An old idea with a new problem
The notion of a screw that pulls itself through a fluid is very old. Ancient engineers used the Archimedean screw to lift water, and the Renaissance sketches attributed to Leonardo da Vinci include an “aerial screw” that looks like an early helicopter rotor. These are important as ideas, but there is no good evidence that such designs actually flew. The path to a working aircraft propeller came largely through ships and through experiment.
In the 19th century, inventors fitted rotating screws to ships, and the screw propeller gradually replaced paddle wheels for many vessels. Early aviation thinkers naturally borrowed the idea for lighter-than-air craft and for model aircraft. Some experimenters put steam or elastic-powered screws on airships and models. The first rubber-band powered model airplanes, built in France in the 1870s, used simple propellers and flew successfully, which showed that a spinning blade could drive a small aircraft forward.
Why a ship propeller was not enough
Water is roughly 800 times denser than air, so ship propeller designs do not simply scale to air. A propeller working in air has to be large, light and carefully shaped. Early aircraft builders commonly copied marine propeller shapes, and the result was usually inefficient.
The central insight, associated with the Wright brothers, was to stop treating the propeller as a screw biting into a solid and to treat it as a pair of rotating wings. Each blade is an airfoil. As it turns, it generates a lift-like force, but because it is pointed forward, that force pulls the aircraft along. This means the same questions that apply to wing design, such as angle of attack and cross-sectional shape, also apply to the propeller.
The Wright propeller
The Wrights’ wind tunnel work provided data about airfoils, and they realized that propeller design was complicated by the fact that the blade moves in two ways at once: it spins, and it advances with the aircraft. The angle at which the air meets the blade changes along its length, since the tip moves faster than the root. For efficient thrust, the blade therefore has to be twisted, with a steeper angle near the hub and a shallower one near the tip.
The 1903 Flyer used two wooden propellers, each about eight feet across, made from laminated spruce and shaped by hand. They were driven by chains from the engine, with one chain crossed so that the two propellers turned in opposite directions, which helped cancel torque effects. Their efficiency is often reported as roughly two thirds, which was remarkably high for the time. Exact numbers vary by source and by what was being measured, so it is safest to describe them simply as much better than competing designs of that period.
Wood, metal and fixed pitch
In the 1900s and 1910s, many aircraft used one-piece or laminated wooden propellers. These were lightweight and easy to repair, and they stayed common for decades. As engines grew more powerful, designers began using metal blades, and by the 1920s and 1930s forged aluminum alloy was widespread.
Early propellers had a fixed pitch, meaning that the blade angle was set when it was made. This is a compromise. A propeller set for a good takeoff will be less efficient at high cruising speed, much as a car stuck in a low gear is wasteful on the highway. Designers sometimes chose a “climb” or “cruise” propeller depending on the job.
Variable pitch and constant speed
The solution was a propeller whose blade angle could change. Early adjustable-pitch types could be set on the ground, and later versions allowed pilots to change pitch in flight. In the 1930s, the controllable-pitch propeller, produced by firms including Hamilton Standard in the United States, became a significant improvement, and it was associated with big gains in climbing performance and efficiency. A related development was the constant-speed propeller, which uses a governor to adjust blade angle automatically so that the engine can stay at its most efficient speed while the aircraft changes altitude and airspeed. Many later propeller aircraft used this arrangement. A further feature, feathering, turns the blades edge-on to the airflow when an engine fails, reducing drag.
Beyond the classic propeller
As aircraft engines changed, propellers faced a new limit. When blade tips approach the speed of sound, efficiency falls and noise rises. That limit is one reason jet engines took over for fast flight, and the arrival of jet airliners ended the dominance of propellers on long routes. Propellers survive where they are efficient, such as turboprop airliners, regional aircraft and small private planes, with blade shapes now designed by computer.
The basic idea has therefore held up for over a century: a spinning blade is a wing that moves in a circle. The refinements since 1903 are mainly about managing its shape, angle and speed.