An airfoil is a structure designed to obtain reaction upon its surface from the air through which it moves or that moves past such a structure. This discussion is confined to the airfoils designed to produce lift. Looking at a typical airfoil profile, such as the cross section of a wing, one can see several obvious design characteristics. [Figure 3-6] There is a difference in the curvatures (called cambers) of the upper and lower surfaces: the upper camber is more pronounced than the lower, which is usually somewhat flat. The two extremities of the profile also differ—the end that faces forward in flight is the leading edge, and is rounded; the other end, the trailing edge, is quite narrow and tapered.
A reference line often used in discussing the airfoil is the chord line, a straight line connecting the extremities of the leading and trailing edges. The distance from this chord line to the upper and lower surfaces denotes the magnitude of the camber at any point. Another reference line, the mean camber line, is equidistant at all points from the upper and lower surfaces. An airfoil is constructed so its shape takes advantage of the air’s response to physical laws, developing two actions from the air mass: a positive pressure lifting action from the air below the wing, and a negative pressure lifting action from lowered pressure above. As the airstream strikes the relatively flat lower surface when inclined at a small angle, the air is forced downward, causing an upward reaction (positive lift). At the same time, the airstream striking the upper curved leading edge is deflected upward. If a wing creates a lift force greater than the aircraft’s weight, the aircraft flies. If all the lift came merely from deflection by the lower surface, an aircraft would only need a flat wing like a kite; the balance of the lift comes from the flow of air above the wing.
It is neither accurate nor useful to assign specific percentages of lift to the upper versus the lower surface—these vary with flight conditions and wing design. Many thousands of airfoils have been tested, but none satisfies every flight requirement; the weight, speed, and purpose of each aircraft dictate the shape of its airfoil. The most efficient airfoil for the greatest lift has a concave, “scooped-out” lower surface, but as a fixed design it sacrifices too much speed and is unsuitable for high-speed flight. Today’s high-speed jets use leading-edge (Krueger) flaps and trailing-edge (Fowler) flaps that, when extended, change the airfoil into the concave form to generate much greater lift at low speed. Conversely, a perfectly streamlined airfoil offering little resistance may not have enough lifting power to take off, so modern airplanes use airfoils that strike a medium between extremes. Figure 3-7 shows some of the more common airfoil sections.
3.4.1 Low Pressure Above #
In a wind tunnel or in flight, an airfoil is simply a streamlined object inserted into a moving stream of air. If the profile were shaped like a teardrop, the speed and pressure changes over the top and bottom would be the same on both sides. But if the teardrop were cut in half lengthwise, a form resembling the basic airfoil section would result. If the airfoil is then inclined so the airflow strikes it at an angle (angle of attack, AOA), the air moving over the upper surface is forced to move faster than the air along the bottom. This increased velocity reduces the pressure above the airfoil. Applying Bernoulli’s Principle, the increase in speed across the top produces a drop in pressure, which is a component of total lift. The pressure difference alone does not account for the total lift: the downward-and-backward flow from the top surface creates a downwash that meets the flow from the bottom at the trailing edge, and by Newton’s third law the reaction of this downward-backward flow results in an upward-forward force on the airfoil.
3.4.2 High Pressure Below #
A certain amount of lift is generated by pressure conditions underneath the airfoil. Because of the way air flows underneath it, a positive pressure results, particularly at higher angles of attack. Close to the leading edge the airflow is virtually stopped (stagnation point) and then gradually increases speed; near the trailing edge it again reaches a velocity equal to that on the upper surface. In conformance with Bernoulli’s principle, where the airflow was slowed beneath the airfoil a positive upward pressure was created (as fluid speed decreases, pressure increases). Since the pressure differential between the upper and lower surfaces increases, total lift increases. Both Bernoulli’s Principle and Newton’s Laws operate whenever lift is generated by an airfoil.
3.4.3 Pressure Distribution #
From experiments on wind-tunnel models and full-size airplanes, it has been determined that as air flows along the surface of a wing at different angles of attack, there are regions where the pressure is negative (less than atmospheric) and regions where it is positive (greater than atmospheric). The negative pressure on the upper surface creates a relatively larger force than the positive pressure from air striking the lower surface. Figure 3-8 shows the pressure distribution along an airfoil at three different angles of attack. The average of the pressure variation for any given angle of attack is the center of pressure (CP); aerodynamic force acts through this CP. At high angles of attack the CP moves forward; at low angles of attack it moves aft. [Figure 3-8]
In the design of wing structures, this CP travel is very important, since it affects the position of the air loads imposed on the wing structure in both low- and high-AOA conditions. An airplane’s aerodynamic balance and controllability are governed by changes in the CP.
3.4.4 Airfoil Behavior #
Although specific examples can be cited in which each principle predicts and contributes to lift, lift is a complex subject—more complex than a simple differential pressure between upper and lower surfaces. In fact, many lifting airfoils do not have an upper surface longer than the lower, as with the symmetrical airfoils used on high-speed aircraft and on many helicopter rotor blades, whose upper and lower surfaces are identical; only the relationship of the airfoil with the oncoming airstream (the angle) differs. A paper airplane, simply a flat plate, has top and bottom of exactly the same shape and length, yet it produces lift, and “flow turning” is partly (or fully) responsible. As an airfoil moves through and is inclined against the airflow, it produces a different flow caused by its relationship to the oncoming air. Think of a hand placed outside a car window at speed: inclined one way or another, it moves up or down. This is caused by deflection, which turns the air about the object; the velocity about the object changes in both magnitude and direction, resulting in a measurable force and direction.
