The Ultimate Guide to Airplane Propellers

Since the first airplane by the Wright Brothers in 1903 took to the skies, aviation has continued to evolve in ever more exciting ways. The invention of powered flight has progressed immensely over the years, including the transformation into many aircraft designs, wing types, and various airplane propeller types.

Propellers are responsible for absorbing an aircraft’s engine power output to deliver thrust forces under various aircraft operating situations while meeting standards for aerodynamic performance, physical forces in multiple directions, and maintenance. While most people today think of modern aircraft engine designs, propellers still serve a vital role in aviation. Read on to learn more about the what, why, and how of propeller designs. 

What is a Propeller?

Airscrews, or what you probably know as a propeller, leverage the centrifugal energy from engines to create a rotational slipstream that can either generate forward or reverse thrust with a propeller’s blades. On a basic level airplane propellers consist of a rotating hub and divergent blades set at specific blade angles, or pitch, such that the quantity of air they displace is optimized for thrust across a variety of flying conditions. You can think of it similar to a wine cork screw wherein the blades are able to move forward by displacing oncoming air. Except that in a propeller that “corkscrew” path created by the blade tips is called a helix and that spinning generates forward thrust. The displacement of air over the blades of a propeller function similar to airfoils, or wings, in that the displacement created by incoming air creates lift, but in the case of propellers that lift is directed and referred to as thrust.

Although most modern aircraft use jets to propel themselves in the air, due to their efficiency, speed, power, and altitude, many planes still use propellers with turboprops and piston engines to generate thrust. However, it’s not rare to find planes using fixed propellers even today.

History of Propellers

The first references to human devised vertical flight date back to 400 BC in China with bamboo flying toys. These bamboo-copters were built with a propeller on a stick, and once spun they created enough lift to fly away. You are probably familiar with a wooden or plastic variety given away at carnivals and arcades as children’s toys today. While this early innovation spread across the world by the time of the Renaissance, it’s worth noting that the eponymous Archimedes’ screw no doubt served as a valuable model for the modern propellers devised by Western inventors. This  hydrodynamic screw had an impact on early propeller ships and boats, which then impacted airscrew designs. No doubt the first helicopter devised, Leonardo Da Vinci’s aerial screw from the late 1480s, was also inspired by the physics displayed by Archimedes’ screw.

There are too many innovators in the development of propellers to delve into here, but for a few key highlights, it’s important to understand that floating airships predated airplanes by a significant margin and it wasn’t until the 1800s that serious innovation began to lay the groundwork for what we think of as aviation today. Indeed it was not until 1852 that Jules H. Giffard built the first full-sized motorized airship with a three-horsepower steam engine and a three-bladed propeller. In addition, Alberto Santos-Dumont, another father of aviation, constructed steerable aluminum airships propelled by enormous wooden propellers in 1873. If you are interested in reading more about the early pioneers of flight you can study the work of Jean Baptiste Marie Meusnier, Jean-Pierre Blanchard, Sir George Cayley, William Bland, Alphonse Pénaud, Dupuy de Lome, & Hiram Maxim. Before moving onto our next section it is worth noting that the key conceptual innovation introduced by the Wright brothers was the twisted airfoil. Rather than base their designs on boat propellers as many had done before, they correctly understood that a twist along the propeller blades created a uniform angle of attack across the blades length.

How Does a Propeller Work & Produce Thrust?

A propeller’s major function is to transform an airplane’s engine horsepower into thrust as it spins around a central axis. Similar to how your hands and feet displace water as you push against them when swimming, propeller blades function by “cutting” through and pushing the air in front of them backwards. Without diving too far into Newtonian physics, propeller blades on a basic level are designed to create more air behind them to push against while they’re rotating across a central axis. This rotational force creates an area of lower pressure in front of the propeller, and high pressure behind it which allows the propeller to create a helical slip stream, or corkscrew motion, as it’s flying through the air. Because there are many more forces acting on propellers in real life than in our basic explanation there are distinct use cases for specific configurations and many propeller types currently in use. 

What Are the Different Kinds of Propellers?

The different types of propellers used in airplanes are dictated by the use intended in their original specification and the conditions they will face. Planes may have two, three, or even more blades, but that matters less than the specific mechanisms they’re utilizing to maximize efficiency. The key concept to understand with the number of blades on a propeller, is that a higher number can provide more surface area which can mean more displacement, but it also introduces tradeoffs with engine performance, weight, and increased drag. The differences in propeller types are actually distinguished by the angle at which they cut through the air. Since planes have different needs and are exposed to different forces when they are taking off, climbing, or cruising they need to displace air differently for maximum performance. Generally propellers can be split into fixed pitch or various variable-pitch configurations. Click this link to read more in our guide to different types of propellers.

Materials Used To Make Aircraft Propellers

As plane technology has evolved so have the materials used in blade construction. Below are some of the most common materials used to make airplane propellers.

• Wood Blades: Before World War II, engineers used wood as the main material to construct airplane propellers. They bonded together 5-9 layers of wood to increase the blade’s strength, resilience, and inability to warp. 
• Metal blades: As ever larger aircraft were needed wooden blades became less  practical. The use of wooden propellers became antiquated once advanced metal alloys like aluminum and stainless steel were available. Aluminum alloy blades for example are significantly better than wood because they are more robust, lighter, and can be easier to repair. Since they were lighter they allowed for increased engine efficiency and performance.
• Composite blades: Modern propeller blades can be made from a mixture of materials, including futuristic composites like carbon fiber which make them ever lighter and quieter.

Parts of an Aircraft Propeller

Although there are myriad propeller parts depending on the manufacturers and model specifications, there are a few key terms you should know:

1. Blade: One arm of a propeller from root to the tip. Propellers usually have two or more blades to balance out the centrifugal forces created when they are rotating. 
2. Blade Back: The surface of the blade when standing in front of the plane.
3. Blade Face: The surface of the blade when standing directly behind the propeller, or “facing” it as the pilot. 
4. Root or Shank: The thickened portion of the blade near the hub of the propeller.
5. Tip:  The “end” of the blade or the portion which is furthest from the hub and traveling the fastest when rotating.
6. Leading Edge: The portion of the blade nearest to the direction in which it is turning.
7. Trailing Edge: The edge of the blade furthest from where it is turning.
8. Hub: The central portion of the propeller which forms the basis of the longitudinal access  where the blades are affixed.
9. Spinner Dome: The spinner dome is positioned in front of the propeller and covers the hub; it improves aerodynamics, and assists cooling. 
10. Spinner Bulkhead: The component that protects any pitch changing mechanisms and connects the spinner dome to the rest of the hub. 
11. Propeller Retaining Nut: The nut which secures the propeller shaft and hub.
12. Pitch Change Mechanism: An assembly that converts engine pressure into force that can alter the blade angles on the hub.
13. Governor or Continuous Speed Unit: A mechanical or electronic mechanism which automatically controls the pitch of propeller blades to maintain the most efficient pitch.  

Key Aircraft Propeller Terms

Although propeller design has undergone numerous changes over the past century, the basic principles of this relatively straightforward component of an airplane have essentially not changed. The basic terminology used in relation to how airplane propellers work is listed below.

• Blade Pitch: The pitch or blade angle commonly refers to the angle between the blade chord line and the plane of rotation.
• Propeller Pitch: The theoretical distance a prop would move forward in one rotation under ideal conditions.
• Slip: The ideal geometric pitch of a prop is greater than the effective pitch under real world conditions, this difference is referred to as slip. 
• Chord Line: A propeller’s chord line is an illustrative line drawn through the blade’s center from its hub, otherwise known as the leading edge, to the tip or trailing edge.
• Blade Angle: Measured in degrees, blade angle refers to the angle between the rotation plane and chord line. Although pitch and angle are frequently used synonymously, pitch technically does not refer to a propeller blade’s angle. Usually, a rise or drop in one is accompanied by an equivalent rise or fall in the other.
• Angle of Attack: The specific angle incoming wind hits a propeller blade.
• Single-acting Systems: Leverage springs, valves, and a variety of other mechanisms in additional to oil pressure from the engine to change the blade pitch in one direction. Double-acting systems are similar except that they can alter pitch in both directions. 
• Longitudinal Access: An axis intended to denote the direction of flight. 
• Plane of Rotation: The plane at which the blades are rotating.
• Centrifugal Forces: The force blades are subjected to in outward direction “pulling” them away from the hub. 
• Torque Bending: The tendency of blades to move in the opposite direction of rotation as the force produced by air pressure rushing against the blades distorts them. 
• Thrust Bending: The forward bending of the blades as they are displacing air behind them while generating thrust. 

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