However, historical evolution of the term 'moment' and its use in different branches of science, such as mathematics, physics and engineering, is unclear. Since the center of pressure for symmetric airfoils is relatively constant for small angle of attack, missile engineers typically speak of the complete center of pressure of the entire vehicle for stability and control analysis.
The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment.
For positive stability in missiles, the total vehicle center of pressure defined as given above must be further from the nose of the vehicle than the center of gravity. Federico Commandinointranslated into Latin from Archimedes : The center of gravity of each solid figure is that point within it, about which on all sides parts of equal moment stand.
This direction of movement of the center of pressure on a reflex-cambered airfoil has a stabilising effect.
This behavior is known as the "helm" and is either a weather helm or lee helm. The horizontal stabilizer contributes extra stability and this allows the center of gravity to be a small distance aft of the aerodynamic center without the aircraft reaching neutral stability.
Too much of either helm is not good, since it forces the helmsman to hold the rudder deflected to counter it, thus inducing extra drag beyond what a vessel with neutral or minimal helm would experience.