- Center of pressure
The center of pressure is the point on a body where the sum total of the aerodynamic
pressurefield acts, causing a forceand no moment about that point. Mathematicallyit can be said that the net pressure force on the body acts through this point. The net force applied at the center of pressure point produces the equivalent moment as the pressure field about any arbitrary point. Typically the center of pressure is referenced to the nose of the vehicle but other points may be selected.
Center of pressure is used in
sailboatdesign to represent the position on a sailwhere its force is concentrated.It is desirable that a sailboat be stable in the sense that the wind should naturally push the bow of the ship away from the wind rather than facing the wind such that the boat would be blown backward.To accomplish this, the center of pressure should be forward of the ship's center of massaround which it will pivot.
A stable configuration is not only desirable in sailing, but in aircraft design as well. Aircraft design therefore borrowed the center of pressure term. But unlike a sail, a rigid non-symmetrical airfoil not only produces lift, but a moment.The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment. A similar idea is the
aerodynamic centerwhich is the point on a wing where the pitching momentproduced by the aerodynamic forces is constant with angle of attack.cite web
last = Preston
first = Ray
year = 2006
url = http://selair.selkirk.bc.ca/aerodynamics1/Stability/Page7.html
title = Aerodynamic Center
work = Aerodynamics Text
publisher = Selkirk College
accessdate = 2006-04-01]
aerodynamic centerplays an important role in analysis of the longitudinal static stabilityof aircraft and other flying vehicles. It is desirable that when the pitch angle and angle of attack of an aircraft are disturbed (by, for example turbulence) that the aircraft returns to its original trimmed pitch angle and angle of attackwithout a pilot or autopilotchanging the control surface deflection. For an aircraft to return towards its trimmed attitude, without input from a pilot or autopilot, it must have positive longitudinal static stability.
Missiles typically do not have a preferred plane or direction of maneuver and thus have symmetric airfoils. Since the center of pressure for symmetric airfoils is relatively constant with angle of attack, missile engineers typically speak of the complete center of pressure of the entire vehicle for stability and control analysis. In missile analysis, the center of pressure is typically defined as the center of the additional pressure field due to a change in the angle of attack off of the trim angle of attack. For unguided rockets the trim position is typically zero angle of attack and the center of pressure is defined to be the center of pressure of the resultant flow field on the entire vehicle resulting from a very small angle of attack (that is, the center of pressure in the limit as angle of attack goes to zero). For positive stability in missiles, the total vehicle center of pressure defined as given above must be further from the nose of the vehicle than the center of gravity. In missiles at lower angles of attack, the contributions to the center of pressure are dominated by the nose, wings, and fins. The
normal forcederivative with respect to the angle of attack of each component multiplied by the location of the center of pressure can be used to compute a centroid representing the total center of pressure. This center of pressure of the added flow field is behind the center of gravity and the additional force "points" in the direction of the added angle of attack, this produces a moment that pushes the vehicle back to the trim position. In guided missiles where the fins can be moved to trim the vehicles in different angles of attack, the center of pressure is the center of pressure of the flow field at that angle of attack for the undeflected fin position. This is the center of pressure of any small change in the angle of attack (as defined above). Once again for positive static stability, this definition of center of pressure requires that the center of pressure be further from the nose than the center of gravity. This ensures that any increased forces resulting from increased angle of attack results in increased restoring moment to drive the missile back to the trimmed position. In missile analysis, positive static margin implies that the complete vehicle makes a restoring moment for any angle of attack from the trim position.
Movement of center of pressure
The center of pressure on a symmetric
airfoiltypically lies close to 25% of the chord length behind the leading edge of the airfoil. (This is called the "quarter-chord point".) For a symmetric airfoil, as angle of attackand lift coefficientchange, the center of pressure does not move. It remains around the quarter-chord point for all angles of attack and lift coefficients. The role of center of pressure in the control characterization of aircraft takes a different form than in missiles.
On a cambered airfoil the center of pressure does not occupy a fixed location. For a conventionally cambered airfoil, the center of pressure lies a little behind the quarter-chord point at maximum
lift coefficient(large angle of attack), but as lift coefficient reduces (angle of attack reduces) the center of pressure moves toward the rear. When the lift coefficient is zero an airfoil is generating no lift but a conventionally cambered airfoil generates a nose-down pitching moment, so the location of the center of pressure is an infinite distance behind the airfoil. This direction of movement of the center of pressure on a conventionally cambered airfoil is de-stabilising, necessitating a horizontal stabiliser to provide the aircraft with longitudinal static stability. Aircraft tend to use cambered wings because they have relatively benign flights with preferred flight orientations as compared to missiles.
For a reflex-cambered airfoil, the center of pressure lies a little ahead of the quarter-chord point at maximum
lift coefficient(large angle of attack), but as lift coefficient reduces (angle of attack reduces) the center of pressure moves forward. When the lift coefficient is zero an airfoil is generating no lift but a reflex-cambered airfoil generates a nose-up pitching moment, so the location of the center of pressure is an infinite distance ahead of the airfoil. This direction of movement of the center of pressure on a reflex-cambered airfoil is stabilising, and a horizontal stabiliser is not necessary. A tailless aircraftwith a straight wing can be designed to have positive longitudinal static stabilityif the wing has reflex camber.
The way the center of pressure moves as lift coefficient changes makes it difficult to use the center of pressure in the mathematical analysis of
longitudinal static stabilityof an aircraft. For this reason, it is much simpler to use the aerodynamic centerwhen carrying out a mathematical analysis. The aerodynamic center is a slightly more difficult concept to comprehend, but the aerodynamic center occupies a fixed location on an airfoil, typically close to the quarter-chord point.
The aerodynamic center is the conceptual starting point for longitudinal stability. Providing the center of gravity of an aircraft lies forward of the aerodynamic center the aircraft will have positive longitudinal stability. The horizontal stabilizer contributes extra stability and this allows the center of gravity to be a small distance aft of the aerodynamic center without the aircraft reaching neutral stability. The position of the center of gravity at which the aircraft has neutral stability is called the neutral point.
Longitudinal static stability
Wikimedia Foundation. 2010.