If you've ever watched pilots performing acrobatic stunts, you know they do some amazing and complicated maneuvers. Yet, no matter how complex they are, all the movements of the plane can be described in terms of just four types of motion. The first, when the aircraft is flying straight and level, is forward motion, or airspeed. But airplanes don't just travel in straight and level paths; they move in three dimensions. The remaining three types of motion are movement around the aircraft's three axes of rotation: the longitudinal axis, the lateral axis, and the vertical axis. Anytime an aircraft rotates around one of these axes, it changes the way the air flows over the aircraft's surface, causing it to turn, climb, roll, and so forth.
You can think of the longitudinal axis as an imaginary line running from the aircraft nose to its tail, passing through the center of gravity. When one wing goes up and the other goes down, the airplane is rotating around the longitudinal axis, and this kind of motion is called roll.
The lateral axis can be visualized as a line from wingtip to wingtip, again passing through the center of gravity and perpendicular to the longitudinal axis. If the aircraft starts to climb, the nose goes up and the tail drops, so the aircraft rotates around the lateral axis. Pilots call this kind of rotation pitch.
The vertical axis is much the same, except it is straight up and down, passing through the center of gravity at right angles to the other two axes. When the aircraft's nose swings to the left or right, it is rotating around the vertical axis, which is called yaw.
The important and interesting thing about rotating an aircraft around its axes of rotation is that this is how pilots control the aircraft. To understand how this works, first you need to know what is meant by aircraft stability. Aircraft are carefully designed to be as stable as possible. If the equilibrium (balance of forces) on the aircraft is disturbed, it always tends to return to its original equilibrium. Anytime the pilot wants to turn or perform any other maneuver, a force of some sort must be applied.
There are three basic types of control surfaces that pilots use to do this: ailerons, rudder, and elevators. Each is moved to change the way the air hits the aircraft as it flies, producing a particular force on some part(s) of the aircraft.
The ailerons are located on the trailing edge of the wings. To turn the aircraft to the left, the pilot will raise the left aileron and lower the right one. As air strikes the raised left aileron, it creates a downward force on the left wing. The lowered right aileron does the same thing in reverse, pushing the right wing upward. The result is the left wing dips, the right rises, and the aircraft rotates around the longitudinal axis. This creates an imbalance in the airflow on the aircraft, causing it to turn to the left.
However, these motions increase the drag on the aircraft. The rudder is used to control the drag on the aircraft, so that it makes a smooth, level turn and doesn't skid or sideslip through the air. The rudder is not used to steer the aircraft, as is the rudder on a ship.
In order to make the aircraft climb or descend, the pilot uses the elevators. These are located on the horizontal stabilizers at the rear of the aircraft. When the elevators are raised, the airflow striking them creates a downward force, pushing the tail down. Here, the aircraft is rotating around the lateral axis. When the tail drops, the nose rises and the aircraft starts to climb. Again, the reverse is rue if the elevators are lowered: the tail rises, the nose drops, and the plane starts to descend.
There are e other control surfaces, but the most important of these for controlling movement around the axes of rotation are the trim tabs. A number of things can make slight changes in the aircraft's balance (for instance, if a passenger walks down the length of an airliner, this changes the plane's center of gravity slightly). The trim tabs are used to correct for small disturbances like this.
Not all aircraft use this particular system of control surfaces. Some military aircraft, for instance, are designed with a different set of control surfaces, depending on their function. However, their function is exactly the same: to control the aircraft's movement around the three axes of rotation.