The science of flight can be somewhat daunting, with so many different terms and complex mechanics involved. Aircraft require several important ingredients to perpetuate flight, but directional control is most definitely one of the most important functions in flying. Control ranges from altitude changes, pitches and rolls, but in aviation it is simply considered the "Axis of Rotation".
To fully understand the axis of aircraft in flight, you have to imagine that a plane has a series of invisible lines that extend from the nose to tail, wing tip to wing tip, and then a vertical line that extends at the intersect point around the center of a plane.
The Axis of Control explained:
The first line is considered to be the Longitudinal Axis or "Roll". This line begins at the nose cone, traveling through the fuselage of an aircraft and extending just past the tail or aft section of a plane. The Longitudinal Axis or roll is what allows a plane to stay balanced with the horizon or stabilize during landing and take off. The control is determined by the position of the ailerons (flap) that are located on the trailing edge of the wing tips. Each aileron is proportionate to the opposing position, which means; as one is deflecting downward, the opposite aileron moves in the opposite upward direction.
The best way to demonstrate the longitudinal axis is to take a simple paper airplane and make two simple cuts on each of the wing tips. This should create a rectangular tab that can be bent either up or down. If you take the right wing and position the tab downward, and take the left wing and position in the upward position, this will cause the plane to spiral, although most paper airplanes do not exhibit the same properties of a typical airplane due the defined arrow shape and missing tail section.
The second line is considered to be the Lateral Axis or "Pitch". This line extends from the entire wingspan from wingtip to wingtip. This particular line controls diving and climbing, which is the factor that controls the altitude of the aircraft. By moving the control stick or yolk, either forward or backward it causes the elevators to deflect down or up depending on the position of the stick. The elevator can be found on the tail section of most planes, and it is fixed upon the horizontal stabilizer. These elevators can appear to be attached to a smaller wing that is fixed to the aft of a plane, however some planes such as the F-117 Nighthawk or the Concord use a much more complex method to stabilize or utilize lateral pitch control.
If you want to demonstrate the lateral axis you can revert back to the paper airplane, but now change the elevators or rectangular tabs into equal upward positions. Notice how the paper airplane now appears to float or hold longer flights. This reaction is due to the air being deflected off the tabs, which will cause the lateral pitch to elevate the plane. Unfortunately most designs will not demonstrate perpetual flight due to drag, and size, however if you reverse the tabs, the plane will almost take an immediate nosedive.
Lastly we have the final axis called the Vertical Axis or "Yaw". This invisible line extends from below the plane, through the center point; possibly the cockpit, all the way to just above the plane. Now, in flying terminology the Yaw is equal to turning, and is controlled by a vertical fin that is fixed upon the tail of a plane. The vertical axis control will affect the heading the plane is traveling by shifting the nose from left to right. The control of the yaw is dependent on two pedals that control either left or right movement of the fin or rudder section of the plane. Only a handful of aircraft such as the B2-Spirit a.k.a. Stealth bomber don't utilize the same form of aft control for Yaw and Pitch, but instead they utilize more complex flap or rudder configurations to aid in air control.
The Axes are what makes flight control possible, and are utilized to properly navigate or to maintain a specific course of flight. The Axis is also a critical component to both the take off and landing process, which ensure that equal contact is maintained during touchdown or lift off. If you ever see a plane during take off or landing, you may see slight corrections on the approach or just before the wheels leave the surface of the ground. This minor correction is so the plane can level off to the horizon line, since the pilot has no other way to view his approach or see if his wheels are both touching down at the same time.
There you have it, the three basic axes of flight control! The principles are quite simple when you get a chance to apply them, such as in my paper airplane experiment examples. However when you're up in the air it certainly isn't quite the same thing.
Flying truly requires thorough understanding of wind sheer, turbulence and other environmental conditions that can combat basic flight control. The axis of control is actually more of a virtual model to understand how a plane operates, but it does not account for other conditions that directly impact a pilot during flight. Flying is wonderful experience whether you are simply gliding or taking out a single engine for the first time, but it must be taken with the utmost respect and understanding, because what goes up, can certainly come down!