Aircraft Center of Gravity and Three Axes of Rotation

The center of gravity (CG) is one of the most important factors affecting aircraft stability and control. It represents the point through which the total weight of the aircraft acts and serves as the aircraft's balance point. Understanding the location of the CG and the three axes of rotation is essential for understanding how an aircraft maintains stability and responds to pilot control inputs.

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 through which the total weight of the aircraft acts. If the aircraft was supported at its exact CG, it would balance in any position. The CG is of major importance because its position has a significant effect on aircraft stability.

The CG is determined by the general design of the aircraft. The designers estimate how far the center of pressure (CP) moves during various flight conditions. The CG is positioned relative to the center of pressure to provide adequate longitudinal stability and a restoring moment during flight.

The Axes of an Aircraft

Whenever an aircraft changes its attitude in flight, it must turn about one or more of three axes. Figure 1 shows the three axes, which are imaginary lines passing through the center of the aircraft.

Figure 1. Motion of an aircraft about its axes

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 from wingtip to wingtip is called the lateral axis, about which the aircraft pitches.

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 the aircraft's primary flight 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.

Quick Review: Center of Gravity and Axes

How do aircraft designers determine the ideal position for the center of gravity (CG)?

Designers position the CG based on how far the center of pressure (CP) moves during various flight conditions. The CG is strategically placed relative to the CP to provide adequate longitudinal stability and ensure a restoring aerodynamic moment during flight.

What are the three principal axes of rotation and where do they intersect?

The three principal axes are the longitudinal axis (running from nose to tail), the lateral axis (running wingtip to wingtip), and the vertical axis (running top to bottom). All three imaginary lines are perpendicular to each other and intersect exactly at the aircraft's center of gravity.

Which primary flight control surfaces govern the movements about each axis?

Movement about the axes is managed by primary controls: the ailerons produce roll about the longitudinal axis; the elevators control pitch about the lateral axis; and the rudder governs yaw about the vertical axis.

Aircraft Balance and Stability Principles →

Aerodynamic Fundamentals Aerodynamics & the Laws of Physics Review how forces and moments act on an aircraft based on Newton’s laws of motion. Open Article → Aircraft Stability Stability & Control Understand how center of gravity position affects static and dynamic stability in flight. Open Article → Flight Control Systems Primary Flight Controls Examine how control surfaces generate moments about the aircraft’s axes. Open Article → Aircraft Design Airfoil Learn how lift distribution affects center of pressure and aircraft balance. Open Article → Flight Dynamics Thrust & Drag Understand how thrust line and drag forces influence pitching, yawing, and rolling moments. Open Article → Advanced Stability High-Speed Aerodynamics Explore how CG location affects stability at high-speed and compressibility conditions. Open Article →