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…

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The Development of Air Power

By the end of the Second World War, the air was the key battleground of modern warfare; with control of the air, one could move small detachments of troops to deep behind enemy lines, gather valuable reconnaissance and, of course, bomb one’s enemies into submission/total annihilation. But the air was also the newest theatre of war, meaning that there was enormous potential for improvement in this field. With the destructive capabilities of air power, it quickly became obvious that whoever was able to best enhance their flight strength would have the upper hand in the wars of the latter half of the twentieth century, and as the Cold War began hotting up (no pun intended) engineers across the world began turning their hands to problems of air warfare.

Take, for example, the question of speed; fighter pilots had long known that the faster plane in a dogfight had a significant advantage over his opponent, since he was able to manoeuvre quickly, chase his opponents if they ran for home and escape combat more easily. It also helped him cover more ground when chasing after slower, more sluggish bombers. However, the technology of the time favoured internal combustion engines powering propeller-driven aircraft, which limited both the range and speed of aircraft at the time. Weirdly, however, the solution to this particular problem had been invented 15 years earlier, after a young RAF pilot called Frank Whittle patented his design for a jet engine. However, when he submitted this idea to the RAF they referred him to engineer A. A. Griffith, whose study of turbines and compressors had lead to Whittle’s design. The reason Griffith hadn’t invented the jet engine himself was thanks to his fixed belief that jet engines would be too inefficient to act as practical engines on their own, and thought they would be better suited to powering propellers. He turned down Whittle’s engine design, which used the forward thrust of the engine itself, rather than a propeller, for power, as impractical, and so the Air Ministry didn’t fund research into the concept. Some now think that, had the jet engine been taken seriously by the British, the Second World War might have been over by 1940, but as it was Whittle spent the next ten years trying to finance his research and development privately, whilst fitting it around his RAF commitments. It wasn’t until 1945, by which time the desperation of war had lead to governments latching to every idea there was, that the first jet-powered aircraft got off the ground; and it was made by a team of Germans, Whittle’s patent having been allowed to expire a decade earlier.

Still, the German jet fighter was not exactly a practical beast (its engine needed to be disassembled after every use), and by then the war was almost lost anyway. Once the Allies got really into their jet aircraft development after the war, they looked set to start reaching the kind of fantastic speeds that would surely herald the new age of air power. But there was a problem; the sound barrier. During the war, a number of planes had tried to break the magical speed limit of 768 mph, aka the speed of sound (or Mach 1, as it is known today), but none had succeeded; partly this was due to the sheer engine power required (propellers get very inefficient when one approaching the speed of sound, and propeller tips can actually exceed the speed of sound as they spin), but the main reason for failure lay in the plane breaking up. In particular, there was a recurring problems of the wings tearing themselves off as they approached the required speed. It was subsequently realised that as one approached the sound barrier, you began to catch up with the wave of sound travelling in front of you; when you got too close to this, the air being pushed in front of the aircraft began to interact with this sound wave, causing shockwaves and extreme turbulence. This shockwave is what generates the sound of a sonic boom, and also the sound of a cracking whip. Some propeller driver WW2 fighters were able to achieve ‘transonic’ (very-close-to-Mach-1) speeds in dives, but these shockwaves generally rendered the plane uncontrollable and they invariably crashed; this effect was known as ‘transonic buffeting’. A few pilots during the war claimed to have successfully broken the sound barrier in dives and lived to tell the tale, but these claims are highly disputed. During the late 40s and early 50s, a careful analysis of transonic buffeting and similar effects yielded valuable information about the aerodynamics of attempting to break the sound barrier, and yielded several pieces of valuable data. One of the most significant, and most oft-quoted, developments concerned the shape of the wings; whilst  it was discovered that the frontal shape and thickness of the wings could be seriously prohibitive to supersonic flight, it was also realised that when in supersonic flight the shockwave generated was cone shaped. Not only that, but behind the shockwave air flowed at subsonic speeds and a wing behaved as normal; the solution, therefore, was to ‘sweep back’ the shape of the wings to form a triangle shape, so that they always lay ‘inside’ the cone-shaped shockwave. If they didn’t, the wing travelling through supersonic air would be constantly being battered by shockwaves, which would massively increase drag and potentially take the wings off the plane. In reality, it’s quite impractical to have the entire wing lying in the subsonic region (not least because a very swept-back wing tends to behave badly and not generate much lift when in subsonic flight), but the sweep of a wing is still a crucial factor in designing an aircraft depending on what speeds you want it to travel at. In the Lockheed SR-71A Blackbird, the fastest manned aircraft ever made (it could hit Mach 3.3), the problem was partially solved by having wings located right at the back of the aircraft to avoid the shockwave cone. Most modern jet fighters can hit Mach 2.

At first, aircraft designed to break the sound barrier were rocket powered; the USA’s resident speed merchant Chuck Yeager was the first man to officially and veritably top 768mph in the record-breaking rocket plane Bell X-1, although Yeager’s co-tester is thought to have beaten him to the achievement by 30 minutes piloting an XP-86 Sabre. But, before long, supersonic technology was beginning to make itself felt in the more conventional spheres of warfare; second generation jet fighters were, with the help of high-powered jet engines, the first to engage in supersonic combat during the 50s, and as both aircraft and weapons technology advanced the traditional roles of fighter and bomber started to come into question. And the result of that little upheaval will be explored next time…