The weight problem
For aircraft, the key figure of merit for an engine is not simply power but power compared with weight. A heavy engine needs a bigger wing to lift it, which needs more power, and so on. Early experimenters found that the steam engines and early gasoline engines available around 1900 were too heavy for the power they gave. Samuel Langley’s team, working on a large piloted aircraft in the early 1900s, had to commission a custom light radial gasoline engine, and it was a considerable achievement in its own right, though the airframe it powered did not succeed.
The Wright motor
The Wright brothers faced the same problem. No manufacturer offered what they needed, so, with their shop mechanic Charlie Taylor, they built a four-cylinder, water-cooled gasoline engine for the 1903 Flyer. It had an aluminum crankcase, which kept its weight down at a time when aluminum was expensive and unusual. Commonly quoted figures give it around 12 horsepower, though exact output varies by source and test. The engine drove two chain-linked propellers, and it was simple, with no carburetor in the modern sense and a basic fuel feed. Its modest power was just enough because the brothers had paid so much attention to wings and propellers.
The gasoline engine itself was by then a few decades old, and its development for cars supplied much of the technology. The wider story of the gasoline engine is a useful backdrop, since aircraft engines were adapted from it and then pushed to extremes.
Rotary and V-type engines
In the years before the First World War, designers experimented with several layouts. Light V8 engines such as the Antoinette, used in early French aircraft, offered good power for their weight. The rotary engine, developed notably by the Gnome company in France, took an unusual approach: the crankshaft stayed fixed while the cylinders and crankcase rotated with the propeller, which provided cooling without a heavy water system. Rotary engines powered many First World War fighters but had drawbacks, including gyroscopic effects that affected handling and high oil consumption.
Water-cooled in-line and V engines remained important, since they allowed a streamlined nose. However, radiators, pumps and coolant added weight and were vulnerable to damage.
Radials and the interwar years
During the 1920s, the air-cooled radial engine, with cylinders arranged in a circle around the crankshaft, became a leading design. Because it needed no radiator, it was lighter and more robust. The Wright Whirlwind radial powered Charles Lindbergh’s Spirit of St. Louis on his solo Atlantic crossing in 1927, an event that made reliability a selling point for engines. Pratt & Whitney’s Wasp and Hornet engines were also widely used, and large radials powered many airliners and bombers of the 1930s and 1940s.
At the same time, liquid-cooled engines continued to develop. The Rolls-Royce Merlin, introduced in the 1930s, is famous for its use in British fighters and bombers in the Second World War, and its power was increased by supercharging, which compresses intake air to allow performance at altitude. Fuel quality also mattered: higher-octane aviation gasoline allowed more boost without engine knock. Engines for large piston-powered aircraft became very complex, with multiple cylinder banks and intricate superchargers.
The gas turbine arrives
By the late 1930s, piston engines were approaching practical limits, especially at high speed, since propeller efficiency falls near the speed of sound. A different idea emerged in which a compressor, a combustion chamber and a turbine are used to produce a fast stream of hot gas. In Britain, Frank Whittle patented a turbojet design in 1930 and developed it with great determination. In Germany, Hans von Ohain worked independently on a similar concept. The Heinkel He 178 made the first turbojet flight in August 1939, and the British Gloster E.28/39 flew in 1941. The Messerschmitt Me 262 and the Gloster Meteor entered service late in the Second World War.
After the war, jets spread through military aviation and then airlines. The de Havilland Comet entered airline service in 1952, though early structural failures taught the industry hard lessons about metal fatigue in pressurized cabins. Later, the Boeing 707 and others established jet transport as normal, as the story of jet airliners explains.
Turboprops and turbofans
Jet engines are not the only gas turbines used in flight. A turboprop uses a turbine to turn a propeller, combining efficiency at moderate speeds with the compactness of a turbine. Turbofans, which use a large fan to push extra air around the core, are quieter and more fuel-efficient than early turbojets at airliner speeds and are standard on modern jetliners. Smaller piston engines remain in use for light aircraft, and electric and hybrid designs are being tested, though their progress depends on battery weight.
The pattern
The thread running through all of this is a steady search for more power from less weight, with fuel economy and reliability growing in importance. Wright’s modest four-cylinder engine and a modern turbofan share almost nothing in construction, yet both were shaped by the same early demand from aircraft design: lift the engine, and everything attached to it, off the ground.