Angle of Attack: Effect of AOA on Center of Pressure (CP)

Angle of Attack: Effect of AOA on Center of Pressure, the Critical or Stalling Angle of Attack

Angle of Attack (AOA) & Center of Pressure

The angle formed by the chord line of the wing and the relative wind direction is known as Angle of Attack (AOA).

It is imperative to understand the words "chord" and "center of pressure" before attempting to discuss AOA and its effect on airfoils (CP).


The chord line of an airfoil or wing section is an imaginary straight line that passes through the section from the leading edge to the trailing edge. The chord line is one side of an angle that eventually results in the AOA. A line denoting the direction of the relative airstream forms the other side of the angle. As a result, AOA is defined as the angle formed by the chord line of the wing and the relative wind direction. This should not be confused with the angle of incidence, which is the angle formed by the wing's chord line and the aircraft's longitudinal axis.

A small force exists on each part of an airfoil or wing surface. Any forces operating on other places forward or backward from this point are of a different magnitude and direction. All of these minor forces can be added analytically. The amount is referred to as the "resultant force" (lift). A vector can be used to indicate the magnitude, direction, and position of the resulting force.


The Pressure Center

The Centre of pressure is the place where the resultant force line intersects the chord line of the airfoil (CP). As the AOA varies, the CP moves along the airfoil chord. Throughout the majority of the flying range, the CP goes forward as the AOA increases and backward as the AOA decreases. As the aircraft's attitude varies, so does the AOA.

Because the AOA plays such a large role in determining lift, it is given top priority when designing airfoils. The lift increases as the AOA increases in a well-constructed airfoil.

Effect of AOA on Center of Pressure

When the AOA is steadily increased toward a positive AOA, the lift component grows rapidly up to a point before abruptly dropping off. The drag component of this movement increases slowly at first, then rapidly as lift decreases.

The burble point is reached when the AOA reaches the maximum lift angle. The crucial angle is what it's called. When the critical angle is achieved, the air stops flowing smoothly over the top surface of the airfoil and burbles or eddyes. This signifies that the air brakes away from the wing's top camber line. This burbling air has filled the space where there was previously a drop in pressure. When this happens, the amount of lift decreases and drag increases. The force of gravity takes effect, and the plane's nose sinks. There is a stall here. As a result, the stalling angle is the burble point.

With increasing AOA, the distribution of pressure forces over the airfoil changes. As a result, the application of the resultant force, or CP, varies. The CP goes forward as the angle increases, and backward as the angle drops. Almost all airfoils have a CP that travels in an unstable manner.

Critical or Stalling Angle of Attack

The critical angle of attack is the angle at which the maximum lift coefficient is produced. The "stall angle of attack" is another name for this. The lift coefficient decreases as the angle of attack decreases below the critical angle of attack. Conversely, as the angle of attack increases over the critical angle of attack, the air begins to flow less smoothly across the upper surface of the airfoil and begins to separate from it. As the angle of attack increases, the upper surface separation point of the flow advances from the trailing edge to the leading edge on most airfoil forms. The upper surface flow is more divided at the critical angle of attack, and the airfoil or wing is creating its maximum lift.

The aircraft is said to be stalling over this critical angle of attack. By definition, a fixed-wing aircraft stalls at or above the critical angle of attack, rather than at or below a specific airspeed. The weight of the aircraft, the load factor, the aircraft's centre of gravity, and other factors all influence the airspeed at which it stalls. The plane, on the other hand, stalls at the same critical angle of attack every time.

Stalling Angle

For many airfoils, the critical or stalling angle of attack is between 15° and 18°. When a maximum angle of attack is achieved, certain aircraft include a built-in flight computer that prevents the aircraft from increasing the angle of attack any further, regardless of pilot input.

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