Once the free-stream Mach number is above M = 1, the C of P (center of pressure) moves backward and settles down at about 50% of the chord at supersonic speeds. It moves forward rapidly as soon as the airflow over the aircraft becomes sonic. For reasons which will become apparent when airfoil theory is studied, it is advantageous to dene the standard location for the moment reference point of an airfoil to be at its quarter-chord location, or xref c/4. The center of pressure tends to move backward as the aircraft's speed increases from low subsonic to high subsonic. situation can easily occur in practice, so the center of pressure is rarely used in aerodynamics work. The linear distance the center of pressure travels over the operating range of Mach numbers in supersonic aircraft. At this point, the center of pressure generally moves backward along the chord.

The center of pressure moves forward as the angle of attack increases until the stalling angle is reached. Each center of pressure is relatively at around a value of 1.5 inches. The flat plates center of pressure is very nearly stationary for angles if attack between positive and negative stalls and is located very near 25 of the mean aerodynamic chord. As observed in Figure 7, all three responses exhibit similar shapes. The center of pressure for a flat plate or any symmetrical airfoil is at the aerodynamic center. When center of pressure moves, a twisting force is exerted on the rotor blades. The center of pressure for a thin airfoil is defined as the position along an airfoil where the moment is equal to zero. Nonsymmetrical airfoils were not used in earlier helicopters because the center of pressure location moved too much when angle of attack was changed. The linear distance the center of pressure travels along a chord over the entire range of angles of attack, ignoring the effects of compressibility and subsonic flow. The advantages of the nonsymmetrical airfoil are increased lift-drag ratios and more desirable stall characteristics. Position of center of pressure as percentage of mean aerodynamic chord.The ratio of the distance of the center of pressure from the leading edge of an airfoil to its chord length.

Airfoil center of pressure