Material Marvels: Looking at the evolution of the wing in aircraft innovation and technology
From changing its shape to varying the use of materials, the wing is constantly evolving to improve aircrafts’ capabilities to build speed and save fuel. Shardell Joseph looks at some key innovations incorporated into wing design, and what to expect in the future.
The wing is a critical component that lifts an aircraft, giving it flight, and is also used for turning, landing and control. Dating back to Newton’s law of motion defined in the 17th Century, followed by the Bernoulli principle in the 18th Century, the idea of lift and the principles of forces were conceptualised, creating a foundation for the innovation of the man-made wing.
Several pioneering engineers that followed began to bridge the gap between conceptual and practical – such as George Cayley (18th Century) and Otto Lilienthal (19th Century), experimenting with fixed-wing design and its capacity to imbue lift, propulsion and control. In 1902, the Wright glider was designed – a breakthrough machine whose wings enabled the Wright brothers to take the final steps in the invention of the aeroplane.
Since the creation of the aircraft in the 20th Century, advancements in wing design have been endless. Focusing on building speed and fuel efficiency, engineers have worked on large and small alterations to both the shape and materials used in wing design. Since the 1960s, three major redesigns have improved the functionality of the wing – the supercritical airfoil, the use of composites, and wingtips – many more new technologies are expected in the future.
The supercritical airfoil
Thought of by many as the inverse of a conventional wing design at the time, the supercritical airfoil was a new tailoring of an airfoil – shape of a wing – design that delayed the formation and reduced the strength of the shockwave over the wing just below and above the speed of sound.
In principle, a supercritical wing is an airfoil designed to delay the onset of wave drag in the transonic speed range. Typical features, when compared with more traditional airfoil shapes, are a flattened upper surface, a highly cambered or curved aft section and greater leading edge radius.
Conceptualised in the early 1960s by NASA Transonic Aero Branch Chief, Richard T Whitcomb at Langley Research Centre, USA, supercritical wing research showed that aircraft using the concept could increase cruising speed, improve fuel efficiency, and reach greater flight range. From what was initially a radical design, supercritical wings are now commonplace on virtually every modern subsonic commercial transport.
Recognised for his contributions to aviation by receiving the prestigious 1954 National Aeronautic Association’s Collier Trophy, Whitcomb unexpectedly came across the concept of supercritical wing design when observing an airfoil meant for a vertical takeoff jet. Known for his preference for visualisation and intuition over calculations, Whitcomb, by his own account, had been sitting at his desk chain smoking and imagining wind as pipelines sweeping over the surface of his model when the idea hit him. The initial design for a supercritical wing was produced in 1964, and Whitcomb and his colleagues spent the next five years working through different models and concepts.
The concept was first studied in the 2.4m transonic pressure tunnel and other wind tunnels at Langley Research Centre before research with an aircraft began. The first flight tests were carried out between 1971-1973 on the Vought F-8, a modified aircraft and the first to use supercritical wing design. On the first flight, Lead Project Pilot Tom McMurtry reached an altitude of 2,804m and a maximum speed of 418.43km/h.
Research showed that the supercritical wing had increased the F-8 speed by as much as 15% and proved passenger transports with supercritical wings could increase profits by 2.5% over conventional wings. This equated to US$78mln a year (in 1974 dollars) for a 280 fleet of aeroplanes of 200 passenger airlines.
The US Air Force joined NASA to test the supercritical wing on the highly manoeuvrable F-111, with test flights continuing into 1975. Results showed the wing created up to 30% more lift than the conventional wing design.
‘Because supercritical airfoils enable less-swept, wider wing spans, low-speed, maximum lift and low-speed, lift-to-drag ratio has improved,’ Boeing Chief Aerodynamicist Robert Gregg told NASA Technology.
‘Improved maximum lift reduces the speed required to lift or land, so takeoff and landing field lengths can be reduced. A better lift-to-drag ratio reduces the thrust required to fly at a given speed. These principles apply to any aircraft.’
Since the testing proved the supercritical airfoil a success, there was great interest from commercial aviation companies. They decided that, instead of using the wing design to achieve transonic cruising speeds, they would use it to save fuel while continuing to cruise at around Mach 0.8. The supercritical wing is considered to still have a large impact in aviation today as by reducing fuel consumption, the wing saves carbon dioxide and other greenhouse gas emissions.
From aluminium to composites
As innovation is determined by the search for lighter weight aircraft design, advanced composite materials were introduced, largely overtaking the predominantly aluminium structures that have dominated models since the 1960s.
Aluminium in aircraft can be traced back to the first Wright glider. At the time, automobile engines were extremely heavy and didn’t deliver enough power to takeoff, so they built an engine in which the cylinder block and other parts were made from aluminium. However, aluminium was prohibitively expensive and not very accessible, so the wings were constructed with wood and the surfaces covered with fabric.
The golden age of aviation saw the emergence of all-aluminium aircrafts. During the 1920s, Americans and Europeans competed in aeroplane racing, leading to innovations in design and performance. Not only were biplanes replaced by ore streamline monoplanes, the era saw a transition to all-metal frames made from aluminium alloys. Nicknamed The Tin Goose, Henry Ford had designed and produced the 4-AT Ford Trimotor in 1925, a three-engine plane using aluminium corrugated sheet-metal for the body and wings.
Since the end of World War II, aluminium became an integral part of aircraft manufacture, as it produced lightweight craft that could carry heavy loads, use less fuel, and it was impervious to rust. Aluminium was widely used, and to some extent still is, for the wing panes and parts such as exhaust pipes, the doors and floors and engine turbines. The metal has also been invaluable in spacecraft, where low weight coupled with maximum strength is even more essential.
Aluminium still holds many benefits for aviation today. However, with the persistent continuation of innovation, composites have created a shift from aluminium to a lighter alternative.
Considered the most important materials to be adapted for aviation since the use of aluminium, composites have reduced the weight of aircraft wings in comparison to its predecessor. Since the introduction of carbon-fibre composites in the 1960s, they have proven their merit in high-performance applications in aircraft.
According to NASA, the high-performance applications demonstrated that primary aircraft structures made from carbon-fibre composites could achieve weight savings of 20%-30% over similarly designed metal structures. Because they are composites, there is a secondary benefit in the ability to tailor them to specific design loads, strengths and tensions for different wing and aircraft models.
In recent years, nanomaterials have been incorporated into this process, applying them to composites during manufacturing. By integrating these tiny materials into the structure, they can be used to make the composites even more specific to job functions in the wing. An example of this is incorporating electrical conductive nanoparticles into structural components that can guard against lightning strikes, or high-strength nanoparticles that can improve the damage resistance of the outer wing laminate.
Aeroplanes are more than capable of flying without wingtips, however, the curved tip has been crucial for safety, lowering emissions, and reducing noise pollution along flight paths. Wingtips, as the name applies, are miniature wings, and as such they generate their own tip vortices that degrade their performance, reducing the effects of the wake.
Recognised more than a century ago, wingtips, also named winglets, were described by English Engineer Fredrick W Lanchester, who had made significant contributions to automotive engineering and aerodynamics. Dating back to 1897, Lanchester patented wing end-plates as a method for controlling wingtip vortices. In 1910, Scottish-born William E Somerville patented the first functional winglets in the USA.
After the huge hike in oil prices hit aviation in the 1970s, winglets emerged into mainstream aircraft design, significantly improving aerodynamic performance of an aircraft, and therefore reducing fuel and lowering costs. At the forefront of this innovation was Whitcomb, moving the design from the drawing board to a practical reality.
Shifting his focus toward wingtips of an aircraft, Whitcomb chose to address the wingtip vortex problem – the turbulent air found at the end of an aeroplane wing as a result of differences in air pressure generated on the upper and lower surfaces of the wing. Whitcomb sought a way to control the wingtip vortex with a new aeronautical structure, a vertical wing-like surfaces that extend above and sometimes below the tip of each wing. Research, conducted by Whitcomb and his team, examined the drag-reducing properties and proved that the reduction, approximately 20%, generated by a pair of winglets also enabled higher cruising speeds.
Apart from small wingtip plates seen on military aircraft during the wars, the first winglet appeared on an enthusiast’s aircraft called the Rutan VariEze in 1975 – designed by Virgin Galactic Founder Burt Rutan. The Learjet 28 shortly followed suit in winglet innovation in 1977. The two-engine, high-speed business jet, successor to the Learjet 25, was originally a test aircraft. Whitcomb then fitted winglets to large tanker aircraft and realised impressive efficiency gains of up to 5%.
‘On aeroplanes without winglets the air flowing over the wing forms a vortex at the wingtip,’ said British Airline Pilots Association spokesperson and former pilot, Stephen Draper.
‘These are like mini tornados effectively wasting energy and increasing the aeroplane’s drag through the air, and therefore increasing the amount of fuel needed. The winglets you can see on many modern aeroplanes reduce these wing tip vortices increasing their efficiency reducing fuel burn.’
Applied to new aircraft design and even retrofitted to old models, these alterations at the wingtip have realised dramatic drag and fuel efficiencies. Boeing, for example, has shown that, when applied to the 767 aircraft, it realised a 4-5% fuel burn improvement, which translates to approximately 1.89 million litres of jet fuel and 4,790 tonnes of CO2 per plane per year.
Flying into the future
From the wood-and-wire flying machines of the Wright brothers, huge innovations in wing design have changed the capabilities of aircrafts, and the continual experimenting with shape and materials is propelling innovation into the future. With the aviation industry eager to reduce greenhouse gas emissions, it is hoped that novel wing designs and morphing mechanisms can help burn less fuel.
One example of collaborative innovation on wing design is a joint project between NASA engineers and MIT researchers, who have created a new aeroplane smart wing that changes shape in flight, making it cheaper. The wing – hundreds of identical cube-like structures bolted together, covered in a thin polymer material – has the capacity to change shape in order to control the aircraft flight. The flexibility means the wing can optimise shape for each stage of the flight, which could boost jet performance.
NASA has also collaborated with Boeing to create the Transonic Truss-Braced Wing, unveiled earlier this year. Boeing stated that the new concept would reduce fuel burn by 60% compared with an aircraft from the early 2000s. The high wing alone unlocks more potential for engine technology, allowing for bigger engines with greater bypass ratios providing they are not too big to the point where its unnecessary drag.
With the goal of improving speed capabilities, increasing fuel efficiency, reducing weight and carbon emissions, the aviation industry is continuing to take advantage of new technologies and design methodologies to improve aerodynamic performance.