Throughout the whole body of an aircraft there are many different forces created by pressure differences. the center of pressure is the average location of where the pressure force is applied. think of it like the center of gravity, the location where the average weight of an object is, except this time it is the location of average pressure. the center of pressure is where the forces of lift and drag are exerted. it is important for engineers to know the center of pressure since it allows them to evenly balance the lift on aircraft.
As the airfoil angle of attack changes, the pressure field changes. Due to this, the center of pressure changes with variation in the angle of attack. In the airplane’s normal range of flight attitudes, if the angle of attack is increased, the center of pressure moves forward; and if decreased, it moves rearward.
Movement of Center of Pressure
As the center of gravity (the point where the weight of the body acts) is fixed, this movement of center of pressure affects the stability of the aircraft.
Aerodynamic Center
The resultant (or the pressure forces) also cause a moment on the airfoil. As the angle of attack increases, the pitching moment at a point (for example, the center of gravity) also changes. However, the pitching moment remains constant at a particular point, which is called the aerodynamic center.
For symmetric airfoils in subsonic flight the aerodynamic center is located approximately 25% of the chord from the leading edge of the airfoil. This point is described as the quarter-chord point.Thus the aerodynamic center does not change with variation in angle of attack. Due to this, the aerodynamic center, rather than the center of pressure is used in the analysis of longitudinal stability.Neutral Point Consider the whole aircraft. The lift of the wing acting through the center of pressure is in front of the center of gravity of the aircraft. This causes a destabilizing motion (increase in lift to increase in angle of attack causes a nose up moment, further increasing angle of attack). This is counteracted by the moment produced by the lift of the horizontal stabilizer (which is a small wing), acting behind the center of gravity.
**Note
Aerodynamic centre (AC) is found at 25% of the airfoil chord line usually infront of the Centre of Pressure in subsonic incompressible flow (speed: <Mach 0.4). This is the point where the pitching moment of the aircraft remains constant. Think of AC as a pivot point for the CP. Say CP produces pitch down moment at a particular angle of attack through the AC.Now, if we increase the airfoil angle of attack, the CP moves forward and the lift the airfoil generates increases. But the moment will remain the same as before, because though there is an increase in lift, the distance between CP and AC reduced. This will chop off any moment gain due to the increase in lift vector.Centre of Pressure (CP) is the point on the airfoil where most of the lift is concentrated on. It unlike AC moves with the angle of attack. As angle of attack increases CP moves forward, until the stall angle is reached. At the point of stall, the CP drops back the airfoil.The pitching moment itself various as the square of the speed. It is the pitching moment COEFFICIENT that is constant.
Author – Vander Stone
