Understanding ‘Three-Axis’

An aircraft rotates in three axis: Yaw, Pitch and Roll. Here is a quick recap and summary.

HOW IS CONTROLLING AN AIRCRAFT DIFFERENT THAN CONTROLLING YOUR CAR?

Cars only move in two dimensions: forward, backward, and left and right. Aircraft have a third dimension: up and down. Stability and control are much more complex for an aircraft, and it is through the Center of Gravity (CoG) that all three axis rotate.

• Rotation around the side-to-side lateral axis is called PITCH.
• Rotation around the front-to-back longitudinal axis is called ROLL.
• Rotation around the vertical axis is called YAW.

PITCH

Let’s start with the pitching motion around the lateral axis. The pitch attitude can be measured visually in relation to the horizon, or by degrees of pitch on an attitude indicator. Imagine if you were holding each end of a miniature aircraft, as if a line were going through wingtip to wingtip. Well, that is where the lateral axis is. Rolling the ends of this imaginary line would also cause the nose of the aircraft to pitch up or down. This is how the pitch attitude rotates along the lateral axis. We control this motion within the cockpit with the flight controls. The control surface moving the pitch attitude is the elevator, and you move that surface with the yoke, stick or the control wheel.

ROLL

The rolling motion rotates around the longitudinal axis. The longitudinal axis, it extends from the nose to the tail of the aircraft. Imagine a line going through the airplane along its longitudinal axis. If you were to rotate along this axis, you would see the wings roll or rock from one side to the other. The control surface which allows this movement is the ailerons, which is moved by the stick or control wheel. When you initiate a turn, let’s say to the left for example, you would turn the control wheel to the left, which moves the left aileron up and the right aileron down. The right aileron is now going to create more lift on the right wing, which allows the aircraft to roll along its longitudinal axis to the left side. Thus, it’s initiating a turn. Next time you’re doing your pre-flight, look at which aileron is going up and down when you move the control wheel.

YAW

The yawing motion rotates through the vertical axis runs which runs through the top and the bottom of the cabin of the aircraft, through the CoG. This is controlled with the rudder at the rear of the aircraft and is controlled by your feet. You can see this movement when you are flying and that’s in relation to the nose of the aircraft, where it’s pointed. During a turn, we want the aircraft to remain coordinated, meaning we want both the nose and the toe of the aircraft to be flying in the same direction. So, if you’re turning to the left and the nose is pointed to the right, then the aircraft is yawing in the wrong direction, and we call that adverse yaw. To correct this, we use the rudder which yaws the nose of the aircraft back to the left.

ADVERSE YAW

Adverse yaw is the tendency for the nose of an airplane to yaw in the opposite direction of a turn, especially during banking. The increased lift of the raised wing also results in increased drag, which causes the airplane to yaw or swing toward the side or direction of the raised wing.

What do you do to help compensate for adverse yaw?

Pick a prominent point on the horizon and roll into a bank in either direction. Apply coordinated rudder to compensate for adverse yaw. If the nose yaws in the direction of the turn, too much rudder was used.

DIFFERENTIAL AILERONS

Some aircraft manufacturers use differential ailerons to help compensate adverse yaw.

Differential ailerons have one aileron deflected more than the other. The aileron on the inner wing of a turn deflects up more than the aileron that deflects down on the top outer wing. There are many types of ailerons fitted to aircraft.

Check which type your aircraft has before you go flying.