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According to several resources, the "Center of Pressure" is where the four aerodynamic forces intersect (Lift, Weight, Drag and Thrust). (Not to be confused with "Center of Gravity" where the three axes meet)
[/ QUOTE ] I would be interested to know what these sources are. From what I know, this is incorrect.
A pressure differential is created over the entire surface of an airfoil in flight and the 'center of pressure' is a theoretical point where the total force created by that surface is considered to be concentrated. In this way it is very similar to the center of gravity, even though the weight of the aircraft is spread throughout its structure, we consider it to be concentrated in the CG.
Both the top and bottom of an airfoil have their own center of pressure. The CP on the lower surface of the wing is smaller than the upper surface and is farther forward. This creates a nose down pitching moment during normal flight.
The center of pressure on the top of the wing moves forward as the angle of attack increases. It is rather hard to describe, but in general most lift is created by the forward third of the wing. After about the first 1/3 of the chord, the flow over the top of the wing becomes detached. We know that as AoA is increased that the coefficient of lift also increases until the critical AoA when the wing stalls. This is a fancy way of saying that as the AoA is increased the pressure differential created by air flowing over the top of the wing increases. As this pressure differential on the foreward part of the wing increases, the COP moves foreward. When a wing approaches stalling AoA, the 'negative pressure' on the lower surface also becomes a positive, upward force. I would recommend that you look at Aerodynamics for Naval Aviators, as it depicts this.
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Now, when the angle of attack is increased, the Center of Pressure moves forward. As long as it stays behind the Center of Gravity, the aircraft will generate lift. If it goes ahead of the CG, the aircraft will stall.
[/ QUOTE ] The CG of a conventional aircraft must always be in front of the COP. The COP is where the 'lift force' that is supporting the aircraft in flight is considered to be concentrated. The CG is forward of the COP and tries to pull the nose down. The tail creates 'tail down force' that balances the downward force created by the CG. So, what we have is a 'teeter-totter' with the COP as the balance point. If the CG moves behind the COP, it is possible that the nose can pitch up into an unrecoverable stall.
You may have heard of flights where the pilots of an improperly loaded aircraft had to push the yokes full forward to keep the nose from pitching up. This occurs when the CG is at or slightly behind the COP and is very dangerous for obvious reasons.
Aircraft with a canard or 'foreward lifting surface' are different, and the CG is in front of the COP on these aircraft. This is why some consider the canard configuration to be superior, as the design does not require performance robbing tail down force.
