Forced Acting on a Propeller
_This text outlines the fundaments operations and aspects of aircraft propellers. It details the components, forces and workings of a propeller as well as discussing the difference between the different propeller types._ Introduction:
Propeller types are defined by blade pitch as being fixed or variable which will be further detailed later in the text. To fully appreciate the differences and understand the advantages of different pitched propellers we must first consider the fundamental characteristics of propellers. Usually propellers have two, three, or four blades; for high-speed or high-powered airplanes, six or more blades are used. In some cases these propellers have an equal number of opposite rotating blades on the same shaft, and are known as dual-rotation propellers. Small single engine aircraft have the propeller mounted on the front as multi-engine aircraft have them set on the wings. Pitch:
What is pitch? Pitch is important as it is the main differential from propeller type to propeller type. Essentially pitch relates to the angle of the blade in respects to a flat plane. It is the helical blade path or simpler the distance the propeller blade covers during a full rotation and the cut it has on the air. Pitch is referred to in two ways, fine and coarse. A fine pitch propeller has a low blade angle, will try to move forward a small distance through the air with each rotation, and will take a 'small' bite of the air. It requires relatively low power to rotate, allowing high propeller speed to be developed, but achieving only limited airspeed. This is like having a low gear in your automobile. (Brandon 2008) A coarse pitch propeller has a high blade angle, will try to advance a long distance through the air with each rotation, and will take a big 'bite' of the air. It requires greater power to rotate, limiting the propeller speed that can be developed, but achieving high airspeeds. This is like having a high gear in your automobile. (Brandon 2008) The Blades:
The propeller blades are in fact aerofoils producing lift and drag. As the propeller spins the leading edge of the blade cut through the atmosphere and accelerates a tube of air or_ relative airflow _the diameter of the propeller moving the aircraft forward. This rotation is able to work because the propeller blades are designed slightly different to wing aerofoils as they have a small twist in them so that the greatest angle is at the blade root and the smallest at the top, due to the different angle and speed that each section of the blade travels. These _blade _elements are in place at different angles because the linear velocity increase towards the tip of the blade as it has a greater distance to travel, the_ _angles prevent bending making each section advance through the air at the same rate. The blade angles combined with the forward motion and the circular rotation of the propeller keep constant the best angle of attack (AOA). The twist causes the blade path to follow an approximate helical path easiest seen in a linear form. This action is similar to a screw being turned in a solid surface, except that in the case of the propeller a slippage occurs because air is a fluid. Forces Acting on the Propeller:
Aircraft that are not jet powered use a propeller which converts the rotational power from an aircrafts engine into aerodynamic forces; thrust power moving the aircraft forward through the atmosphere and propeller torque which acts in the plane of rotation. The plane of rotation is perpendicular to the propeller shaft. Propellers are conventionally placed in front of the engine on the engine drive shaft. During cruising flight the propeller...
Please join StudyMode to read the full document