The propeller system is the earliest thrust generator designed for fully powered aircraft flight. Though they have evolved quite a bit since their first operation by the Wright brothers in 1903, propellers utilize essentially the same principles of motion. Let’s take a look at how they work.
At its most fundamental level, a propeller needs to generate thrust to create upward and forward motion. As a whole, it is a device with twisted blades that are pointed at an angle and extend from a hub that is rotated via the power of an engine or motor. A propeller blade has the capacity to create lift by altering the direction of air that comes into contact with it. This process is representative of Newton’s third law of motion or law of action and reaction. The force of air flow applied to a propeller blade has the potential to create levels of both drag and lift. Lift, by definition, acts perpendicular to the motion of a fluid. Drag applies force in the same direction as fluid movement.
Manipulation of airflow depends on the conservation of momentum, mass, and energy within the propeller system. Air moves as a fluid— it has the ability to redistribute its mass freely while conserving momentum and energy. When airflow interacts with a propeller blade, any change in velocity in one direction can cause a change in velocity in a perpendicular direction.
Propeller blades need to create uniform lift across their length. However, a propeller blade rotates at a lower speed near its hub, and at a higher speed at its tips. This means that the blade tips are traveling a greater distance in the same amount of time as the blade root. The blade must, therefore, account for the difference in speed by incorporating different angles of attack. Propeller blades are designed with a “twist”— they integrate a low angle near their fast-moving tip, and a high angle of attack at the root, ensuring that lift occurs evenly regardless of RPM. Pitch helps adjust the angle of attack along the propeller blade. It is steeper where a blade is moving more slowly (near the hub) and shallow where a blade is moving faster (tip), allowing for differing angles of attack along the length of the blade.
In order to accelerate air downward to create lift, each propeller blade is shaped like an airfoil. Aerofoil stall, or loss of lift, is prevented by the shape of the blade itself. A propeller blade is usually cambered, just like an airfoil wing. Camber, in this instance, refers to the characteristics of the curve of an airfoil's upper and lower surfaces around the blade and the difference in pressure between the two. A pilot, or engineer, can alter the efficiency of a propeller blade by adjusting the pitch.
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