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Winging It

Winging It

When you look at modern aircraft design, it’s amazing to think that human flight once balanced on the fragile skeleton of the Wright brothers’ famous and history-making invention, the Wright Flyer. It was probably beyond even the two pioneering brothers to imagine sets of wings spanning longer in length than their own ground-breaking flight.

The most fundamental component of an aircraft is its wing. In nature, wings most likely evolved first as a means of attraction and intimidation (depending on the situation) and then as a way of getting from A to B. Earth-bound humans, watching birds soar through the skies, decided that flight looked pretty neat and began making use of winged contraptions like kites at least 2,800 years ago in China. Ancient sources depict these early inventions as tools for communication, measuring distance and for testing wind conditions.

Early accounts of piloted gliders, while tough to verify, reported the feats of adventurous spirits like the monk Eilmer of Malmesbury, England who was said to have flown off the roof of his Abbey sometime between 1000 and 1010 AD, gliding about 200 metres. Archival references suggest thatmany flying contraptions like Eilmer’s involved emulations of bird, bat and dragon-like wings, which met limited success.

As aircraft evolved, theorists and mathematicians chiselled away at the problem of wing design. Concepts for wings became less creature-like in character when it was realised that the structure of animal wings had a lot to do with the fact that they needed to fold up against the body when not in use, something early pioneers weren’t concerned about. By the time the Wright brothers arrived on the scene, it was common knowledge that efficient human-made wings needed to be long and slim, rather than animal-like.

The Wright brothers tested their fledgling aircraft as kites in 1899 in the high winds of Kitty Hawk, North Carolina, slowly building up their knowledge of aerodynamic performance as well as developing methods of exerting greater control over the aircraft. However, the brothers were dissatisfied by the performance of their kites and gliders. Returning to their expertise in bicycle technology, they built small models of wing designs and attached them to a bicycle wheel. They attached this wheel to the handlebars of a bike and cruised the streets of Dayton, Ohio, to produce wind over the tester wings.

Though the results of the bike-tests did yield further insight into the workings of aerodynamics, the nature of the test made it hard to get consistent results, so the brothers decided to take nature into their own hands and manufactured an artificial wind tunnel for a more controlled testing environment. Over the course of their testing, the brothers ended up trialling over two hundred different wing configurations.

By the time they arrived at the 1903 Wright Flyer, the brothers had the most comprehensive data on wing design in the world. They established that wing curvature produced lift and noted that changes in the angle of attack, or the angle between the line of the chord of an aerofoil and the relative airflow, caused a variation in lift.  They further reasoned that to control the wing in roll they needed to vary the lift force on each wing independently. The Wrights developed an innovative technology called wing warping to achieve roll control.  Via a series of cables, the Wrights’ design allowed the pilot to twist the wing tips up or down relative to the rest of the wing. This produced an unbalanced force on the wing which caused the aircraft to roll. They then placed an elevator out the front of the wing to control their climb and decent.

Continuing from the dawn of powered flight, the quest for the best profile and plan shape for a wing quickly became a formal scientific pursuit. Given the enormous number of variations possible in aerofoil shape, various agencies commenced systematic empirical testing of a wide variety of wing shapes and then documenting the results. Three key parameters of wing design were identified; airfoil section, wing loading and aspect ratio, each interacting with the other in a careful balance of material science and physics.

The National Physical Laboratories in the United Kingdom for example, produced a series of airfoil sections designated “RAF” (not to be confused with the Royal Air Force) which were employed in aircraft up to and during World War II. Companies like De Havilland used the RAF 34 airfoil for the wings of many of their aircraft including the DH 88 Comet and the Mosquito.

Formed in 1915 in the USA, the National Advisory Committee on Aeronautics (NACA) did endless experiments to determine the properties of airfoil shapes. By this time, ailerons had almost completely replaced other forms of lateral control, such as wing warping, well after the function of the rudder and elevator flight controls had become standardised. Tailored to low speed, high speed and supersonic purposes respectively, the various wing shapes developed by NACA were published and manufacturers were able to employ these standard airfoil sections rather than start from scratch themselves.

Described in detail in NACA Report 460, published in November 1933 and entitled The Characteristics of the Seventy-Eight Related Airfoil Sections from Tests in the Variable Density Wind Tunnel, NACA airfoils continue to be used as a foundation for wing design into the modern era. A modern example of this is seen in the wings of Jabiru Aircraft. As Sue Woods of Jabiru Aircraft explains, the wings produced by Jabiru Aircraft use “…a very conventional constant chord (rectangular) wing using a reliable and long favoured 4-digit NACA aerofoil (the NACA 4412)”.

Of course, wings are crucial in determining the performance and overall flying characteristics of an aircraft. “Wing design fundamentally drives the overall efficiency of an aircraft” says Ms Woods. “In arriving at a final wing design, other aerodynamic elements of the design tend to fall into place such as tail sizing and fuselage length (which enable stability and control of the wing). By changing the most basic geometric properties of a wing planform (span, aspect ratio and wing area) changes in an aircraft stall and glide performance are significantly affected”.

One of the most significant evolutions in wing technology is in manufacturing materials. Aircraft typically feature a main spar and, in some cases, secondary spars to provide torsional strength while the actual airfoil shape is defined by the ribs of the wing. Early aircraft designers had only steel tube, fabric and wood at their disposal for wing construction. Consequently, they resorted to bi-plane structures predominantly to compensate for the inherent shortcomings of wood.

Aircraft must be as strong as they are light, and the double wing of a bi-plane created more wing area for a lower stall speed and formed a sort of giant wing spar or girder that was able to take on much more of a load. However, the wires and struts required for the configuration added a lot of parasitic drag, reducing speed and performance. When better materials and construction techniques were realised, longer monoplane wings became the dominant arrangement.

Of course, there is no one wing for every situation. As Peter Harlow from Foxbat Australia explains “[wing design] will determine what the aircraft flies like – fast, aerobatic, slow, etc. A wing which is efficient at 60 knots will be terrible at 260 knots and vice versa. A wing for an aerobatic aircraft will not be suitable for an aircraft which is not aerobatic and vice versa…a pilot should look for an aircraft which fulfils the mission s/he has in mind. For example, if they want to fly slow and safely, they should probably avoid a laminar flow wing. If they want to fly fast, they should probably avoid thick wings which are designed to give maximum lift at slower speeds”.

Newer wing models don’t necessarily mean better performance. As Mr Harlow explains “the Foxbat wing profile…is a relatively old profile designed originally in the late 1930’s to be a high-lift, relatively low drag profile. Like many old wing profiles (e.g. the Piper Cub) it has stood the test of time and performs very well on today’s very light aircraft”.

As aviation matured, duralumin became a popular construction material because it was light, weather proof, corrosion resistant and relatively easy to work. Duralumin was the trade name for a specialised alloy of copper and aluminium that was found to make a harder material than straight aluminium. It did have drawbacks in that, as it fatigued, it cracked or failed in key structures. In an effort to combat fatigue, aircraft like the DC3 were over-built and rigorously inspected for cracks. But as the understanding of material science improved, it proved possible to use less metal and therefore reduce weight.

At the cutting edge of modern wing manufacturing today is the use of composite materials. Composite materials are a combination of ingredients with different physical or chemical properties that, when combined, produce a product with improved characteristics over each material on its own. Composites are often lighter when compared to traditional materials and usually prove less expensive both from the outset and in the long term.

Composite materials are also incredibly strong. As Ms Woods explains “…probably what makes the Jabiru wing stand out is what’s inside. The full glass fibre, foam and epoxy composite construction is designed for an ultimate load of between 8 and 9g’s depending on the model!”

The individual components that make up a wing, namely the skin and ribs, as well as control surfaces, such as ailerons and flaps need to support different loads. The traditional wing has an internal structure of ribs spars and sub-spars with a skin fitted onto the outside. With the introduction of composite materials, steel and aluminium alloys continued to be used in the manufacture of ribs, while composite materials can be used in the design of the wing skin and the control surfaces, maximising efficiency while simultaneously minimising cost and improving safety.

As Ms Woods explains “[w]hen the first Jabiru aircraft was built in the late 1980’s, glass fibre composite construction was not yet a fully-fledged aircraft construction technique. The Jabiru wing utilised this up-and-coming technology in the structure as with the rest of the airframe to produce a very safe and robust product. This material also allowed accurate and consistent wings (and other parts) to be produced quickly without the need for expensive dies and tooling, allowing a decent production rate with a minimum of overheads”.

Wings also have external refinements. Clever devices such as turbulators are fitted to some aircraft such as certain models of Learjet. Their purpose is to draw down energy from the airflow around the aircraft to the boundary layer on the surface of the wing. The boundary layer in aerodynamics is the part of the air flow near the surface of a body where friction slows down the local flow. Turbulators energise the boundary layer and improve stall characteristics.

On the subject of the boundary layer, it’s interesting to note that polishing an aircraft does more than make it look good. A polished aircraft, particularly in the case of a fast one, is more fuel efficient than a non-polished one. This is because the boundary layer is essentially thinner causing the envelope of air that surrounds the flying aircraft to be appreciably lighter.

On the wingtips, many aircraft feature specialised devices. Some aircraft employ tip-tanks to store additional fuel and to restrict the formation of tip-turbulence which results from a spilling over of the low-pressure area above. Other aircraft have vertically mounted winglets that serve the same purpose, reducing drag from wingtip vortices. Winglets have become popular additions to high speed aircraft to increase fuel efficiency, and although lower speed aircraft don’t benefit as much from their installation, slow speed short take-off and landing aircraft may use wingtips to shape airflow for greater control at low airspeeds. The upcoming Boeing 777X will feature 3.5 metre folding wingtips supplied by Liebherr Aerospace from Lindenberg.

But it isn’t only the larger aircraft that are benefiting from modern tech. CNC (computer numerically controlled) machinery is the automation of machine tools with computers executing programmed sequences of machine commands. Rather than machinery manually operated by hand wheels or levers, or mechanically automated by cams alone, CNC is highly automated. A required part’s mechanical dimensions are defined by using a computer-aided design (better known as CAD) program, which is then translated into manufacturing directives by computer-aided manufacturing (sometimes referred to as CAM) software.

Originally developed for the construction of military and large commercial aircraft, the technology has trickled down into the general aviation, recreational aviation and light sports aircraft markets, keeping costs down and increasing safety. As Mike Blythe of The Airplane Factory, producer of the Sling aircraft, explains “the real impact of recent technology on the wing design is in the structure and manufacturing methods. CNC machines have allowed The Airplane Factory to produce parts and assemblies to very close tolerances making the assembly easy and accurate”.

One of the most critical requirements of aerodynamic lift is a smooth wing surface. Even the smallest irregularity increases drag and reduces lift, which can be enough to destabilize or reduce the efficiency of an aircraft in flight. For aircraft that fly in subzero temperatures, keeping ice in check is critical. Typically, ice is removed from general aviation craft with either “weeping wing” liquid de-icing systems or inflatable rubber bladders, called pneumatic boots, installed along the leading edge of the wings. The main drawbacks of these two automated solutions are the limited capacity for on-board de-icing liquid and the additional weight and power usage of the pneumatic boots.

Many modern civil fixed-wing transport aircraft use anti-ice systems on the leading edge of wings, engine inlets and air data probes using warm air. This is bled from engines and is ducted into a cavity beneath the surface to be anti-iced. The warm air heats the surface up to a few degrees above freezing point, preventing ice from forming. The system may even operate autonomously, switching on and off as the aircraft enters and leaves icing conditions.

One of the most recent developments in de-icing technology is incorporated on the 787 Dreamliner and combines composite material technology with conductive elements under the leading-edge surface to heat the wing. During manufacture, liquid metal is sprayed on a fibre fabric to create an electrically conductive surface. The sprayed metal acts as an electrothermal element that transfers heat to the skin of the wing. On the 787, the heater mats deliver de-icing at a balmy temperature range of 7.2°C to 21.1°C.  

Another scientific advancement pilots operating in colder conditions can get excited about is a development that could see chunks of ice slide right off the skin of an aircraft wing without the pilot having to do a thing. In 2016, scientists reported that they had developed a liquid-like substance that can make wings and other surfaces so slippery that ice cannot adhere to them at all. Researchers at the American Chemical Society released the results of their research into liquid-secreting materials called self-lubricating organogels, or SLUGs. Research Director at the National Institute of Advanced Industrial Science & Technology in Japan, Atsushi Hozumi, Ph.D. explained that the SLUGs technology “has a host of formulations and applications, including in a gel form that can be encapsulated in a film coating on the surface of a wing or other device”.

Like aircraft wings themselves, the inspiration for the SLUGS project came from findings in nature. “We came upon this idea when we observed real slugs in the environment,” Chihiro Urata, Ph.D., said. “Slugs live underground in soils when it is daytime and crawl out at night. But we never see slugs covered in dirt. They secrete a liquid mucus on their skin, which repels dirt, and the dirt slides off. From this, we started focusing on the phenomenon called syneresis, the expulsion of liquid from a gel.”

The gel and the liquid repellent substance are held in a matrix of silicone resin and, as Urata explains, “…the mix is cured and applied to a surface as a nearly transparent and solid film coating”. Both Urata and Hozumi, explained that the material’s thermo-responsive secretion properties came as a surprise and further testing revealed that the secretion was also reversible. The chemical process is triggered when temperatures fall below freezing, meaning ice can still form but it isn’t able adhere to a surface and slides off.  Once the temperature conditions rise above freezing, the liquids neatly return to the film. The development is definitely a promising prospect for cold climate aviation endeavours in the future.

Wings have come a long way in a short period of time. Not so long ago, humans were looking at birds and wondering how many feathers were necessary for us to flap our arms and fly off into the sunset. In just over 100 years, we have progressed from ungainly wooden configurations to streamlined wings spanning longer than humanity’s first powered flight.

An improved understanding of chemistry may soon allow aircraft to better weather cold climates using the secret chemistry of slugs. And manufacturing techniques continue to move towards even safer, more economical aircraft. Even new construction materials are re-defining the capabilities of modern aircraft. When those first ambitious humans looked up in envy at winged creatures traversing the skies, not even they could have guessed that the contraptions they envisioned would eventually result in the precision wings that carry us humans into the skies today.

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