Center of pressure
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/* Aircraft */ Added link to [[pitching moment]]
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The Center of Pressure (or CoP) is the point on a body where the sum of the total pressure acts. Pressure acting on a surface causes a force. The point at which the sum of these forces, from the various surfaces of the body, passes through the body is the CoP. Mathematically it can be said that the net pressure force on the body acts through this point.
Aircraft
The CoP must be managed with respect to the center of mass (CoM), or center of gravity, for stability and efficiency during flight. The distance between the location of the two forces, weight and pressure, defines a moment arm which creates torque across the aircraft and attempts to pitch it. Since the CoP often moves with changes in attitude or speed, most aircraft need trim adjustments to balance this force.
In general terms the CoP must be greater than and aft of the CoM for stable flight of aircraft without adjustable control surfaces, such as simple rockets or hand-thrown darts. Otherwise the force from the CoP would be greater than the inertial force of the CoM which would tend to move the CoP aft of the CoM. Darts are very heavy at the front to move the CoM forward, whereas rockets place fins at the rear to move the CoP rearward, both having the same net effect. Model rockets often add small weights at the front if they prove to be unstable after being built.
Most aircraft place the CoM in front of the CoP. A typical general aviation design, for instance, places the engine, fuel and passengers all at or in front of the CoP. This results in a strong downward pitching moment, which is counteracted by a downward force behind the CoM by the tail surfaces. Since the rear surfaces are effectively pulling the aircraft down, this adds to the overall weight the wings need to lift. Reducing the trim force as much as possible is a goal for most larger aircraft. For instance, large jetliners often include fuel tanks in the vertical tail, pumping fuel into them after takeoff to allow the amount of downforce of the tail to be reduced.