Center of Gravity (CG) and the Axes of an Aircraft

in Aerodynamics, Aircraft Assembly, and Rigging

Center of Gravity (CG)

Gravity is the pulling force that tends to draw all bodies within the earth’s gravitational field to the center of the earth. The CG may be considered the point at which all the weight of the aircraft is concentrated. If the aircraft were supported at its exact CG, it would balance in any position. CG is of major importance in an aircraft, for its position has a great bearing upon stability.

The CG is determined by the general design of the aircraft. The designers estimate how far the CP travels. They then fix the CG in front of the CP for the corresponding flight speed in order to provide an adequate restoring moment for flight equilibrium.

The Axis of an Aircraft

Whenever an aircraft changes its attitude in flight, it must turn about one or more of three axis. Figure 2-10 shows the three axes, which are imaginary lines passing through the center of the aircraft.

Figure 2-10. Motion of an aircraft about its axes.

Figure 2-10. Motion of an aircraft about its axes. [click image to enlarge]

The axes of an aircraft can be considered as imaginary axles around which the aircraft turns like a wheel. At the center, where all three axes intersect, each is perpendicular to the other two. The axis that extends lengthwise through the fuselage from the nose to the tail is called the longitudinal axis. The axis that extends crosswise from wing tip to wing tip is the lateral, or pitch, axis. The axis that passes through the center, from top to bottom, is called the vertical, or yaw, axis. Roll, pitch, and yaw are controlled by three control surfaces. Roll is produced by the ailerons, which are located at the trailing edges of the wings. Pitch is affected by the elevators, the rear portion of the horizontal tail assembly. Yaw is controlled by the rudder, the rear portion of the vertical tail assembly.