#47 The Aerofoil

There is perhaps no simpler nor more elegant idea in our list than the aerofoil. And what an idea! Just a little gentle curvature, a curvature that makes a wing – and on those wings we can soar into the sky far above the ground, we can glide over high mountains, we can cross the world’s widest oceans in hours. Even for those used to flying, the moment when your accelerating plane finally gains the speed to lift off with a kick beneath you is still exhilarating. How can such a vast weight, something so heavy that it would take a crane to lift, suddenly become so feather-light, so fantastically defiant of gravity that it can bear not only its own weight but yours and your fellow passengers’ rapidly up into the air? It seems like magic.

The magic, of course, is in the physical interaction between the wings and the air. If the shape of the wing – the aerofoil – is right and the plane is moving fast enough, the wing is pushed into the air as it slices through it. It seems like magic because our all-too-literal brains tell us that because air is invisible it must be insubstantial too. But air is not nothing; air is a substance, chock-full of gas. Think of a wing slicing through water rather than air and you can begin to imagine how air might provide the upward push that aeronautical scientists call ‘lift’.

The key to the aerofoil’s lift is the flow of air around it. Of course, still air is neither moving nor flowing; air flows around an aerofoil because the aerofoil is moving, just as the bow of a boat creates a flow in still water. What matters is the way the curved shape of the aerofoil diverts the flow around it. To really see why, it’s worth playing about with knives and spoons under a running tap.

Hold a knife flat under the stream and the water flows straight and undisturbed past the blade. Twist the blade slightly at an angle to the stream and you can see how it begins to block and split the flow, breaking it into turbulent eddies – and you may see the turbulence increase as you increase the angle of the knife. Hold a spoon under the stream instead, however, and something different happens. Unlike the flat knife, the spoon diverts the water, but does not disrupt it. You have to twist the spoon at a much steeper angle before it disrupts the flow. Like the curvature of the spoon, the curvature of the aerofoil ensures the flow of the air around it is diverted but not broken up.

It is the way that the flow is made to curve like the aerofoil that is crucial. Far above or below the aerofoil, the airflow is undisturbed, but the closer it is to the aerofoil the more its flow is bent to follow the aerofoil’s shape. As the flow changes direction it begins to push in a different direction too, and the more it bends the greater the change. Right on top and underneath the aerofoil the pressure of the airflow turns effectively at right angles, pushing the aerofoil upwards and creating lift.

Since it is the way that the airflow is distorted that creates lift, it is clear that the pattern of the airflow distortion is important. This depends on the angle that the aerofoil moves through the air – its ‘angle of attack’. The steeper the angle of attack, the greater the lift, up to the ‘stall’ point where the angle is so steep that the airflow is broken up altogether and all lift is lost.

The shape of the aerofoil is also crucial. A gentle, thin, flat curve provides the best lift, and this is the shape of bird wings and the shape that the flexible wings of hang-gliders and microlights bow into. But it is hard to make a large wing strong enough in this shape. So the wings of most large aircraft are a narrow teardrop shape in profile. This doesn’t give us much lift, which is why the wings have to be huge, but is easier to make strong, and the hollow inside the wing provides room for fuel storage. Elevator flaps on the rear of the wings swing up or down to alter the aerofoil curvature and its effective angle of attack and so allow the pilot to vary the lift to climb or descend.

Of course, the wings of birds were the original inspiration for the aerofoil. Countless thinkers in the distant past must have marvelled at birds gliding through the sky and guessed that they were held aloft by their outstretched wings. And maybe some even guessed it was the shape of the wings that mattered, such as the fifth-century Greek philosopher Archytas who is said to have built a mechanical bird that flew. Brave pioneers like the ninth-century Cordoban, Abbas ibn-Firnas, were bold (or foolish) enough to strap artificial wings to their arms and leap from high places. Ibn-Firnas was successful (or lucky) enough to glide through the air for ten minutes before crash-landing and almost breaking his neck.

Yet the first person who really began to explore the shape of wings methodically was the British engineer Sir George Cayley (1773–1857), and it is to Cayley that we owe the idea of the curved aerofoil. Yorkshire-born Cayley was an extraordinary and inventive man, and he is credited with developing self-righting lifeboats, wire-spoked wheels, seatbelts and even an internal combustion engine. But it is mostly as the ‘father of aviation’ that he is remembered, and it is he who pioneered much of the theory of flight. He carried out many experiments with wings on whirling arms to discover the forces acting on them, and what shapes and angles produced the greatest lift. In his analyses, he developed the names for the four key forces involved in flight – weight, lift, drag and thrust – still used by scientists today. Flight itself involves a balance between these four forces.

Cayley wasn’t just a theorist, though. In the early 1800s, he began to build model gliders to try out his ideas. Then in 1849, he built a miniature biplane in which a ten-year-old boy is said to have flown a short distance, using ‘flappers’ to propel himself along. Most famously, however, in 1853 the by then 80-year-old Cayley built a full-size glider in which his terrified coachman or butler is said to have been launched out across Brompton Dale near Scarborough on Cayley’s estate. The butler survived and thus made the world’s first aeroplane flight. Cayley had clearly mastered lift, but for a successful aeroplane you need both power and control, which is why it took another half-century before the Wright brothers made their historic flight at Kittyhawk on 17 December 1903.

The development of air travel since that pioneering day has been astonishing. According to the travel organisation IATA, 2.3 billion people flew on 35 million flights in 2009 alone. It is a remarkably safe way to travel. Of those 35 million flights, only nineteen came to grief in accidents, and fewer than 700 of those 2.3 billion passengers were killed in air accidents – in other words 1 in 30 million.

Flight has transformed the way we experience the world. It seems a smaller, more connected place and many of us now frequently visit on brief holidays and business trips places that before air travel we may have travelled to only once in a lifetime. Millions of Britons, for instance, hop on a plane for weekend breaks in European cities, or travel right the way around the world for a short holiday in Thailand. Many ordinary people know about many foreign places and cultures not just because they have looked them up on the internet or watched a TV documentary, but because they have actually travelled there by plane.

Air travel is not essential. Indeed, there are plenty of critics who argue that it is a wasteful luxury, and the global warming debate has focused attention on just how much we should be flying. The high energy cost of getting a plane into the air means that air travel is a major contributor to the greenhouse gases that are triggering global warming. Aeroplanes are very noisy, too, as anyone who lives under the flight-path near an airport will no doubt testify.

Yet whatever level of flying ultimately proves sustainable, there is no doubt that the simple shape of the aerofoil has introduced something remarkable into our lives. It has given us all the chance of a magical experience. According to Plato, writing long before the aerofoil became a reality: ‘The natural function of the wing is to soar upwards and carry that which is heavy up to the place where dwells the race of gods. More than any other thing that pertains to the body it partakes of the nature of the divine.’

For the pioneering aviator Charles Lindbergh, that divine quality was perhaps even too much: ‘Sometimes, flying feels too godlike to be attained by man. Sometimes, the world from above seems too beautiful, too wonderful, too distant for human eyes to see.’ (Spirit of St Louis, 1953)