Aeronautics: Aircraft Propeller Design

Today we’re focusing on the basic design of propeller blades. The use of propeller propulsion systems extends over a century and have paved the way for modern air travel as we know it. Let’s take a look at how standard propeller blade design helps to creates sufficient airflow in order to generate lift.

A propeller blade is a small-scale airfoil. Like an airfoil wing, they have a leading edge (which is the surface of the blade that interacts with air first), and a trailing edge. Both also have a cambered cross-sectional airfoil shape, in order to manipulate the shape of airflow around them. The force created by this process generates lift by changing the direction of air that they come into contact with.  

A propeller differs from an airfoil wing in multiple ways because of its threaded rotation engineering. The blades travel on their plane of rotation thanks to power from an engine or motor system. The spinning propeller will create differing velocity along the blade due to their distance from the center of rotation. Therefore, the blade tip is travelling much faster than the blade shank, or root. In order to account for this, a blade is designed with two primary requirements— angle of attack and pitch angle.

A specified angle of attack is the angle between the chord of a blade element and the relative wind. The efficiency of a blade at differing RPMs and altitude depends on the angle of attack along the length of the blade. The angle of the attack of a propeller blade is engineered to be steeper at the hub where the blade is moving slowest, and shallower near the tip where it is moving fastest. This design creates a twist along the length of an airfoil blade to facilitate uniform lift.

The pitch angle of a blade refers to the position each element along the length of the blade in relation to the hub of the propeller. An efficient pitch angle ensures that a propeller can accelerate air downward to create lift without creating too much drag. Propeller blades are installed at an angle from the hub, based on the most efficient placement for the needs of the aircraft. This ensures that the angle of attack of the blade extending from the hub can compensate for applied speed and force.

A propeller encounters force and stress when in operation. It is susceptible to thrust force, centrifugal force, and torsion (twisting) force. Thrust force can result in bending stresses, which encourage forward bending of the blade as it travels through air. Centrifugal force creates tensile stress, which puts pressure on the blades, pushing them outward from the center of the hub. Torsional stress is created by twisting forces within the blade resulting from interaction with the air flow, which tends to twist the blade toward a lower blade angle.

Overall, an incredible amount of engineering is involved in aircraft propeller blade design. The field of aeronautics has advanced propeller technology to what it is today. As a result, you can find a variety of propeller blade designs that meet the former specifications and serve various applications within the aerospace industry.