Aircraft Motions and the Principle Axes
As we explored on the last two pages, flaps and slats enable a pilot to move an aircraft through three-dimensional space. In other words, the pilot alters the plane's orientation around its own center of gravity, producing torque. Imagine this center of gravity as a fixed point in the middle of the fuselage. Next, imagine an invisible horizontal line that travels straight through the plane's nose, center of gravity and tail. We call this the roll axis.
By adjusting the plane's ailerons (or spoiler) a pilot can cause the lift to increase in one wing and decrease in the other. One wing rises, the other descends. This causes the body of the plane to rotate along its roll axis, which results in a maneuver known as a roll. When a plane makes a complete rotation of its roll axis, the maneuver is called a barrel roll. However, when a pilot merely rolls enough to tilt the angle of the airfoil, the aircraft banks or turns.
Now imagine an invisible vertical line intersecting the center of gravity, shooting down through the top of the aircraft and out through the belly. This is called the yaw axis, and it comes into play when a pilot manipulates the aircraft's rudder. The rudder's deflection results in a side force, rotating the tail in one direction and the nose in the other. This is called a yaw motion, which helps the pilot to maintain course.
Finally, imagine an invisible horizontal line moving through the sides of the aircraft's center of gravity, roughly parallel to the wings. This is the pitch axis, which necessitates the pitch motion due to changes in the airplane's elevator. When the tail tilts down, the nose rises and the plane ascends -- and vice versa. Some aircraft can actually perform complete loops in this manner.