There is an art, or rather, a knack, to flying…

The aerofoil is one of the greatest inventions mankind has come up with in the last 150 years; in the late 19th century, aristocratic Yorkshireman (as well as inventor, philanthropist, engineer and generally quite cool dude) George Cayley identified the way bird wings generated lift merely by moving through the air (rather than just by flapping), and set about trying to replicate this lift force. To this end, he built a ‘whirling arm’ to test wings and measure the upwards lift force they generated, and found that a cambered wing shape (as in modern aerofoils) similar to that of birds was more efficient at generating lift than one with flat surfaces. This was enough for him to engineer the first manned, sustained flight, sending his coachman across Brompton Dale in 1863 in a homemade glider (the coachman reportedly handed in his notice upon landing with the immortal line “I was hired to drive, not fly”), but he still didn’t really have a proper understanding of how his wing worked.

Nowadays, lift is understood better by both science and the general population; but many people who think they know how a wing works don’t quite understand the full principle. There are two incomplete/incorrect theories that people commonly believe in; the ‘skipping stone’ theory and the ‘equal transit time’ theory.

The ‘equal transit time’ theory is popular because it sounds very sciency and realistic; because a wing is a cambered shape, the tip-tail distance following the wing shape is longer over the top of the wing than it is when following the bottom surface. Therefore, air travelling over the top of the wing has to travel further than the air going underneath. Now, since the aircraft is travelling at a constant speed, all the air must surely be travelling past the aircraft at the same rate; so, regardless of what path the air takes, it must take the same time to travel the same lateral distance. Since speed=distance/time, and air going over the top of the wing has to cover a greater distance, it will be travelling faster than the air going underneath the wing. Bernoulli’s principle tells us that if air travels faster, the air pressure is lower; this means the air on top of the wing is at a lower pressure than the air underneath it, and this difference in pressure generates an upwards force. This force is lift.

The key flaw in this theory is the completely wrong assumption that the air over the top and bottom of the wing must take the same time to travel across it. If we analyse the airspeed at various points over a wing we find that air going over the top does, in fact, travel faster than air going underneath it (the reason for this comes from Euler’s fluid dynamics equations, which can be used to derive the Navier-Stokes equations for aerofoil behaviour. Please don’t ask me to explain them). However, this doesn’t mean that the two airflows necessarily coincide at the same point when we reach the trailing edge of the wing, so the theory doesn’t correctly calculate the amount of lift generated by the wing. This is compounded by the theory not explaining any of the lift generated from the bottom face of the wing, or why the angle wing  is set at (the angle of attack) affects the lift it generates, or how one is able to generate some lift from just a flat sheet set at an angle (or any other symmetrical wing profile), or how aircraft fly upside-down.

Then we have the (somewhat simpler) ‘skipping stone’ theory, which attempts to explain the lift generated from the bottom surface of the wing. Its basic postulate concerns the angle of attack; with an angled wing, the bottom face of the wing strikes some of the incoming air, causing air molecules to bounce off it. This is like the bottom of the wing being continually struck by lots of tiny ball bearings, sort of the same thing that happens when a skimming stone bounces off the surface of the water, and it generates a net force; lift. Not only that, but this theory claims to explain the lower pressure found on top of the wing; since air is blocked by the tilted wing, not so much gets to the area immediately above/behind it. This means there are less air molecules in a given space, giving rise to a lower pressure; another way of explaining the lift generated.

There isn’t much fundamentally wrong with this theory, but once again the mathematics don’t check out; it also does not accurately predict the amount of lift generated by a wing. It also fails to explain why a cambered wing set at a zero angle of attack is still able to generate lift; but actually it provides a surprisingly good model when we consider supersonic flight.

Lift can be explained as a combination of these two effects, but to do so is complex and unnecessary  we can find a far better explanation just by considering the shape the airflow makes when travelling over the wing. Air when passing over an aerofoil tends to follow the shape of its surface (Euler again), meaning it deviates from its initially straight path to follow a curved trajectory. This curve-shaped motion means the direction of the airflow must be changing; and since velocity is a vector quantity, any change in the direction of the air’s movement represents a change in its overall velocity, regardless of any change in airspeed (which contributes separately). Any change in velocity corresponds to the air being accelerated, and since Force = mass x acceleration this acceleration generates a net force; this force is what corresponds to lift. This ‘turning’ theory not only describes lift generation on both the top and bottom wing surfaces, since air is turned upon meeting both, but also why changing the angle off attack affects lift; a steeper angle means the air has to turn more when following the wing’s shape, meaning more lift is generated. Go too steep however, and the airflow breaks away from the wing and undergoes a process called flow separation… but I’m getting ahead of myself.

This explanation works fine so long as our aircraft is travelling at less than the speed of sound. However, as we approach Mach 1, strange things start to happen, as we shall find out next time…

War in Three Dimensions

Warfare has changed a lot in the last century. Horses have become redundant, guns become reliable, machine guns become light enough to carry and bombs have become powerful enough to totally annihilate a small country if the guy with the button so chooses. But perhaps more significant than just the way hardware has changed is the way that warfare has changed itself; tactics and military structure have changed beyond all recognition compared to the pre-war era, and we must now fight wars whilst surrounded by a political landscape, at least in the west, that does not approve of open conflict. However, next year marks the 100th anniversary of a military innovation that not only represented massive hardware upgrade at the time, but that has changed almost beyond recognition in the century since then and has fundamentally changed the way we fight wars; the use of aeroplanes in warfare.

The skies have always been a platform to be exploited by the cunning military strategist; balloons were frequently used for messaging long before they were able to carry humans and be used for reconnaissance during the early 20th century, and for many years the only way of reliably sending a complicated message over any significant distance was via homing pigeon. It was, therefore, only natural that the Wright brothers had barely touched down after their first flight in ‘Flyer I’ when the first suggestions of a military application to such a technology were being made. However, early attempts at powered flight could not sustain it for very long, and even subsequent improvements failed to produce anything capable of carrying a machine gun. By the First World War, aircraft had become advanced enough to make controlled, sustained, two-person flight at an appreciable height a reality, and both the Army and Navy were quick to incorporate air divisions into their structures (these divisions in the British Armed Forces were the Royal Flying Corps and the Royal Naval Air Service respectively). However, these air forces were initially only used for reconnaissance purposes and ‘spotting’ for artillery to help them get their eye in; the atmosphere was quite peaceful so far above the battlefield, and pilots and observers of opposing aircraft would frequently wave to one another during the early years of the war. As time passed and the conflict grew ever-bloodier, these exchanges became less friendly; before long observers would carry supplies of bricks into the air with them and attempt to throw them at enemy aircraft, and the Germans even went so far as to develop steel darts that could reportedly split a man in two; whilst almost impossible to aim in a dogfight, these darts were incredibly dangerous for those on the ground. By 1916 aircraft had grown advanced enough to carry bombs, enabling a (slightly) more precise method of destroying enemy targets than artillery, and before long both sides could equip these bombers with turret-mounted machine guns that the observers could fire on other aircraft with; given that the aircraft of the day were basically wire and wood cages covered in fabric, these guns could cause vast amounts of damage and the men within the planes had practically zero protection (and no parachutes either, since the British top brass believed this might encourage cowardice). To further protect their bombers, both sides began to develop fighter aircraft as well; smaller, usually single-man, planes with fixed machine guns operated by the pilot (and which used a clever bit of circuitry to fire through the propeller; earlier attempts at doing this without blowing the propeller to pieces had simply consisted of putting armour plating on the back of the propeller, which not infrequently caused bullets to bounce back and hit the pilot). It wasn’t long before these fighters were given more varied orders, ranging from trench strafing to offensive patrols (where they would actively go and look for other aircraft to attack). Perhaps the most dangerous of these objectives was balloon strafing; observation balloons were valuable pieces of reconnaissance equipment, and bringing them down generally required a pilot to navigate the large escort of fighters that accompanied them. Towards the end of the war, the forces began to realise just how central to their tactics air warfare had become, and in 1918 the RFC and RNAS were combined to form the Royal Air Force, the first independent air force in the world. The RAF celebrated its inception three weeks later when German air ace Manfred von Richthofen (aka The Red Baron), who had 80 confirmed victories despite frequently flying against superior numbers or hardware, was shot down (although von Richthofen was flying close to the ground at the time in pursuit of an aircraft, and an analysis of the shot that killed him suggests that he was killed by a ground-based AA gunner rather than the Canadian fighter pilot credited with downing him. Exactly who fired the fatal shot remains a mystery.)

By the time the Second World War rolled around things had changed somewhat; in place of wire-and-fabric biplanes, sleeker metal monoplanes were in use, with more powerful and efficient engines making air combat faster affair. Air raids themselves could be conducted over far greater distances since more fuel could be carried, and this proved well suited to the style of warfare that the war generated; rather than the largely unmoving battle lines of the First World War, the early years of WW2 consisted of countrywide occupation in Europe, whilst the battlegrounds of North Africa and Soviet Russia were dominated by tank warfare and moved far too fluidly for frontline air bases to be safe. Indeed, air power featured prominently in neither of these land campaigns; but on the continent, air warfare reigned supreme. As the German forces dominated mainland Europe, they launched wave after wave of long distance bombing campaigns at Britain in an effort to gain air superiority and cripple the Allies’ ability to fight back when they attempted to cross the channel and invade. However, the British had, unbeknownst to the Germans, perfected their radar technology, and were thus able to use their relatively meagre force of fighters to greatest effect to combat the German bombing assault. This, combined with some very good planes and flying on behalf of the British and an inability to choose the right targets to bomb on behalf of the Germans, allowed the Battle of Britain to swing in favour of the Allies and turned the tide of the war in Europe. In the later years of the war, the Allies turned the tables on a German military crippled by the Russian campaign after the loss at Stalingrad and began their own orchestrated bombing campaign. With the increase in anti-aircraft technology since the First World War, bombers were forced to fly higher than ever before, making it far harder to hit their targets; thus, both sides developed the tactic of ‘carpet bombing’, whereby they would simply load up as big a plane as they could with as many bombs as it could carry and drop them all over an area in the hope of at least one of the bombs hitting the intended target. This imprecise tactic was only moderately successful when it came to destruction of key military targets, and was responsible for the vast scale of the damage to cities both sides caused in their bombing campaigns. In the war in the Pacific, where space on aircraft carriers was at a premium and Lancaster Bombers would have been impractical, they kept with the tactic of using dive bombers, but such attacks were very risky and there was still no guarantee of a successful hit. By the end of the war, air power was rising to prominence as possibly the most crucial theatre of combat, but we were reaching the limits of what our hardware was capable of; our propellor-driven, straight wing fighter aircraft seemed incapable of breaking the sound barrier, and our bombing attacks couldn’t safely hit any target less than a mile wide. Something was clearly going to have to change; and next time, I’ll investigate what did.

The Conquest of Air

Everybody in the USA, and in fact just about everyone across the world, has heard of Orville and Wilbur Wright. Two of the pioneers of aviation, when their experimental biplane Flyer achieved the first ever manned, powered, heavier-than-air flight on the morning of December 17, 1903, they had finally achieved one of man’s long-held dreams; control and mastery of air travel.

However, what is often puzzling when considering the Wright brothers’ story is the number of misconceptions surrounding them. Many, for instance, are under the impression that they were the first people to fly at all, inventing all the various technicalities of lift, aerofoil structures and control that are now commonplace in today’s aircraft. In fact, the story of flight, perhaps the oldest and maddest of human ambitions, an idea inspired by every time someone has looked up in wonder at the graceful flight of a bird, is a good deal older than either of them.

Our story begins, as does nearly all technological innovation, in imperial China, around 300 BC (the Greek scholar Archytas had admittedly made a model wooden pigeon ‘fly’ some 100 years previously, but nobody is sure exactly how he managed it). The Chinese’s first contribution was the invention of the kite, an innovation that would be insignificant if it wasn’t for whichever nutter decided to build one big enough to fly in. However, being strapped inside a giant kite and sent hurtling skywards not only took some balls, but was heavily dependent on wind conditions, heinously dangerous and dubiously useful, so in the end the Chinese gave up on manned flight and turned instead to unmanned ballooning, which they used for both military signalling and ceremonial purposes. It isn’t actually known if they ever successfully put a man into the air using a kite, but they almost certainly gave it a go. The Chinese did have one further attempt, this time at inventing the rocket engine, some years later, in which a young and presumably mental man theorised that if you strapped enough fireworks to a chair then they would send the chair and its occupants hurtling into the night sky. His prototype (predictably) exploded, and it wasn’t for two millennia, after the passage of classical civilisation, the Dark Ages and the Renaissance, that anyone tried flight again.

That is not to say that the idea didn’t stick around. The science was, admittedly beyond most people, but as early as 1500 Leonardo da Vinci, after close examination of bird wings, had successfully deduced the principle of lift and made several sketches showing designs for a manned glider. The design was never tested, and not fully rediscovered for many hundreds of years after his death (Da Vinci was not only a controversial figure and far ahead of his time, but wrote his notebooks in a code that it took centuries to decipher), but modern-day experiments have shown that his design would probably have worked. Da Vinci also put forward the popular idea of ornithopters, aircraft powered by flapping motion as in bird wings, and many subsequent attempts at flight attempted to emulate this method of motion. Needless to say, these all failed (not least because very few of the inventors concerned actually understood aerodynamics).

In fact, it wasn’t until the late 18th century that anyone started to really make any headway in the pursuit of flight. In 1783, a Parisian physics professor, Jacques Charles, built on the work of several Englishmen concerning the newly discovered hydrogen gas and the properties and behaviour of gases themselves. Theorising that, since hydrogen was less dense than air, it should follow Archimedes’ principle of buoyancy and rise, thus enabling it to lift a balloon, he launched the world’s first hydrogen balloon from the Champs du Mars on August 27th. The balloon was only small, and there were significant difficulties encountered in building it, but in the design process Charles, aided by his engineers the Roberts brothers, invented a method of treating silk to make it airtight, spelling the way for future pioneers of aviation. Whilst Charles made some significant headway in the launch of ever-larger hydrogen balloons, he was beaten to the next significant milestones by the Montgolfier brothers, Joseph-Michel and Jacques-Etienne. In that same year, their far simpler hot-air balloon designs not only put the first living things (a sheep, rooster and duck) into the atmosphere, but, just a month later, a human too- Jacques-Etienne was the first European, and probably the first human, ever to fly.

After that, balloon technology took off rapidly (no pun intended). The French rapidly became masters of the air, being the first to cross the English Channel and creators of the first steerable and powered balloon flights. Finally settling on Charles’ hydrogen balloons as a preferable method of flight, blimps and airships began, over the next century or so, to become an accepted method of travel, and would remain so right up until the Hindenburg disaster of 1937, which rather put people off the idea. For some scientists and engineers, humankind had made it- we could now fly, could control where we were going at least partially independent of the elements, and any attempt to do so with a heavier-than-air machine was both a waste of time and money, the preserve of dreamers. Nonetheless, to change the world, you sometimes have to dream big, and that was where Sir George Cayley came in.

Cayley was an aristocratic Yorkshireman, a skilled engineer and inventor, and a magnanimous, generous man- he offered all of his inventions for the public good and expected no payment for them. He dabbled in a number of fields, including seatbelts, lifeboats, caterpillar tracks, prosthetics, ballistics and railway signalling. In his development of flight, he even reinvented the wheel- he developed the idea of holding a wheel in place using thin metal spokes under tension rather than solid ones under compression, in an effort to make the wheels lighter, and is thus responsible for making all modern bicycles practical to use. However, he is most famous for being the first man ever, in 1853, to put somebody into the air using a heavier-than-air glider (although Cayley may have put a ten-year old in a biplane four years earlier).

The man in question was Cayley’s chauffeur (or butler- historical sources differ widely), who was (perhaps understandably) so hesitant to go in his boss’ mental contraption that he handed in his notice upon landing after his flight across Brompton Dale, stating  as his reason that ‘I was hired to drive, not fly’. Nonetheless, Cayley had shown that the impossible could be done- man could fly using just wings and wheels. He had also designed the aerofoil from scratch, identified the forces of thrust, lift, weight and drag that control an aircraft’s movements, and paved the way for the true pioneer of ‘heavy’ flight- Otto Lilienthal.

Lilienthal (aka ‘The Glider King’) was another engineer, making 25 patents in his life, including a revolutionary new engine design. But his fame comes from a world without engines- the world of the sky, with which he was obsessed. He was just a boy when he first strapped wings to his arms in an effort to fly (which obviously failed completely), and later published works detailing the physics of bird flight. It wasn’t until 1891, aged 43, once his career and financial position was stable and he had finished fighting in the Franco-Prussian War, that he began to fly in earnest, building around 12 gliders over a 5-year period (of which 6 still survive). It might have taken him a while, but once he started there was no stopping him, as he made over 2000 flights in just 5 years (averaging more than one every day). During this time he was only able to rack up 5 hours of flight time (meaning his average flight time was just 9 seconds), but his contribution to his field was enormous. He was the first to be able to control and manoeuvre his machines by varying his position and weight distribution, a factor whose importance he realised was absolutely paramount, and also recognised that a proper understanding of how to achieve powered flight (a pursuit that had been proceeding largely unsuccessfully for the past 50 years) could not be achieved without a basis in unpowered glider flight, in recognising that one must work in harmony with aerodynamic forces. Tragically, one of Lilienthal’s gliders crashed in 1896, and he died after two days in hospital. But his work lived on, and the story of his exploits and his death reached across the world, including to a pair of brothers living in Dayton, Ohio, USA, by the name of Wright. Together, the Wright brothers made huge innovations- they redesigned the aerofoil more efficiently, revolutionised aircraft control using wing warping technology (another idea possibly invented by da Vinci), conducted hours of testing in their own wind tunnel, built dozens of test gliders and brought together the work of Cayley, Lilienthal, da Vinci and a host of other, mostly sadly dead, pioneers of the air.  The Wright brothers are undoubtedly the conquerors of the air, being the first to show that man need not be constrained by either gravity or wind, but can use the air as a medium of travel unlike any other. But the credit is not theirs- it is a credit shared between all those who have lived and died in pursuit of the dream of fling like birds. To quote Lilienthal’s dying words, as he lay crippled by mortal injuries from his crash, ‘Sacrifices must be made’.