In this blog, we will talk about propeller technology and how they are implemented on modern aircraft. The operating principle of propellers can be described using basic physics, particularly Newton's Third Law, which states that for every action, there is an equal and opposite reaction. That being said, propellers convert rotational energy into forward thrust by pulling air through the propeller. This act of displacing air creates a pressure differential which is met with force in the opposite direction, thus pushing the aircraft forward. The relationship between the two opposing forces is constant, in that more thrust is created as more air is pulled through the propellers.
That basic mechanistic description helps in understanding propeller theory, which outlines the various forces acting on a propeller and thus the aircraft. The first of these forces is centrifugal force, which pulls the propellers away from the central hub. Vibration from the engine and flight turbulence also acts on the propeller blades. While this force typically does not cause any adverse aerodynamic consequences, it can lead to pilot fatigue if left unchecked. Finally, when torque bending occurs, the propeller blades are pulled slightly in the opposite direction than they are traveling.
The most ubiquitous propellers found on modern aircraft are wooden and metal fixed-pitch propellers and adjustable propellers. Fixed-pitch refers to the inability to adjust the propeller blade's angle after installation. In a fixed-pitch configuration, the propellers' efficiency is only optimal at a particular angle of attack and speed. As a result, this propeller design is preferentially found on smaller aircraft where a slight loss in efficiency is less costly than in a larger model. For the smallest aircraft, such as those used for personal commute or training, the wooden fixed-pitch propeller is typically chosen due to its price and ability to be easily serviced. Such propellers are constructed from several layers of hardwoods, including birch, walnut, oak, or mahogany. When molded and set, the propellers become waterproof and rugged.
Metal fixed-pitch propellers are lighter and cheaper to maintain than their wooden counterparts but are more expensive initially. First composed of an aluminum-copper alloy, nearly every modern metal propeller is manufactured from an anodized aluminum alloy. This new material is much more durable and is heat-treated, thus protecting the propeller from temperature extremes. Additionally, novel composite blades made from carbon fiber are beginning to saturate the market. These blades are promising because they are lighter than metal and wooden propellers and produce less noise and vibration.
Adjustable or variable-pitch propellers are either ground-adjusted or in-flight-adjustable. Both types allow the pilot to change the pitch of the propellers' blade to a more optimal angle of attack. This unique ability far outperforms fixed-pitch propellers in terms of efficiency, particularly during takeoff and landing. They are also beneficial on multi-engine aircraft during instances of engine failure. Through a feathering process, pilots can adjust the propellers to reduce the drag created by a failed engine, thus allowing the other engines to operate better.
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