x Abu Dhabi, UAEThursday 27 July 2017

The secret to airplane flight? No one really knows

Scientists disagree on the exact explanation of why airplanes can fly.

Orville Wright controls the 'Wright Flyer', as his brother Wilbur looks on during the plane's first flight at Kitty Hawk, North Carolina on December 17, 1903.  Made of wood, wire and cloth by two bicycle mechanics, the plane remained aloft for 12 seconds and travelled 36.6 metres.
Orville Wright controls the 'Wright Flyer', as his brother Wilbur looks on during the plane's first flight at Kitty Hawk, North Carolina on December 17, 1903. Made of wood, wire and cloth by two bicycle mechanics, the plane remained aloft for 12 seconds and travelled 36.6 metres.

Etihad Airways, the UAE national carrier, has signed a deal with Boeing to become the world's biggest customer for the revolutionary 787-9 Dreamliner.

Featuring pioneering use of composite materials and impressive fuel efficiency, the craft certainly fits well with the UAE's reputation for investing in cutting-edge technology.

By all accounts, Etihad's chief executive, James Hogan, is getting a fleet of outstanding aircraft for his US$2.8 billion (Dh10bn).

But it is intriguing to ponder his response if, pen poised above chequebook, he had been told that there is still bitter dispute about how the 787-9 - or indeed any aircraft - gets off the ground.

Mr Hogan and those currently waiting in departure lounges may wish to stop reading at this point, but more than a century after the Wright brothers' historic first flight, it is still possible to ask three aerodynamicists to explain how aircraft fly and get four different answers.

This may come as a shock to anyone familiar with the seemingly straightforward explanation given on many websites and textbooks.

According to this, aircraft fly because the air flowing over the top of their wings moves faster than that underneath, producing a net upwards pressure over the wing, resulting in the force known as "lift".

The trouble starts with the attempts to explain precisely how wings achieve this difference in flow speeds. It is often claimed that air molecules flowing over the top of the "cambered" wing clearly have further to go, so must speed up to ensure they meet up their former companions travelling underneath. Quite why air molecules cannot bear to be separated from their friends in this way is not clear - and is known to be nonsense in any case.

A barely less ludicrous explanation is that the camber somehow squashes the air flowing over the top surface, compelling it to go faster - a bit like how water travels faster and further through a garden hose if its end is squeezed.

Again, exactly what the camber is squeezing the air "against" is not obvious. This did not stop Einstein himself coming up with this kind of explanation during his brief and inglorious stint as an aerodynamics consultant, which led to an aircraft that, according to its test-pilot, flew "like a pregnant duck".

A somewhat better account of lift focuses on Newton's laws, arguing that lift is just the result of the underside of the wing deflecting the oncoming air downwards, producing an equal and opposite force upwards. This sounds impressive until one realises that it means a plank should then be as effective as a cambered wing - which is not so.

Yet surely someone must know how aircraft stay aloft. After all, apart from the occasional - and impressively rare - mishap, they can and do. But as a fascinating new study of the history of aerodynamics shows, the science of wings is like the physics of the atom: there is an impressively reliable theory - but one whose foundations are not as solid as one might like.

In The Enigma of the Aerofoil (University of Chicago Press), science philosopher Professor David Bloor, of the University of Edinburgh, shows how early aerodynamicists came to terms with this disturbing truth after a decades-long dispute of almost religious intensity among some of the most brilliant mathematical minds of the day.

At the heart of the problem was the physics of fluid flow. About a century ago, this seemed well-established: the law showing that fluid pressure drops with increasing speed had been published by the great Swiss mathematician, Daniel Bernoulli, in 1738.

This seemed to hold the key to understanding lift, reducing it to the challenge of explaining why air travelled faster over the top of the wing than underneath. But there was a problem. The law strictly holds only for fluids that have zero viscosity, or "stickiness", which is not really true for air.

Scientists routinely encounter such challenges, and one time-honoured approach is to just carry on and hope the "technicality" can be ignored.

That is pretty much what early aerodynamicists in Germany did - with great success. They claimed that wings created a circulating flow of air over their surface, which generated the crucial speed difference needed for lift.

Yet British experts dismissed all this as little more than wishful thinking. They pointed to a fundamental theorem that proved that air could not circulate in the way claimed and began a quest for the "real" explanation.

The bad news was that this meant confronting the notorious Navier-Stokes equations, which can cope with viscosity, but only at the price of appalling mathematical complexity.

Meanwhile, over in Europe, aircraft design based on the dodgy explanation went from strength to strength. In an attempt to heal the rift, German theorists, notably Ludwig Prandtl, developed arguments to explain their success, based on kicking all the messy viscosity effects into something they called the "boundary layer".

Their wind-tunnel studies even revealed the existence of the "impossible" circulating air.

Yet the British remained unimpressed and stuck doggedly to their holy mission of trying to extract the "real" explanation from the Navier-Stokes equations.

By the mid-1930s, however, they had thrown in the towel. They might be using the right mathematics but they still struggled to explain the wind-tunnel results. So grudgingly they accepted the pragmatic, if scientifically questionable, "explanation" of the Germans.

The timing of their capitulation is significant. At about the same time, physicists were developing quantum theory, a description of the subatomic world that worked wonderfully well but whose foundations were - and remain - deeply mysterious. Meanwhile, mathematicians had discovered questions that were probably forever beyond their grasp. In short, the limits of human understanding were becoming painfully clear on many fronts.

So how do aircraft fly? Some will point to Bernoulli's Law, others to Prandtl's boundary layer theory and some to the Navier Stokes equations.

But in the end, all aircraft are carried aloft on wings made from metaphors, none of which capture the true nature of reality.

 

Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England.