How does the location of the center of gravity (CG) relative to the center of pressure (CP) affect longitudinal stability in a fixed-wing UAV?

The location of the center of gravity (CG) relative to the center of pressure (CP) is a critical factor in determining the longitudinal stability of a fixed-wing UAV. Longitudinal stability refers to the UAV's tendency to return to its original pitch attitude after being disturbed. The center of gravity (CG) is the point where the entire weight of the UAV is concentrated. The center of pressure (CP) is the point where the total sum of aerodynamic forces acts. For a fixed-wing UAV to be longitudinally stable, the CG must be located ahead of (forward of) the CP. When the CG is ahead of the CP, any disturbance that causes the UAV's nose to pitch up will result in a restoring moment that pushes the nose back down. This is because the aerodynamic force acting at the CP will create a nose-down moment about the CG. Conversely, if the nose pitches down, the aerodynamic force at the CP creates a nose-up moment, restoring the original attitude. This self-correcting behavior ensures stability. If the CG is located behind (aft of) the CP, the UAV becomes longitudinally unstable. In this scenario, if the nose pitches up, the aerodynamic force at the CP creates a nose-up moment, further exacerbating the pitch-up motion. This leads to a divergent oscillation, where the UAV's pitch attitude becomes increasingly unstable. Similarly, a nose-down disturbance would be amplified, leading to a rapid dive. A UAV with the CG directly at the CP is neutrally stable. It will maintain its current pitch attitude after a disturbance, without actively returning to its original state, but also without diverging. While seemingly harmless, this is generally undesirable as it lacks the inherent stability required for controlled flight, making it difficult to fly precisely, especially in turbulent conditions. For example, if a UAV's payload is loaded in such a way that the CG shifts too far aft, it will become increasingly difficult to control, potentially leading to instability and a crash. Therefore, ensuring that the CG is located slightly ahead of the CP is essential for achieving stable and controllable flight in a fixed-wing UAV. The exact distance between the CG and CP is typically small (a percentage of the wing chord) and is carefully determined during the design and testing phase.