In a span of just four years, 1914–1918, all of the basic principles of military aviation combat, all still relevant today to some degree, were set. The context was the bloody trial-and-error aerial combat of the First World War, and the fliers were often only 18, 19, or 20 years old.
This period began only 11 years after the Wright Brothers' flight.
The airplanes of the First World War were rickety affairs made mostly of linen stretched over wooden frames. They were difficult to fly. Half of all the pilot deaths of the war occurred during take-off. Many early models did not have brakes; they simply slowed to a stop upon landing on the grass runways.
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At first during the war, airplanes were used only for reconnaissance. They had no weaponry. Opposing pilots sometimes waved in greeting upon encountering each other in the the sky even as their respective nations' armies clashed murderously below them the ground.
The first recorded instance of an aircraft bringing down an enemy aircraft was when an Austro-Hungarian Albatros B.II reconnaissance airplane was deliberately rammed by a Russian Morane-Saulnier G on September 8, 1914. Both planes crashed.
In time, pilots started to shoot at each other from their open-seat cockpits with shotguns or pistols. Eventually, not surprisingly, a machine gun was tried.
Ground-based anti-aircraft weapons and techniques improved, too.
Pilots were not allowed parachutes; it was thought they'd encourage pilots to bailout instead of finishing their missions. Airplanes damaged or set alight by enemy fire or mechanical trouble in the air became diving death traps.
British pilots' average life expectancy was only 18 hours of flying time at one point during the war when the Germans possessed a particularly superior model of plane, the Albatross, and Germans began referring to the pilots of the Britain's Royal Flying Corps (RFC) as kaltes Fleisch, "cold meat."
During April 1917, RFC aviators' life expectancy was at its worst during the war: only 11 hours flying time.
Starting in 1917, pilots started to focus more on group formations of airplanes and formation tactics. Aerial warfare became less about pilots operating as lone airborne hunters, less about their individual pilot heroics (and a certain degree of recklessness) and more about dog fights between groups of opposing pilots. These fights were often of breath-taking complexity.
But one reality remained throughout: bringing down an opposing aviator generally meant maneuvering to get very close to his airplane. The technique of RFC pilot Albert Ball, but rendered obsolete in the formation flying tactics of later 1917 and 1918, was to spot his target, drop out of the sun, position himself only about 15 feet behind and beneath it, and fire his plane's machine gun upward—directly into the German plane's cockpit.
Josha Levine, writer, actor, historian, and author of Fighter Heroes of WWI:
"Dog fights were a very common occurrence in the First World War, but they never really ever happened again, because by the Second World War...airplanes were simply too quick. And one airplane only had another in its sights for a split second at a time.... If you watch a science fiction movie,... Star Wars for example, and watch the X-fighters take on the TIE fighters, you'll notice that what they're really having is a First World War dog fight. And yet that never really happened again after 1918."
I recommend the Channel 4 program Fighting the Red Baron (2010), which offers a humanizing look at the military aviators of WWI. (See a promo video here.) Some of the British and German aces (i.e, five or more air victories) mentioned in Fighting the Red Baron are show here, from top to bottom:
Albert Ball, VC, DSO & Two Bars, MC; Royal Flying Corps (RFC); killed in action aged 20.

Cecil Arthur Lewis, MC; Royal Flying Corps (RFC); he survived the war and went on to co-found the BBC; died in 1997 at the age of 98.
Arthur Rhys-Davids, DSO, MC & Bar; Royal Flying Corps (RFC); killed in action aged 20.
Werner Voss; Deutsche Luftstreitkräfte; killed in action aged 19.
Manfred von Richthofen, "the Red Baron"; Deutsche Luftstreitkräfte; killed in action aged 25. (Pictured at left.) Richthofen scored more aerial kills than any other ace of the First World War: 80. Canadian flier Captain Roy Brown was credited with shooting him down.
A List of the Top Aces of the First World War
Top photo: a formation of recreated Fokker DR-1s at a 2005 airshow (via The Vintage Aviator Ltd of New Zealand).
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