center of pressure

Center of pressure

A point on a plane surface through which the resultant force due to pressure passes. Such a surface can be supported by a single mounting fixture at its center of pressure if no other forces act. For example, a water gate in a dam can be supported by a single shaft at its center of pressure. See Resultant of forces

Center of Pressure

the point at which the line of action of the resultant of the forces due to pressure on a body at rest or in motion in an ambient medium intersects some plane drawn in the body. The ambient medium may be a liquid or a gas.

For example, the center of pressure for an airplane wing (Figure 1) is the point of intersection of the line of action of the resultant aerodynamic force and the plane of the wing chord. For a body of revolution—such as the fuselage of a rocket or the body of a dirigible or mortar shell—the center of pressure is the point of intersection of the resultant aerodynamic force and the body’s plane of symmetry perpendicular to the plane passing through the axis of symmetry and through the velocity vector of the body’s center of gravity.

Figure 1. Location of the center of pressure for a wing: (b) chord length, (α) angle of attack, (v) free-stream velocity vector, (R) resultant aerodynamic force, (X_cp) distance from the center of pressure to the leading edge of the wing

The location of the center of pressure depends on the shape of the body; for a moving body, it may also depend on the direction of motion and on the properties of the ambient medium, for example, its compressibility. Thus, depending on the shape of the wing section, the location of the center of pressure for an airplane wing may vary with the angle of attack a or may be constant. If the location of the center of pressure is constant, the wing is called a constant center of pressure wing and x_cp ≈ 0.25b (Figure 1). During supersonic motion, the center of pressure is shifted substantially toward the trailing edge of the wing because of the effect of the compressibility of air.

A change in the location of the center of pressure has a considerable effect on the dynamic stability of a moving body, such as an airplane, a rocket, or a mortar shell. For the motion to be stable upon a random variation of the angle of attack α, the center of pressure should shift in such a way that the aerodynamic moment causes the body to return to its original position. For example, as α increases, the center of pressure should shift toward the tail. To ensure dynamic stability, a body intended for flight is often provided with an appropriate tail assembly.

REFERENCES

Loitsianskii, L. G. Mekhanika zhidkosti i gaza, 3rd ed. Moscow, 1970.
Golubev, V. V. Lektsiipoi teorii kryla. Moscow-Leningrad, 1949.

center of pressure

[′sen·tər əv ′presh·ər] (aerospace engineering) The point in the chord of an airfoil section which is at the intersection of the chord (prolonged if necessary) and the line of action of the combined air forces (resultant air force). (fluid mechanics) For a body immersed in a fluid, the point through which the resultant of the forces on the surface of the body due to hydrostatic pressure acts.

center of pressure (C of P)

The point, usually on the chord line, through which the total reaction (sum total of all aerodynamics forces) may be considered to act. The center of pressure is not a fixed point but moves with changes to the angle of attack.