From the Wright Brothers's "Flyer", a frail contraption of twisted wood, ropes and canvas, using a pusher configuration and canard, planed went to the late WW1 models, either superfast, well-profiled and powerful fighter capable of 250 kph, sturdy all-duty two-seaters, or heavy four engine bombers. The hart of the plane was, as today, its engine. In 1903 Taylor built a light four-cylinder for the Wright's brother plane. In 1906 Levavasseur built a streamlined water-cooled V8 engine. But most importantly in 1908 Seguin's Gnome Omega, became the world's first rotary engine, mass-built until the end of the war and used by all. This was the ancestor of fixed star-shaped engines (or radial). There would be a long and protracted "battle" between the supporters of the air-cooled radial and watercooled inline engines. Both had their advantages and drawbacks, and both were maintained in production until the coming of the jet. In this era of daring pioneers however, even more radical solutions were studied. The first ramjet engine was patented as early as 1908, first Coandă ducted fan aircraft in 1910, first Rateau turbocharger in 1914 (perfected by Moss in 1918 and later built in serie by Armstrong Siddeley as the Jaguar IV).
Rotary engines has been the mainstay of propulsion in WW1, alongside inline engines. By quantity the first easily outmatched the second. They were more compact, easy to manufacture and maintain. But they also provoked a high drag. The inline engine however was compact in another way: The inline cylinders made for a much more reduced cut, and helped to create streamline fuselages with more reduced drag, and ultimately better speeds. Much costlier and requiring more maintenance and technical skills they were mostly used by the Germans. One important characteristic was inline engines were immobile, while the rotary ones, as their name suggest, turned at the very same rate as the propeller. This cause excessive fatigue and some gyration problems that made some flight characteristics tricky for untrained pilots (which was often the case). But in the end, and giving the life expectancy of planes and pilots on the Western front, the main concern became production.