HOW IT WORKS
HOW A WING CREATES LIFT
Airfoils and how a wing creates lift
An airfoil, or a wing, as pilots more commonly refer to it, when viewed from the wingtip looking back at the fuselage, normally offers a curved shape, with the front, or leading edge, being thicker than the rear, or trailing edge. Airfoils are, of course, also used in the construction of propellers and helicopter rotor blades.
Airfoils are created in a variety of shapes, many of which originated with the National Advisory Committee for Aeronautics (NACA) in the late 1920s and through the 1930s. Some NACA airfoils come with razor-thin leading edges. Some are flat on the bottom, while others are nearly concave at the trailing edge, with others being symmetrical above and beneath the wing. Each offers very different flight characteristics and peculiarities from which engineers choose depending upon the desired performance characteristics of the aircraft they’re designing.
Some airfoils create great climb performance, while others produce faster cruise speeds. No matter the shape, all airfoils are created for the same purpose: to create lift as air moves over both the upper and lower surfaces. Physics tells us that the faster the air moves across the surface of an airfoil, the more lift a wing creates. An understanding of how an airfoil accomplishes its work does demand a grasp of some fundamental terms, in addition to leading and trailing edge.
Engineers create an imaginary reference point in the design process, called the chord line, that runs from the
leading edge straight through to the trailing edge. The airfoil’s overall curvature is known as the wing’s camber. The angle at which a wing is attached to the fuselage that offers a slightly upward tilt is referred to as the angle of incidence.
However, understanding how an airfoil actually creates lift is much like studying weather; simply knowing the terminology and the shapes is only a starting point. So important is a deeper comprehension of what makes an airfoil work that Wolfgang Langewiesche, a famed test pilot for Cessna Aircraft and Chance Vought, explained the “why” in the first few pages of his 1944 aerodynamics classic, Stick and
Rudder. “At this very moment, thousands of men trying to learn to fly are wasting tens of thousands of air hours simply because they don’t really understand how an airplane flies; because they don’t see one fact that explains just about every single thing they are doing; because they lack the one key that with one click unlocks most of the secrets of the art of flying.” The key element to which Langewiesche was referring was angle of attack. An inadequate understanding of angle of attack’s role in the performance of a wing is one reason that today, loss of control is the largest cause of fatal accidents in aircraft of all shapes and sizes.
To understand how angle of attack fits into the role of an airfoil demands defining a few more important terms. One is relative wind, the name attached to the air moving opposite the direction the aircraft is traveling. The angle measured between the wing’s chord line and the relative wind then is the true definition of angle of attack. As the angle of attack increases, so does the lift generated by the airfoil. However, the angle of attack can only be increased to a point. The critical angle of attack defines the region in which the wing begins to stop producing lift because air no longer flows smoothly above and beneath the wing. This point at which a wing stops producing lift is called a stall. Wings can stall in any flight attitude, from nearly level flight to descents, climbs and in turns.
Avoiding stalls demands a thorough understanding of both Bernoulli’s theorem and Sir Isaac Newton’s three laws of motion, although Langewiesche saw Bernoulli’s theorem as nothing more than “an elaboration and a more detailed description of just how Newton’s law fulfills itself” in the production of lift.