turn

Here is our little red airplane making a left turn.

Because both ailerons are deflected equally, albeit in different directions, they both create the same amount of form drag and cancel each other out. However, the right aileron is creating lift because it is effectively changing the "camber" of the wing.

Here's a wing in the "normal" position:

And here's the same wing with the aileron "down":

"Camber" is, in effect, what creates lift. The more camber, the more lift. So, when an aileron is deflected in the "down" position it increases the camber of the wing and creates more lift. The same principal applies to the flaps on an airplane. For all intents and purposes they are just ailerons that "drop down" together and create more lift- and drag.

Back to our left turn. The left aileron is "up", and the right aileron is "down". The right wing has more lift than the left one does because it has more lift. Because the right wing has more lift it will rise up, banking the airplane to the left and initiating the left turn.

So? What's that got to do with the rudder?

Ah, here's where it finally all comes together. Because the right aileron is creating more lift that its cousin on the left side it is creating more (induced) drag. (Remember that both ailerons are creating equal "form" drag, so they cancel each other out.) Because the right wing has more lift, it has more drag. Because it has more drag it will yaw (skew) the airplane to the right. Now don't get confused here, it's easy enough to do. The airplane is banking to the left, but the nose is yawing to the right. If the nose is yawing to the right, the tail is yawing to the left. Because the tail is yawing to the left you will see that the ball in the Turn and Bank instrument is also to the left.

The T&B is a yaw indicator that is related to the yaw of the tail in the airplane. If the tail is yawed left, the ball in the T&B will be to the left.

When making a turn, anticipate that you will need rudder in the turn and apply some going into the turn. Check with the T&B to see if you have the appropriate amount. In a real airplane (sorry), the pilot can sense the sideways motion of the yaw. In a simulator it's much more difficult. You just have to refer to the T&B to see how you're doing.

Aha! So that's why you need to use rudder in a turn?

Yep, plain and simple. You can stop reading right here, and you have the answer to the question. Or, keep on going and we'll look at some aspects of this roll and yaw thing.

So the airplane yaws in a turn. So what? What's the big deal?

Four prime reasons.

1. When the airplane is yawing the air coming into the pitot tube that drives the airspeed indicator is at an angle. When the air entering the pitot tube is at an angle, the indicated airspeed is no longer accurate. You, as a pilot, always need to know what the airspeed of your aircraft is- it is critical information to safe operation.

2. When an airplane is yawing, it is not efficient- in effect, it is flying sideways. When an airplane is not efficient it flies at a lower airspeed (because you have more "form" drag), and burns more fuel.

3. Your stall speed increases because the wing of the aircraft is flying "sideways" and is not as efficient.

4. It's uncomfortable. Your inner ear that provides the balance senses to your mind realizes that you're not "straight and level", and so does your stomach.

Yaws can be good

What? Yep, you can use the yaw control, the rudder, to put the airplane in a mode that will allow you to lose a lot of altitude rapidly- it's called a "slip".

Yaw dampeners

Because the yawing motion is so unsettling to most people high performance aircraft are usually equipped with a "yaw dampener" that is a part of the autopilot. The yaw dampener "senses" yaw and automatically feeds in the correct amount of rudder to counteract the yawing moment. In most installations the yaw dampener can be utilized without the other functions of the autopilot being engaged.

As an aside, when the company that I worked for bought a new airplane we had a yaw dampener installed along with the autopilot. There was a nice little annunciator that lit up when the dampener was turned on. The insert for the annunciator was accidentally installed backwards so that when we made the test flight for the avionics installation it read "NO WAY" when turned on.

Slip sliding along

A "slip" is when the airplane is "cross-controlled'. To enter a slip, first make certain that you are below the aircraft's maneuvering speed. Feed in rudder on one side or the other, we'll use the left side for example. As soon as you push in the left rudder the right wing will start to bank up because you have skewed the airplane to the left. As the wing starts to come up, turn the wheel to the right to keep the airplane from changing heading. At some point you will have fed in as much rudder as possible- depending on the airplane, you will run out of either rudder or aileron authority to counteract the other. At this point the airplane will be shuddering quite a bit as it is buffeted by the air turbulence created by your sideways flying.

And you will be descending big time.

Try this at altitude first so that you know what your stall speed will be- remember that your airspeed indicator is not accurate in this mode. Be aware that in this "Cross-controlled" situation the stall may be spetacular- a snap over to a spin is not unusual, it depends on the aircraft.

Can you use flaps in a slip?

Yes you can, and the descent rate is even more incredible. However it is quite possible that the extended flaps will blanket out the elevator or the rudder- in other words the tail may stall. If this happens, you have become a test pilot. The stall will most likely be a deep one, and you will have to retract the flaps to get out of it. The altitude loss in a stall recovery from this configuration can be staggering.

It's best not to slip with flaps unless you are very familiar with the aircraft.

Why would you want to slip an airplane?

Because it is a very effective way to lose altitude. Many older planes didn't have flaps- using the slip for landing was standard fare for their drivers. Even with flaps available many pilots prefer to use the slip because you can enter and leave the configuration so rapidly, whereas with flaps it takes longer for them to extend and retract.

Last question. Can you steer the airplane with just the rudder?

Yep, you can. It's not pretty, and it's kind of just an exercise for fun, but you can. Push in rudder, establish the desired bank, push in opposite rudder to stop the banking and use elevator as necessary to maintain altitude.

Fly safely.

November, 2002