Most aircraft have a main wing and a separate stabilizer. On the previous page we learned that the stabilizer is a major contributor to the longitudinal stability of the aircraft (hence the name.) On this page we will consider the contribution of the main wing.
If the main wing was well behind the c of g, as the stabilizer is, it would be just as stabilizing. Unfortunately, the wing must be much closer to to the c of g, since we need to generate a lift force which will oppose weight. Remember that weight acts at the c of g by definition.
The movie below shows how the wing is stable if it is behind the c of g.
The movie below shows how the wing will react if it is ahead of the C of G. As you can see the wing will be totally unstable in this situation.
Aerodynamic Center - Introduction
In the above movie we learned that if the wing is well behind the c of g it is stable. We also learned that if the wing is ahead of the c of g it is unstable. We can guess that if the wing was exactly at the c of g it would display neutral stability.
When we say the wing is ahead of or behind the C of G we must be talking about a specific point on the wing. This would be the point where lift effectively acts. This point is called the Aerodynamic Center (ac). We will discuss the in detail below.
Center of Pressure
The center of pressure (c of p) is the point along the chord, at which the lift of the wing acts. On a cambered airfoil the c of p is always behind the ac. As the angle of attack increases the c of p moves forward. But, the c of p never quite reaches the ac.
At the point of stall the c of p moves quickly backwards, causing a nose down pitching moment.
Use the movie below to see how the c of p changes on a cambered airfoil.
The c of p can be ahead of the ac if the wing is reflexed. This will be required if the aircraft has no tail (see stability discussion elsewhere.) The movie below shows how the c of p on a reflexed airfoil changes as the elevators are deflected, and how this gives the pilot pitch control.
As the angle of attack increases c of P moves toward the ac on all wings. Thus, the lift force increases, but the arm at which the force acts, as measured from the ac, gets smaller.
The aerodynamic center (ac) is the point on the wing about which the coefficient of pitching moment is constant.
The Aerodynamic Center is the point on the wing about which all changes in lift effectively act. In other words when the angle of attack is increased or decreased the total lift and center of pressure move. But, the net effect is as though the change in lift happens at the ac.
On all airfoils the ac very close to the 25% chord point (+/- 2%) in subsonic flow. It is worth noting that the ac moves back to the 50% chord point in supersonic flow.
The movie below shows what the ac is.