Propellers are a common assembly found on various types of aircraft, and they feature two or more blades protruding radially from a central hub, powered by a motor or an engine. Propeller blades sport both twists and angles for their design, with the slope differing throughout the length so that the blade is moving faster at the tips than the hub. Propeller assemblies work according to the principle of Newton's third law of motion, which states that every action has an equal and opposite reaction, and this is exemplified by the propeller acting like a turning screw. This blog aims to elucidate the minutiae of propeller structures, types, and functions to make their role clearer in aircraft safety and control.
An aircraft's engine powers the rotation of the propeller blades so that they spin at high speeds, where the engine's rotary power is transformed into forward thrust for the plane. The cross-section of a typical propeller blade is akin to that of an aircraft wing, where one surface is curved or cambered while the other is flat. Moreover, blades have a trailing or leading edge with a chord line running throughout their length. The blade twist and pitch are varied because the angular speed is most prominent at the tip compared to the relative airflow. The blade angle is also greater at the hub and smaller at the end, corresponding to the decreasing thickness from the center.
Propeller aerodynamics work much in the same way as aircraft wings. The static pressure generated due to the propeller movement gets reduced in front of the blade, while a reduction in airflow at the blade face increases static pressure. According to Newton's Third Law, this differential air pressure around the rotating blades sucks in the air within the central rotating disc, causing a rearward motion of the air column and a forward thrust of the aircraft. The amount of propeller thrust depends on the air mass at the aircraft's rear multiplied by its acceleration. Therefore, smaller propellers deliver lesser thrust due to the small mass of air they displace as compared to large propellers. The angle at which the wind strikes the propeller blade is called its angle of attack. At the same time, more considerable dynamic pressure is produced at the engine side of the blade with relation to the atmospheric pressure, thereby generating thrust from the overall pressure difference.
Different propeller designs exist along a gradient of complexity, ranging from ground-adjustable and fixed-pitch types to controllable-pitch and reverse-pitch systems. The different characteristics of each propeller type are discussed below:
As the name suggests, fixed pitch propellers have a fixed blade pitch or blade angle at a given airspeed or RPM. These propellers were extensively used in planes during World War II, and they are a feature of some vintage planes today. Since fixed pitch propellers are directly linked to their engines, where their speed is proportional to the engine. Moreover, the propeller's fixed-angle directly influences the plane's takeoff, flight, and landing capability.
Test club propellers are used for engine testing by providing an appropriate load to the engine. Ideally, all test club propellers should be checked for their general working conditions before being used, particularly paying attention to their alterations to pitch settings in the case of using adjustable test club propellers. Test club propellers also provide a load for cylinder head cooling, making them useful for ground running and bench testing applications. Furthermore, test club propellers should be calibrated and re-calibrated during events of deterioration or suspicious settings.
Ground adjustable propellers have a central clamping mechanism that fixes the angle of the blade around the central hub, which can be changed only when the plane is on the ground. Therefore, the basic functionality of these planes is similar to that of fixed-pitch airplanes since their pitch angle cannot be altered during flight. To alter the assembly’s configuration, the clamping is loosened to readjust the blade's pitch angle, followed by the realignment of the blades to the desired angle before tightening the clamp.
Reverse-pitch propellers are feathering-type propellers, which are the hallmarks of turboprops. Usually, these propellers are designed with a negative angle to produce opposing thrust by working against the plane's forward motion. Pilots employ propeller-reversal during a touchdown to slow their aircraft on the runway and prevent wearing down landing gear.
Unlike fixed pitch propellers, controllable pitch propellers can have their blade angles or pitch adjusted during flight, making them ideal for various applications across different ranges of aircraft weight. However, the options for adjusting the pitch angle are limited in the case of two-blade propellers. Hydraulic pumps and pistons are used to change the in-flight propeller pitch, being a design improvement over the earlier systems that were too simple to handle the evolving sophistication of aircraft maneuverability. Running an engine at continuously high speeds is neither practical nor economical. This is why controllable pitch propellers are used to control the pitch and RPM of the rotating blades.
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