Wing

• 1. Introduction

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  A wing, in its oldest sense, meant only one thing: the foremost limb of a bird. The word itself comes from the Old Norse vaengr, and for centuries it also did double duty as an architectural term, describing the aisle of a building. Only over recent centuries did wing stretch to cover the lift-producing limbs of insects, bats, and pterosaurs, the arms of a boomerang, the sails of certain boats, and eventually the fixed surfaces bolted to an aircraft or a race car. Stripped of that history, a wing is simply a structure that produces both lift and drag as it moves through air, shaped by two things: an airfoil section and a planform, with its overall efficiency measured as a lift-to-drag ratio. The same basic idea works underwater too, showing up as the hydrofoils that lift racing sailboats and power vessels clear of the surface, and as the diving planes that tilt a submerged submarine up or down. How does a shape this simple generate enough force to lift tons of aircraft into the sky? Why do engineers keep adding new devices to what should be a single smooth surface? And how did such a specific solution to the problem of flight end up evolving from scratch again and again over the long history of life?

• 2. How A Wing Lifts

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  Solving exactly how air behaves around a moving wing means working through the Navier-Stokes equations of fluid dynamics, a set of equations difficult to solve except in the simplest geometries, which is why simpler explanations are usually offered instead. For a wing to generate lift, it has to meet the airflow at a suitable angle, known as the angle of attack, deflecting the air downward as it passes. Because the wing pushes the air, the air pushes back on the wing with equal and opposite force, showing up as a pressure difference between the wing's upper and lower surfaces: pressure drops on top and rises on the bottom. That pressure difference can be measured directly with instruments, or calculated from airspeed using Bernoulli's principle, which links changes in air speed to changes in air pressure. The lower pressure above the wing exerts a smaller downward push than the higher pressure below exerts upward, and the net result is the upward force called lift. Because pressure differences, airflow velocity, changes in air direction, and lift itself are all just different ways of describing the same event, any one of them can be used to calculate the others, and different mathematical routes to the same answer are sometimes mistaken by outsiders for genuine disagreement about how flight actually works.

• 3. Shape And Control

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  Most wings built for flight slower than the speed of sound use an asymmetrical cross-section, though a symmetrical one can still generate lift by holding a positive angle of attack to push air downward; symmetrical airfoils stall at higher speeds than curved ones but fly identically whether an aircraft is upright or inverted, which is why aerobatic planes favor them. For flight near the speed of sound, engineers turn to supercritical airfoils, flat on top and curved on the bottom, designed specifically to blunt the sharp rise in drag that shows up in transonic flight. A working wing tends to accumulate hardware: leading-edge slats, slots, or extensions and trailing-edge flaps or flaperons that let a pilot reshape the wing in flight, winglets that curb the wingtip vortices which otherwise add drag and cut lift, and a deliberate tilt called dihedral, or its opposite, anhedral, that respectively increases or decreases a plane's natural roll stability. Ailerons near the wingtips roll the aircraft, spoilers on the upper surface dump lift and add drag during descent and braking, vortex generators keep transonic airflow from separating, and wing fences block that same separation from spreading sideways across the wing. Some naval aircraft fold their wings entirely to fit more of them onto a carrier's hangar deck, and variable-sweep, or swing wings, that stretch out for slow flight and swing back for supersonic speed have appeared on the F-111 Aardvark, the F-14 Tomcat, the Panavia Tornado, the MiG-23, the MiG-27, the Tu-160, and the B-1B Lancer.

• 4. Wings Beyond Airplanes

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  Fixed-wing aircraft are only one use of the shape. Hang gliders range from the fully flexible fabric of paragliders to rigid framed designs, kites rely on their own variety of lifting surfaces, and helicopters use a rotating wing whose pitch angle can be adjusted to steer thrust in different directions, the same underlying principle that lets a propeller's blades generate lift for propulsion instead of altitude. NASA's Space Shuttle used its wings for one purpose only, gliding unpowered to a runway landing during its descent, earning that category of vehicle the name spaceplane. Formula One cars mount wings upside down, turning the same lift-generating shape into downward force that presses tires into the track for extra grip, while sailboats put the idea to entirely different use, treating a sail as a vertical wing that can change its fullness and direction to move a boat across open water. The flexible end of the spectrum has its own inventors. In 1948, Francis Rogallo built the first fully limp flexible wing, and Domina Jalbert later developed a flexible, unsparred wing inflated into shape by ram air, the design underlying the modern parafoil.

• 5. Evolution's Repeat Invention

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  Wings are not a single evolutionary event. They have arisen independently across insects, dinosaurs, mammals, fish, and reptiles, each time solving the same problem from a different starting point. In birds, bats, and pterosaurs, wings evolved out of limbs the animals already had, reshaping existing forearms into airfoils, while insect wings evolved as a completely separate structure with no equivalent limb behind them. Wherever they appeared, wings opened up new possibilities for movement, for spreading into new territory, and for splitting into new species. Not every wing stays airborne, either. Several species of penguins, along with other water birds capable of flight or not, including auks, cormorants, guillemots, shearwaters, eider and scoter ducks, and diving petrels, use the same limb to swim, turning what began as a flight surface into an efficient paddle underwater.