MIAMI – The heart of an airplane, aeronautical engineers will tell you, is the wing. It generates the majority of the lift in conventional aircraft, with the fuselage and tail contributing only a small percentage of the overall lift.
To get some measure of the evolution of wing design, Orville Wright’s first flight, which lasted 12 seconds and covered 120 feet, was shorter than the wingspan of a Boeing 747.
The airplane wing has evolved from the wooden and fabric twin-wing design of the Wright brothers’ Flyer to the composite materials utilized in today’s models from giants Airbus and Boeing. Quite different from the infamous Wright Patent Wars, there is nevertheless stiff competition between the companies when it comes to efficient wing design.
As a primer, aerodynamics involves a combination of four different forces: lift, weight, drag, and thrust. Lift is the opposite force of weight, and it occurs as air moves on wings. Let’s dive in!
Lift and Drag
From the beginning of their work on flying machines, the Wright brothers knew that the wings would generate most of the lift. To that effect, the brothers used the aeronautical data supplied by German aviation pioneer Otto Lilienthal to create the wings of their first gliders in 1900 and 1901.
When they measured the aerodynamic lift on their gliders, however, the Wrights discovered that it was only one-third of the lift calculated using Lilienthal’s data. The issue was Wright’s misreading of the data, which was based on a lack of information concerning Lilienthal’s test model’s wing geometry.
Despite this, the Wrights conducted their tests with a crude wind tunnel they built themselves. They discovered the importance of wing aspect ratio on lift and drag during their wind tunnel testing. The aspect ratio of their rectangular wings is equal to the wingspan divided by the chord. A large aspect ratio wing is similar to the slat from a Venetian blind; a low aspect ratio wing is short and stubby.
A double-wing provided extra lift without making the aircraft too big and provided a stiffer wing structure. At the time, airplanes only had enough power to lift the pilot and the aircraft.
High Aspect Ratio Wing
The Wrights discovered that a high aspect ratio wing produced greater lift and less drag than a low aspect ratio wing based on their wind tunnel findings. The aspect ratio of their next glider, which flew magnificently in 1902, was 6.7. The aspect ratio of the Wright Flyer was 6.4. Many conventional airplanes today have fairly similar aspect ratios, as we can see.
Another key aspect of the flying machines were their wingtips, which may be warped in different directions, creating an asymmetrical lift force on both wings and therefore providing a roll control mechanism. We will look at the latest advancements in wingtip shapes further below.
Of course, one of the Wrights’ most fundamental technological contributions to the airplane was the concept of lateral (roll) control. After a few years, ailerons were used instead of wing warping for roll control, but the Wrights’ contribution was crucial.
Through Thick and Thin: Airfoils
An airfoil is the cross-section of a wing taken in the direction of flight. The shape of an airfoil is an important part of a wing’s design. For example, it has an impact on the wing’s lift and drag, as well as the stalling angle of attack (the angle of attack of the wing beyond which the lift dramatically drops off and the drag suddenly increases).
The Wrights utilized very thin airfoils because their wind tunnel tests showed that very thin forms had lower drag than larger airfoils. During WWI, most planes followed suit and adopted thin airfoils. However, because the wind tunnel models were small and the airflow speeds in the wind tunnels were modest, the early results were misleading.
At angles of attack substantially lower than conventional stalling angles of attack, thin airfoils suffered “thin airfoil stall.” This occurred owing to flow separation across the thin airfoil’s upper surface, resulting in substantially increased drag and a loss of lift.
Thicker airfoils, on the other hand, did not experience flow separation until considerably higher angles of attack, resulting in more lift and less drag at higher angles of attack under the same operating conditions.
German engineers identified this, and thick airfoils were used in the Fokker Triplane and Fokker D-7 near the conclusion of WWI. Because these planes could climb quicker and maneuver more sharply than planes with thin airfoils, the Fokker D-7 became one of the most effective fighters of WWII.
Designers of airplanes began to adopt thick airfoils in the 1920s. With their efficiency, large aspect ratios and thick airfoils were common in wing designs by the 1930s. With its aesthetically pleasing high wing aspect ratio of 9.14 and streamlined 15 percent thick airfoil, the legendary Douglas DC-3 is a good example of this improvement in wing design.
Thick airfoils have both structural and aerodynamic benefits. Fuel tanks and retractable landing gear might be stored under a thicker wing. A thicker wing also allowed for a larger and stronger structural spar on the inside, allowing the wing to be cantilevered from the fuselage without the requirement of external support wires and struts. This aided in the adoption of the current single-wing (monoplane) configuration rather than the older two-wing (biplane) configuration.
After WWII, German scientists Johanna Weber and Dietrich Küchemann collaborated with British scientists at Farnborough to conduct groundbreaking research in the UK. The junction of the wing and the fuselage, as well as the geometry of the wingtip, were recognized as major sources of aerodynamic inefficiency.
Their effort resulted in the development of Concorde’s unique wing in the 1960s and the Airbus A300, the first twin-engined widebody aircraft, in the 1970s.
Breaking the Sound Barrier
With the introduction of jet aircraft in the 1950s, which were capable of speeds approaching and exceeding the speed of sound, airfoil and wing shapes saw yet another drastic transformation. Thinner airfoils permitted subsonic planes to fly closer to the speed of sound before encountering harmful shock waves across the wing, which increased drag and lowered lift significantly.
Additionally, with the jet age, strength and performance requirements changed, resulting in new wing shapes, including swept wings, delta wings, and crescent wings.
Now, the primary design element for supersonic airplanes was to lower the strength of shock waves on the wings, and therefore the supersonic wave drag. The weaker the shocks and the smaller the wave drag, the thinner the airfoils are.
The Lockheed F-104, the first airplane intended for sustained Mach 2 speeds, is an excellent example. The F-104 has a very thin airfoil, around 3.5 percent thick, and a razor-thin leading edge, all to lessen the power of shock waves from the wing’s leading edge.
It’s almost as if the airfoil thickness has gone full circle, reverting to that of the Wright brothers, but for entirely different flight conditions. Many high-speed subsonic and supersonic planes also have swept wings rather than straight wings, which helps to lessen the severity of shock waves and reduce wave drag.
New and demanding flight situations continue to drive the evolution of wing and airfoil forms today. To achieve higher lift-to-drag ratios, new and improved wing arrangements and airfoil shapes are being developed to provide greater fuel economy in flight. Future hypersonic aircraft vehicles with Mach 5 and higher speeds will also necessitate novel wing and airfoil forms.
Weight, Composite Materials, Wingtip Designs
During a flight, the amount of fuel consumed is roughly proportional to the aircraft’s drag. Thus, wing designers and manufacturers have worked on lowering weight. Additionally, aerodynamic improvements minimize the amount of power and fuel necessary to propel the plane through the air, while lighter loads reduce the amount of lifting force required.
In comparison to the primarily aluminum constructions that have dominated the industry since the 1960s, innovative composite materials have lowered the weight of aircraft wings, with the advantage of tailoring specific design loads, strengths, and tensions for different wing and aircraft models. This technology has been improved by the incorporation of nanoparticles that are applied to composites during the manufacturing process.
Technology has also had a secondary impact on aircraft wing design and construction since they are increasingly required to accommodate and store new gear for structural health monitoring. With sensors that monitor important performance characteristics during flight, this advanced technology is being included in wing structure design and production.
Finally, the introduction of wingtip design advancements has greatly increased aircraft aerodynamic performance. The effects of the “wake” – the swirling vortex of air left behind the wing as it flies through the air at high speed – are reduced by sharklets (Airbus) or raked wingtips and later winglets (Boeing).
According to Boeing, without any winglets, the airflow over the tip of every wing rolls up from the high-pressure area under the wing to the low-pressure area above it. Additionally, when the wing is moving forward at high speed, airflow over the tip of the wing is forced back, with the upward and backward flow elements combining to form vortices. These vortices cause lift-induced drag, lowering the efficiency of the wing.
Small upward-pointing extensions at the apex of the wings are the most common way to avoid the above issues. The passage of the airplane is smoother and more efficient as the disturbance to the air is reduced. It provides the same result as a significant increase in the aircraft’s wingspan, but without the additional weight.
Recent Wing Advances from Airbus, Boeing
Airbus is looking into how long, narrow wings can have a high lift-to-drag ratio, which can help save fuel. However, because of airport limitations, wing length is limited. The Airbus team is therefore experimenting with folding wingtips that can be extended before takeoff and folded back on the ground.
Additionally, Airbus said on September 22 that it would develop an “extra performance wing” capable of altering shape during flight to improve efficiency and reduce emissions.
On its part, Boeing unveiled its latest Transonic Truss-Braced Wing (TTBW), which will, according to researchers, fly higher and faster than prior TTBW models. The new configuration is intended to provide unparalleled aerodynamic efficiency while flying at Mach 0.80, a speed comparable to that of many modern jetliners.
Boeing notes that the folding wings are 170 feet long from end to end. The presence of a truss, which supports the ultra-thin wing’s expanded length, allows for the large wingspan.
Furthermore, Boeing’s 737 MAX AT Winglet is the latest advancement in the company’s winglet technology. The new lower aerofoil creates a vertical lift component that is vectored away from the fuselage and slightly forward, in addition to the inward, upward, and slightly forward lift components of the top aerofoil.
These components work together to create a precisely balanced winglet that maximizes the wing’s overall efficiency.
The 737 MAX team went on to include Boeing’s sophisticated natural laminar flow technology into the surface material specification for the MAX AT winglet, believing that there was even more efficiency to be obtained on top of the benefits from this innovative approach.
A Faster Wing Prduction on the Horizon
Nobody knows how the future generation of aircraft wings will look. It could be reshaped or assembled in a new way. It could be made of sophisticated metallic materials or composites. It could fold and reshape. One thing is certain: as aircraft production rates increase, wings will need to be faster, easier, and less expensive to manufacture and assemble.
It remains to be seen whether these novel wing advances in aerodynamic performance by aircraft and wing manufacturers outweigh the additional weight and cost.
Since the Wright Brothers’ first flight, the airplane wing has evolved significantly. Today, OEMs are leveraging new technologies and design approaches to improve aerodynamic performance and reduce component weight throughout the wing structure, resulting in significant fuel efficiency gains for airlines.