Definition: Any of a variety of wing shapes, which provide the lift needed for an airplane to fly and ensure optimum performance in the designated mission of a particular aircraft.
Significance: The most significant part of an airplane, wings generate the lift an airplane needs to overcome its weight and maneuver in flight. Wings can be long and thin or short and stubby. They can be angled or straight. The design of the wing must suit the purpose of the airplane: long-distance flight, aerobatic flight, high- or low-speed flight, or even space flight.
Background
Wings and the concept of flight go hand-in-hand. From the beginning of time, humans have watched birds and insects fly and dreamed of somehow making their own wings and soaring into the skies. Early designs for flying machines, from those of Leonardo da Vinci to those of Otto Lilienthal, copied the wing shapes of birds and bats for the simple reason that these were proven designs. However, the first successful powered aircraft had wings that were nearly rectangular in shape and stacked one above the other, nothing like the designs of nature. Modern airplane wings take almost any shape imaginable, from the long, slender wings of sailplanes to the sleek, highly swept wings of the Concorde.
Many things define the shape or design of a wing, including its span, sweep, taper, dihedral, twist, and its air-foil section or sections. All of these elements can be varied to provide an optimal design for a particular type of airplane. In designing wings, engineers must look first at the mission of the aircraft and determine whether it is to be designed for low-speed, high-speed, subsonic, supersonic, or even hypersonic flight. They must know whether the aircraft will travel long distances or remain close to home. They must know whether it will be used for stunt flying or aerial combat or for slow and comfortable flight movements. They must determine the aircraft’s aerodynamic constraints, structural constraints, and even simpler limitations, such as the width of hangar doors.
Sweep
The sweep of a wing is the angle at which the wing curves. One can define several different sweep angles for a wing, including those for the wing’s leading and trailing edges. This angle is defined as the angle between the relevant part of the wing and the free-stream or oncoming airflow. To an aerodynamicist or a wing designer, the important definition of the sweep is that of the quarter-chord line, a line that would run along the span from the wing root, or centerline, to the wingtip, one-fourth of the way back between the wing’s leading and trailing edges. In other words, a straight or unswept wing would have a quarter-chord line that is perpendicular to the free-stream flow, whereas a swept wing would have its quarter-chord line at an angle other than 90 degrees to the flow. By this definition, a tapered, unswept wing, one with its chord at the tip different from the chord at the center or root, would still have some sweep of its leading or trailing edges.
Wings may be swept for several reasons, but the usual reason is to reduce the drag rise that occurs when a wing nears the speed of sound. Because the airflow accelerates over the top of a wing as an airplane approaches the speed of sound, a region of supersonic flow appears above the wing before the plane actually reaches Mach 1. As that flow moves further back over the wing’s upper surface, it must decelerate back toward the free-stream air speed. However, supersonic flow has a strong tendency suddenly to decelerate through a shock wave. This small shock wave, which often occurs over the wing of an airplane flying at speeds just below the speed of sound, causes a sudden pressure change in the flow over the wing and can result in the flow breaking away from the wing at the shock location. The shock wave itself and the resulting separation of the flow over the wing cause an increase in drag, known as the transonic drag rise, which may double or triple the drag of a wing from its subsonic value.
The onset and magnitude of this drag rise are functions of the Mach number of the flow normal, or perpendicular, to the quarter-chord line of the wing. If this line is swept, the normal component of the Mach number will be lower than the free-stream Mach number. Sweeping the wing will therefore delay and reduce the transonic drag rise, allowing an airplane to fly closer to the speed of sound with less engine thrust or to accelerate past Mach 1 and fly at supersonic speeds with smaller engines. Hence, supersonic aircraft, such as fighters and supersonic transports, have highly swept wings with sweep angles of 45 degrees or more and high-speed, subsonic transports, such those made by Boeing and Airbus, have sweep angles of around 30 degrees to allow them to fly economically at Mach speeds of 0.8 or higher.
Although rearward and forward sweep will provide this drag benefit, aft sweep is usually the design choice, because forward sweep introduces a unique structural problem. The wingtip of a forward-swept wing tends to twist to a higher angle of attack than the rest of the wing, and this can lead to structural failure if the wing is not designed to resist the resulting forces. Thus, the forward-swept wing is often heavier and more expensive to produce than its aft-swept counterpart.
Some airplanes have wing sweep for other reasons such as to have the lift of the wing centered at some point behind or ahead of where the wing root attaches to the fuselage. The Douglas Aircraft DC-3, the world’s first commercial airliner, which cruised below 200 miles per hour, had swept wings. The designers needed to move the wing’s lift a little aft from their original design and wanted to do this without changing the place where the wing mated with the fuselage.
Aspect Ratio
The ratio of a wing’s average chord to its span is known as the aspect ratio. Aerodynamic theory says that a wing with a high aspect ratio will have a very good lift-to-drag ratio, which will, in turn, make it very efficient for both long-range cruising and gliding. The aerodynamic cause of the aspect ratio effect is the flow around the tip of the wing from the higher-pressure area on the lower surface into the region of lower pressure on top. This has the effect of reducing the wing’s lift and increasing its drag. If two wings have the same area, but one has a larger span and smaller chord, these losses near the tip will not affect as much of the wing area on the wing with the higher aspect ratio. Therefore, the wing with the higher aspect ratio will have a higher lift and lower drag than will a lower aspect-ratio wing with the same area and angle of attack. In contrast, a wing with a high aspect ratio will require a stronger structure than one with low aspect ratio and will make the airplane more sluggish in roll. This leads to tradeoffs in the design of aircraft.
Wing designs with a very high aspect ratio, such as 15 to 20, are used for sailplanes that depend solely on glide for flight. In these planes, the added structural weight is more than offset by the improved gliding ability, and there is no need for fast roll rates.
Commercial and military transports and general aviation airplanes designed for longer flights have wings with aspect ratios on the order of 6 to 10. This ratio is sufficient to give excellent long-range cruise capability, a lightweight structure with room for fuel in the wing, and the moderate roll rates needed for comfortable and controllable flight.
Aerobatic and fighter aircraft have wings with smaller aspect ratios of 5 or less, because their ability to roll and do other maneuvers at high rates is more important than efficient, long-range flight. Later variations of the famous Spitfire airplane used by the British in World War II had the span of their efficiently designed wings clipped, or shortened, in order to improve the planes’ roll capability in combat with enemy aircraft.
Twist
Many wings are twisted to make the angle of attack near the wingtip lower than it is at the base. Although wings may be twisted for several reasons, such as to obtain the most efficient aerodynamic loading along the wing’s span or to create effective structural loading, they are often twisted to make sure the inboard part of the wing stalls before the wingtips do.
It is important that the pilot be able to control the roll of an aircraft after the wing begins to stall. Otherwise, the stall can easily turn into a dangerous spin. Because the ailerons, near the wingtips, are the devices that provide roll control, the wing should be designed such that stall will begin on the inboard portion of the wing and progress outward, allowing the pilot to feel the stall while the ailerons are still effective. Although an untwisted wing will have some tendency to stall in this desired manner on its own, due to the three-dimensional flow around the tips, twist is often added to provide an extra degree of certainty of control during a stall.
Dihedral
When looking at an airplane from the front or rear, one may see that the wings are not horizontal but are rather swept slightly upward or even sometimes downward. This angle is called the dihedral, and it is used to improve the roll stability of the airplane.
Two factors in the design of a wing will influence the airplane’s stability in roll. One is the vertical placement of the wing on the fuselage, and the other is its dihedral. Stability in roll means that if the airplane were disturbed from its wings-level position, it would automatically tend to roll back to level. For most aircraft, this stability is achieved by the use of dihedral. In a slight roll, the wing that moves downward toward a level position generates more lift than the opposite wing. The added lift then automatically helps restore the aircraft to equilibrium.
If the aircraft has a high wing placed above the fuselage, the fuselage’s weight hanging below the wing will cause a pendulum-like effect, giving the craft some roll stability. Therefore, less wing dihedral will be needed than for a low-wing design. In fact, if the high-wing plane is a heavy transport, the wing may need to be built with negative dihedral, or anhedral, to ensure that the aircraft is not too stable in roll. Excessive stability would make the airplane resist the roll required during turns or other maneuvers.
Planform
The shape of a wing when viewed from above is known as its planform shape. The area inside this shape, the planform area, is used as a reference area when calculating wing lift, drag, and pitching moment coefficients.
An examination of airplane designs since the beginning of flight will reveal almost every planform shape imaginable. Planform shapes include simple rectangles, basic trapezoids, smooth curves, bird- or bat-wing contours, triangles, and wings with swept leading edges and sawtooth trailing edges.
The simplest wing planform to build is probably the rectangular shape, and sometimes this is the designer’s choice when the cost of construction is more important than other factors. From a structural-efficiency perspective, a wing that is tapered to give a smaller chord at the tip than at the root is a good choice. The tapered planform can give a reasonable aspect ratio while placing the major portion of the lift on the wing’s inboard sections where it is less likely to bend the wing-support spar.
Aerodynamic theory holds that, in addition to the benefits of high aspect ratio, the drag on a wing can be further minimized by optimizing the way that lift acts along the wingspan. The best low-drag lift distribution over the wingspan is an elliptical one, in which the lift tapers from a base value at the center, or root, of the wing to zero at the wingtip in a shape like an ellipse. One way to try to achieve such a lift distribution is to actually vary the chord of the wing elliptically along its span, and many airplane wings have been designed this way. Such designs were particularly prevalent in World War Il-era fighter aircraft, with the best-known example being the British Spitfire.
Elliptically shaped wing planforms are, however, more expensive to build than straight tapered wings, and the wing designer must compare the cost of a purely elliptical wing shape to that of a tapered wing that may approximate the same aerodynamic efficiency. Because it is the wing’s lift distribution and not actually its shape that should be elliptical, there are other alternatives available to the designer. One can design a wing with the right combination of taper, twist, and sweep to give an elliptical load distribution. Another alternative is to employ variations of airfoil section along the wingspan to tailor the lift distribution. In fact, a calculation of the lift distribution on the B-2 flying wing stealth bomber, with its swept wing and unusual saw tooth trailing edge, will reveal a near-elliptical lift distribution.
Wingtip Designs
Over the years, wing designs have included some interesting variations in wingtip shape. Many of these have been accompanied by claims of improved performance due to the reduction or elimination of the drag-producing flow around the wingtip. Although some wingtip shape variations may be capable of slightly altering the structure of the swirling vortex that trails behind a wing, none have ever demonstrated any significant effect. The trailing vortex is a consequence of the lift on a wing, and the only way to reduce or eliminate it is to lower or eliminate the lift on the wing.
The winglet is, however, a wingtip device that is designed to use rather than alter the developing wingtip vortex to produce a thrust. It has been used successfully to improve the performance of many aircraft. Some wing shapes have beneficially altered the wing dihedral at the tip to enhance stability or handling or, as add-on devices, to slightly increase the wing’s aspect ratio and, thus, its aerodynamic performance.
Combinations and Other Variations
Sometimes there is a strong desire to optimize the performance of a wing in seemingly conflicting ways or to design a wing for good flight performance while meeting a non-flying requirement. Variable sweep wings have been used on several fighter and bomber designs to take advantage of the properties of an unswept, high aspect ratio wing and of a swept wing with lower aspect ratio on the same airplane. This increases the cost of manufacturing and maintenance, as well as the weight of the aircraft, factors that must be weighed carefully against the aerodynamic and other performance gains. Over the history of aviation, several designs have employed wings that folded in various ways or extended and retracted either in flight or on the ground in order to optimize wing area or aspect ratio in the air or to fit into tight spaces on the ground.
There have also been designs with interesting combinations of sweep. The scissor-wing concept has a single wing that rotates on an axis, with one side of the wing rotating to a forward sweep and the other moving aft. This design provides a simple but somewhat strange-looking way to achieve variable sweep. The joined wing, with fuselage-mounted aft-swept wing joined at its tips to a forward-swept wing mounted on the vertical tail, claims structural and aerodynamic benefits.
Biplanes, with their wires and struts, are usually considered World War I-era designs. However, modern aerobatic biplanes provide plenty of wing area and lift with a short span for ease of roll. Tandem-wing designs place one wing in front of the other, and at least one past design proposed including a sliding section to fill the space between the tandem wings for added area on takeoff. The channel-wing design wraps part of the wing around the lower half of a propeller supposedly to enhance both wing and propeller performance. Radar reflectivity or stealth considerations may also lead to strange shapes for both the planform and the airfoil sections. Flexible or inflatable wings are often used for “flyable” parachutes and hang gliders and in military applications where wings need to be stored in small places for deployment on demand.
