The Principles of Aircraft Propellers

An aircraft propeller consists of two or more blades mounted on a central hub connected via a shaft to the engine. This can be either an extension of the engine’s crankshaft in low-horsepower engines, or a propeller shaft geared to the crankshaft in high-horsepower configurations. In either case, each blade of the propeller essentially acts as a rotating wing, which produces force that creates thrust to pull or push the aircraft through the air. The vast majority of aircraft have a “tractor” arrangement, where the propeller is mounted towards the front of the fuselage and pulls the aircraft, but there are examples of “pusher” arrangements where the propeller is mounted in the back and pushes the aircraft through the air.

In either case, the engine rotates the airfoils of the blades through the air at high speeds, and the propeller transforms the rotary motion of the engine into thrust. This is a conversion of brake horsepower to thrust horsepower, which is not perfectly efficient. Most propellers operate at roughly 50-87% efficiency.

A propeller can be described as a twisted airfoil of irregular platform. Blades do not remain flat, but twist from base to tip, changing their airfoil along the way. The blade shank is the thick, rounded portion near the hub, designed to give the propeller blade strength. The butt, or base or root, is the end of the blade that fits into the hub, while the tip is designated as the last six inches of the propeller blade.

Blades are shaped similar to aircraft wings, with a bottom side that is flat while the other side is cambered or curved. The flat side is also referred to as the blade face, since it is the side facing the pilot in a conventional tractor arrangement.

There are multiple forces that exert themselves upon propeller blades as they rotate. Centrifugal force tries to throw the rotating propeller blades away from the hub, while torque bending force bends the propeller blades in the opposite direction of rotation. Thrust bending force bends the propeller blades forward as the aircraft is pulled through the air, and aerodynamic twisting force creates a rotational force around the center of pressure, causing the blade to pitch to a lower blade angle. All this means that a propeller must be able to withstand severe stresses, which get stronger near the hub caused by centrifugal force and thrust. It is important to inspect your aircraft’s propeller blades frequently ensure there are no signs of wear or tear upon them. Fluttering, a type of vibration where the ends of the blades twist back and forth at high frequency, is also a sign of failing rigidity in the propeller blade and should be addressed immediately when it is identified.


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