An airfoil is a surface designed to obtain lift from the air through which it moves. Thus, it can be stated that any aircraft surface designed to produce lift from airflow is an airfoil. The profile of a conventional wing is an excellent example of an airfoil. [Figure 1]
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| Figure 1. Airflow over a wing section |
Notice that the top surface of the wing profile has greater curvature than the lower surface.
The curved shape of an airfoil and its angle of attack cause the airflow over the upper surface to accelerate while the airflow beneath the wing remains at a relatively higher pressure. According to Bernoulli’s Principle, the increased velocity of the airflow over the upper surface results in lower static pressure, while the slower-moving air beneath the wing maintains a higher pressure.
At the same time, the wing deflects air downward, producing an upward reaction force in accordance with Newton’s Third Law of Motion. The combination of this pressure difference and the downward deflection of airflow generates the lift force that supports the aircraft in flight.
Within limits, lift can be increased by increasing the angle of attack (AOA), wing area, velocity, density of the air, or by changing the shape of the airfoil. When the force of lift on an aircraft’s wing equals the force of gravity, the aircraft maintains level flight.
Shape of the Airfoil
Individual airfoil section properties differ from those properties of the wing or aircraft as a whole because of the effect of the wing planform. A wing may have various airfoil sections from root to tip, with taper, twist, and sweepback.
The overall aerodynamic characteristics of the wing are determined by the combined effects of the airfoil sections along the wingspan.
The shape of an airfoil influences the amount of drag it produces and, consequently, the overall efficiency of the wing. One important characteristic is the thickness-to-chord ratio, which compares the maximum thickness of the airfoil to its chord length.
Thin airfoils generally produce less form drag but may have structural limitations and reduced low-speed performance. Thicker airfoils can provide greater structural strength and internal space but may generate increased pressure drag at higher speeds. The design of an airfoil therefore represents a compromise between aerodynamic efficiency, structural requirements, and the intended operating characteristics of the aircraft.
The efficiency of a wing is measured in terms of the lift to drag ratio (L/D). This ratio varies with the AOA but reaches a definite maximum value for a particular AOA. At this angle, the wing has reached its maximum efficiency. The shape of the airfoil is the factor that determines the AOA at which the wing is most efficient; it also determines the degree of efficiency. Many efficient airfoils used for general aviation aircraft have their maximum thickness located approximately one-third of the chord length behind the leading edge.
High-lift wings and high-lift devices for wings have been developed by shaping the airfoils to produce the desired effect. The amount of lift produced by an airfoil increases with an increase in wing camber. Camber refers to the curvature of an airfoil above and below the chord line surface. Upper camber refers to the upper surface, lower camber to the lower surface, and mean camber to the mean line of the section.
Camber is positive when departure from the chord line is outward and negative when it is inward. Thus, high-lift wings have a large positive camber on the upper surface and a slightly negative camber on the lower surface. Wing flaps cause an ordinary wing to approximate this same condition by increasing the upper camber and by creating a negative lower camber.
It is also known that the larger the wingspan, as compared to the chord, the greater the lift obtained. This comparison is called aspect ratio. In general, a higher aspect ratio improves aerodynamic efficiency and reduces induced drag, which can enhance lift performance. Despite the aerodynamic benefits of a higher aspect ratio, practical limitations are imposed by structural strength, weight, and operational considerations.
On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the aircraft off the ground. Thus, modern aircraft have airfoils which strike a medium between extremes, the shape depending on the purposes of the aircraft for which it is designed.
Angle of Incidence
The acute angle the wing chord makes with the longitudinal axis of the aircraft is called the angle of incidence, or the angle of wing setting. [Figure 2]
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| Figure 2. Angle of incidence |
The angle of incidence in most cases is a fixed, built-in angle. When the leading edge of the wing is higher than the trailing edge, the angle of incidence is said to be positive. The angle of incidence is negative when the leading edge is lower than the trailing edge of the wing.
Angle of Attack (AOA)
Before beginning the discussion on AOA and its effect on airfoils, first consider the terms chord and center of pressure (CP) as illustrated in Figure 3.
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| Figure 3. Airflow over a wing section |
The chord of an airfoil or wing section is an imaginary straight line that passes through the section from the leading edge to the trailing edge, as shown in Figure 3. The chord line provides one side of an angle that ultimately forms the AOA.
The other side of the angle is formed by a line indicating the direction of the relative airstream. Thus, AOA is defined as the angle between the chord line of the wing and the direction of the relative wind. This is not to be confused with the angle of incidence, illustrated in Figure 2, which is the angle between the chord line of the wing and the longitudinal axis of the aircraft.
On each part of an airfoil or wing surface, a small force is present. This force is of a different magnitude and direction from any forces acting on other areas forward or rearward from this point. It is possible to add all of these small forces mathematically. That sum is called the resultant aerodynamic force. This resultant force has magnitude, direction, and location, and can be represented as a vector, as shown in Figure 3.
The point of intersection of the resultant force line with the chord line of the airfoil is called the center of pressure (CP). The CP moves along the airfoil chord as the AOA changes. Throughout most of the flight range, the CP moves forward with increasing AOA and rearward as the AOA decreases. The effect of increasing AOA on the CP is shown in Figure 4.
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| Figure 4. Effect on increasing angle of attack |
The AOA changes as the aircraft’s attitude changes. Since the AOA has a great deal to do with determining lift, it is given primary consideration when designing airfoils. In a properly designed airfoil, the lift increases as the AOA is increased. When the AOA is increased gradually toward a positive AOA, the lift component increases rapidly up to a certain point and then suddenly begins to drop off. During this action the drag component increases slowly at first, then rapidly as lift begins to drop off.
When the AOA increases to the angle of maximum lift, the burble point is reached. This is known as the critical angle. When the critical angle is reached, the air ceases to flow smoothly over the top surface of the airfoil and begins to burble or eddy. This means that air breaks away from the upper camber line of the wing. What was formerly the area of decreased pressure is now filled by this burbling air. When this occurs, the amount of lift decreases significantly while drag increases rapidly. The wing is then said to be stalled. Thus, the burble point corresponds to the stalling angle.
As previously seen, the distribution of the pressure forces over the airfoil varies with the AOA. The application of the resultant force, or CP, varies correspondingly. As this angle increases, the CP moves forward; as the angle decreases, the CP moves back. The unstable travel of the CP is characteristic of almost all airfoils.
Boundary Layer
In physics and fluid mechanics, the boundary layer is the layer of fluid immediately adjacent to a surface. In relation to an aircraft, the boundary layer is the portion of the airflow closest to the aircraft's surface. In designing high-performance aircraft, considerable attention is paid to controlling boundary-layer behavior in order to minimize pressure drag and skin-friction drag.
The behavior of the boundary layer has a significant influence on lift, drag, and stall characteristics. Maintaining smooth airflow over the wing helps improve aerodynamic efficiency, delay flow separation, and enhance overall aircraft performance.