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Encyclopedia > Center of pressure

The center of pressure is the point on a body where the sum total of the aerodynamic pressure field acts, causing a force and no moment about that point. Mathematically it 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. This article is about pressure in the physical sciences. ... This article is about the physical quantity. ... It has been suggested that this article or section be merged with torque. ... Mathematics is commonly defined as the study of patterns of structure, change, and space; more informally, one might say it is the study of figures and numbers. Mathematical knowledge is constantly growing, through research and application, but mathematics itself is not usually considered a natural science. ...

Contents

Historical usage

Center of pressure is used in sailboat design to represent the position on a sail where 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 mass around which it will pivot. Diagram of Sailboat, in this case a typical monohull sloop with a bermuda or marconi rig. ... A gaff-rigged cutter flying a mainsail, staysail and genoa jib For other uses, see Sail (disambiguation). ... Bow of the Cruise ship Spirit of Endeavour The bows of lifeboat 17-31 (Severn class) in Poole Harbour, Dorset, England The bow (pronounced to rhyme with how) is a nautical term that refers to the forward part of the hull of a ship or boat, the point that is... In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the systems mass behaves as if it were concentrated. ...


Aircraft

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 center which is the point on a wing where the pitching moment produced by the aerodynamic forces is constant with angle of attack.[1] It has been suggested that this article or section be merged with torque. ... The aerodynamic center of an airfoil moving through a fluid is the point at which the pitching moment generated by the airfoil is constant regardless of the angle of attack[1], [2]. This is an aspect of aerodynamics. ... The pitching moment of an airfoil, in aerodynamics, is a moment produced by a vertical force applied at a distance forward or aft from the aerodynamic center of the airfoil, causing the aircraft to pitch up or down[1]. References ↑ Preston, Ray (2006). ... In this diagram, the black arrow represents the direction of the wind. ...



The aerodynamic center plays an important role in analysis of the longitudinal static stability of 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 attack without a pilot or autopilot changing 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. The aerodynamic center of an airfoil moving through a fluid is the point at which the pitching moment generated by the airfoil is constant regardless of the angle of attack[1], [2]. This is an aspect of aerodynamics. ... In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... In this diagram, the black arrow represents the direction of the wind. ... An autopilot is a mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being. ...


Missiles

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 force derivative 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. Fn represents the normal force. ...


Movement of center of pressure

The center of pressure on a symmetric airfoil typically 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 attack and lift coefficient change, 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. Aircraft tend to use cambered wings because they have relatively benign flights with preferred flight orientations as compared to 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. For the kite, see foil kite. ... In this diagram, the black arrow represents the direction of the wind. ... A typical lift coefficient curve. ... The camber in aerospace engineering is the asymmetry between the top and the bottom curves of an airfoil. ... A typical lift coefficient curve. ... In this diagram, the black arrow represents the direction of the wind. ...


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 aircraft with a straight wing can be designed to have positive longitudinal static stability if the wing has reflex camber. The camber in aerospace engineering is the asymmetry between the top and the bottom curves of an airfoil. ... A typical lift coefficient curve. ... In this diagram, the black arrow represents the direction of the wind. ... The DH108 Swallow A tailless or tail-less aeroplane traditionally has all its horizontal control surfaces on its main wing surface. ...


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 stability of an aircraft. For this reason, it is much simpler to use the aerodynamic center when 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 of an airfoil moving through a fluid is the point at which the pitching moment generated by the airfoil is constant regardless of the angle of attack[1], [2]. This is an aspect of aerodynamics. ...


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. The tail of a Lufthansa airliner (Airbus A319) in flight, showing the horizontal and vertical stabilizer For aircraft, the horizontal stabilizer is a fixed or adjustable surface from which an elevator may be hinged, while a vertical stabilizer (also called a fin) is fixed to the aircraft and supports the...


See also

The aerodynamic center of an airfoil moving through a fluid is the point at which the pitching moment generated by the airfoil is constant regardless of the angle of attack[1], [2]. This is an aspect of aerodynamics. ... The Center of lift (or CoP) is the point on a body where the sum of the total pressure acts. ...

References

  1. ^ Preston, Ray (2006). Aerodynamic Center. Aerodynamics Text. Selkirk College. Retrieved on 2006-04-01.
  1. Hurt, Hugh H., Jr. (January 1965). Aerodynamics for Naval Aviators. Washington, D.C.: Naval Air Systems Command, United States Navy, 16-21. NAVWEPS 00-80T-80. 
  2. Smith, Hubert (1992). The Illustrated Guide to Aerodynamics, 2nd ed., New York: TAB Books, 24-27. ISBN 0-8306-3901-2. 
Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 91st day of the year (92nd in leap years) in the Gregorian calendar. ...

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