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I have just started learning some aerospace concepts, and I am not able to understand the difference between the three terms centre of pressure, aerodynamic centre and neutral point. What are their functional differences?

What would be an example that distinguishes these three?

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  • $\begingroup$ In this sentence: As the center of gravity (the point where the weight of the body acts) is fixed, this movement of center of gravity affects the stability of the aircraft. - surely there is a misprint or absent-minded word?? Because it should read: As the center of gravity (the point where the weight of the body acts) is fixed, this movement of center of PRESSURE affects the stability of the aircraft. $\endgroup$ – user14398 Apr 10 '16 at 0:27
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Any body moving in a fluid experiences pressure forces over its surface. The concepts of center of pressure, aerodynamics center and neutral point are useful in understanding the effects of these forces. Let's take an airfoil moving in air with subsonic flow attached to the body.

Center of Pressure The center of pressure is the point where the total sum of a pressure field acts on a body. In aerospace, this is the point on the airfoil (or wing) where the resultant vector (of lift and drag) acts.

Center of Pressure

Source: avstop.com

As the airfoil angle of attack changes, the pressure field changes. Due to this, the center of pressure changes with variation in the angle of attack. In the airplane's normal range of flight attitudes, if the angle of attack is increased, the center of pressure moves forward; and if decreased, it moves rearward.

Movement of Center of Pressure

Source: www.flightlearnings.com

As the center of gravity (the point where the weight of the body acts) is fixed, this movement of center of pressure affects the stability of the aircraft.

Aerodynamic Center The resultant (or the pressure forces) also cause a moment on the airfoil. As the angle of attack increases, the pitching moment at a point (for example, the center of gravity) also changes. However, the pitching moment remains constant at a particular point, which is called the aerodynamic center.

Aerodynamic Center

Source: www.theairlinepilots.com

For symmetric airfoils in subsonic flight the aerodynamic center is located approximately 25% of the chord from the leading edge of the airfoil. This point is described as the quarter-chord point.

Thus the aerodynamic center does not change with variation in angle of attack. Due to this, the aerodynamic center, rather than the center of pressure is used in the analysis of longitudinal stability.

Neutral Point Consider the whole aircraft. The lift of the wing acting through the center of pressure is in front of the center of gravity of the aircraft. This causes a destabilizing motion (increase in lift to increase in angle of attack causes a nose up moment, further increasing angle of attack). This is counteracted by the moment produced by the lift of the horizontal stabilizer (which is a small wing), acting behind the center of gravity.

Forces on aircraft

"AirStability" by User_A1 - SVG derivative of Public Domain work by G J Vigurs. Licensed under GFDL via Commons.

As the center of gravity is moved forward, the stability of the aircraft increases (as the main wing lift arm is reduced) and the aircraft is statically stable.

Statically Stable

Source: quest.arc.nasa.gov

As the center of gravity is moved aft, the main wing lift moment arm increases and the aircraft stability decreases. The aircraft is said to be statically unstable.

Statically Unstable

Source: quest.arc.nasa.gov

As the aircraft is stable when center of gravity is in the nose and unstable when center of gravity is in the tail, there is a position in the middle where the aircraft is neither stable nor unstable i.e the stability is neutral. This point is called the neutral point. The neutral point is fixed for a particular aerodynamic configuration of the aircraft.

Neutrally Stable

Source: quest.arc.nasa.gov

Thus,

  • Center of pressure of an aircraft is the point where the Lift acts.
  • Aerodynamic center is the point in the wing where the pitching moments are constant.
  • The neutral point is where the center of gravity of the aircraft is neutrally stable
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  • $\begingroup$ Notice how a right sized (area) Hstab will help static stability a great deal with very little drag penalty. Ideally it would be near 0 angle of attack at cruise, holding the wing at its optimum AOA. Simple flat plate, low aspect Hstabs are common for a good reason. $\endgroup$ – Robert DiGiovanni Apr 10 at 12:36
  • $\begingroup$ for neutral stability the aircraft's moment slope should be 0. In other words, the pitching moment is constant. The neutral point of an aircraft is its aerodynamic center. Of course, this not the aerodynamic center of its wing. $\endgroup$ – Guha.Gubin May 1 at 13:48
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Center of pressure: the net force of air on a body/surface/wing. The center of pressure changes as an aircraft maneuvers because different parts of the aircraft stick into the airflow as the angle of attack changes. Notice how the center of pressure moves forward as the angle of attack increases (the airflow is right-to-left):


(source: prepware.com)

Aerodynamic center: the location at which the moment does not change regardless of the angle of attack. Wings tend to produce a rotational force due to the way they push air around, and this is the location at which the rotational force stays constant at any angle of attack. It is often easier to consider the aerodynamic center for where you want to place your wings and mass because its location is constant, unlike the center of pressure. For now, think of Mtrim as equivalent to the rotational force the elevators must impart in order to achieve level flight.


(source: charlesriverrc.org)

Neutral Point: the position of center of mass where the aircraft would be neutrally stable. If you put your center of mass behind (aft) of this point, you will get an unstable aircraft that will increasingly turn with any maneuver the pilot makes. At low speeds, this may be tolerable, but computers are needed to fly unstable aircraft at higher speeds.


(source: rchomepage.com)

tl;dr:

  • CoP = where the wings push
  • AC = where the wings rotate
  • NP = where the center of mass should be to make an aircraft that does not tend to pitch up or down
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  • $\begingroup$ Is it accurate to also describe the neutral point as the centre of the couple between the CoG and the AC? $\endgroup$ – Simon Aug 27 '15 at 13:09
  • $\begingroup$ @Simon That may not be the case. In tailless aircraft, the CoP and NP coincide, so a modified airfoil is often used to counter the AC. $\endgroup$ – Kurt Tank Aug 27 '15 at 13:18
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    $\begingroup$ The first sketch is lousy - the center of pressure is generally too far back and should be close to the quarter chord at the high angle of attack. The real Kurt Tank would have noticed that immediately! $\endgroup$ – Peter Kämpf Aug 27 '15 at 15:58
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    $\begingroup$ @PeterKämpf Ah, you see I lost my high-quality drawing tools in the war. Notice that the 3 degree angle and 9 degree one look suspiciously close! $\endgroup$ – Kurt Tank Aug 27 '15 at 16:32

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