Center of pressure
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The Center of Pressure (or CoP) is the point on an aircraft where all of the aerodynamic forces act. It is found by summing the individual forces from the wings, tail surfaces, fuselage, etc.
The CoP must be managed with respect to the center of mass (CoM), or center of gravity, for stability and efficiency. The CoP often moves with changes in attitude or speed, leading to the need for trim adjustments.
The CoP must be greater than and aft of the CoM for stable flight of aircraft without adjustable control surfaces, such as simple rockets. 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.
If the CoM is placed slightly behind the CoP the nose tends to pitch up in level flight, requiring additional lift at the tail which complements other lift forces and increases efficiency (but reduces stability as noted above). For safety, cargo aircraft operators usually adjust the CoM as little forward of the CoP as possible, increasing stability with a small sacrifice in efficiency.