Aircraft Propeller: Fixed vs. Variable
In 1903, The Wright Brothers’ airfoil shaped propeller blades made the first powered flight in history. Nowadays, we still use a similar airfoil design for modern propeller blades, but we have a few options as far as pitch and angle of attack. Two commonly seen propeller types are fixed pitch propellers and variable pitch propellers.
Pitch refers to a pitch angle, which describes the angle at which a propeller blade is fixed to a hub, or angle of attack of each element in the length of the blade. Engineers assign pitch changes along the length of a propeller blade, because the blade is moving faster at its tip than at its root (near the hub). The propeller design must compensate for this change in order to create constant thrust across its entirety. As a result, pitch is designed to be steepest at the hub, and shallowest at the blade tip— allowing the angle of attack to vary across the blade and to account for difference in speed at the root and the tip. If a blade is not designed this way, dispersed speed and airflow would create an immense amount of stress on the mechanism and cause potential failure of the propeller.
Fixed pitch propeller types are most similar to the Wright brothers blade design. They are a simple design seen on smaller aircraft that travel at low speeds. The angle of attack employed on each blade is set by the manufacturer. It cannot be altered after production. These propeller blades are usually designed for optimized cruising or climbing efficiency. While they are considered reliable and cost effective in the proper operating regime, fixed pitch propellers tend to operate inefficiently at stages of the flight cycle its pitch is not designed for.
Variable pitch propellers give a pilot more options for efficiency. They provide the capacity to alter the pitch of a propeller blade to suit differing flight conditions. There are three main types of variable pitch propellers— adjustable pitch, controllable pitch, and constant speed. Adjustable pitch propellers can only be adjusted while on the ground, not during flight. Controllable pitch refers to a propeller that can be adjusted during flight. Constant speed propellers employ hydraulic mechanisms that have the capacity to automatically change the blade pitch when necessary. This allows the propeller to rotate at a constant speed in order to achieve RPM designated by the pilot. Therefore, regardless of what flight cycle stage the aircraft is in, or how fast or slow it is going, it is able to maintain a constant speed via hydraulic manipulation.
Overall, propeller blade design has evolved quite a bit since the Wright Brothers’ first full-powered flight. However, the airfoil structure they employed is still used on the propeller systems of small aircraft, allowing the variations we see today.
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