Why Are Aircraft Propeller Blades Twisted?

Posted on July 25, 2022 Jerold Perkins Aviation

When working with aircraft that feature propeller assemblies, one may notice that each propeller blade features a twisted design. This is an intentional choice during the manufacturing stage, ensuring that blades are able to most optimally perform during flight. To help you better understand this design choice, you will first need to be familiar with the physics of propeller assemblies.

For a typical aircraft with a propeller assembly, all blades will extend outward from a rotating center. As the entire structure spins around during flight, the tips of each blade will generally travel faster than the sections closest to the center. By implementing a specific twist, the blades can compensate for the speed difference, ensuring a more uniform generation of thrust across the entire length of the component. This is crucial for optimal flight as the primary role of the propeller assembly is to convert the torque generated by the engine into a form of linear force that drives the aircraft forward in the atmosphere. While airplane wings are designed to create lift that overcomes weight, propellers create forward force to overcome the drag that acts as the rearward force.
 
Like wings, propellers are also a form of airfoil in how they utilize the shape of a wing to manipulate airflow. The bottom of the blade is the side that faces the cockpit, and it is generally flat in design as compared to the forward facing surface that is much more rounded like a wing. Additionally, the leading edge of each propeller blade is rounded while the trailing edge is pointed, making them even more similar to the airfoil design of airplane wings.
 
One way to understand how propellers work is to familiarize yourself with Bernoulli’s principle that describes how low pressure will develop in front of the propeller as a result of its shape, and the higher the pressure is behind the blade, the more it will push along the length of the assembly to fill the low-pressure void. This translates into the forward momentum of the aircraft as the propeller acts as a screw. Propeller functionality can further be explained by Newton’s law of thrust in that as a vast amount of air is forced backward, an equal and opposite reaction will occur in the form of resulting thrust. While the actual science behind propeller functionality is much more complex, this explanation is suitable for understanding the very basics.
 
For a common propeller aircraft like the Cessna 172, when the propeller assembly is spinning at 2,500 RPM, the outer tip will generally need to travel upwards of four times the distance of the inner section to complete a single revolution. What this means is that the root of the propeller may spin at a speed that is nearly 400 MPH less than the tip. As such, twisting the blades ensures that lift is produced evenly across the entire blade, promoting more efficiency. With the twist, an airfoil surface spinning at 149 MPH will create the same lift as one traveling at 558 MPH. This is done through the setting of the angle of incidence so that the angle of attack of the blade section traveling at 149 MPH is much higher than the one traveling at 558 MPH. This need for twisting is not new by any means, as the first twisted blades were found on the original aircraft flown by the Wright brothers in 1903.
 
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