It’s difficult to define what a propeller blade is. If it’s just the shape of it, then it could be said that a screw is a propeller, though screws are rarely used as a device of propulsion. If it’s just something that converts rotational motion to linear motion, then wheels would be propellers too. Establishing an origin for the propeller is difficult since it’s not something humans invented. If a propeller spins to control fluid flow, then seeds from maple trees can be considered propellers. However, these propeller blades or seeds have an interesting property: their shape. Whether the propellers were developed through natural selection or computer-aided design, these propeller blades all have the same general shape with very little variation. This article will cover why this has come to be, and what the future holds for this seemingly simple mechanism.

Why are Propellers Shaped the Way They Are?

Fundamentally, propellers have one purpose: they convert rotational motion into linear motion. Propellers are different from other mechanisms since they never have to retract in any way. For example, a linear rail can also convert rotational motion to linear motion, but to do so repeatedly, it has to retract itself. Propellers simply move the fluid around them to create thrust, which in turn moves whatever object they’re attached to. A propeller has a few main parts that separate them from each other.

The primary component of a propeller is the blade. It generates thrust by creating a pressure difference on either side, causing fluid on one side to move faster while temporarily slowing it down on the other. This pressure imbalance allows the blade to control the direction of fluid flow, producing forward motion. Various factors, such as blade pitch, length, and twist, influence how a propeller interacts with the surrounding fluid, affecting its overall performance.

A crucial element of blade design is the tip, which moves through the fluid at the highest speed. It is at the blade tip that tip vortices—swirling flows of compressed fluid that resist motion—are most commonly formed. While many innovations have aimed at reducing these vortices, most involve modifications to traditional propeller designs.

The blade itself consists of two main edges: the leading edge and the trailing edge. The leading edge is the part that first meets the oncoming fluid, while the trailing edge is where the fluid exits after passing over the blade.

The root of a propeller blade is the section closest to the hub, where it attaches to the central part of the propeller. It plays a critical role in the overall structural integrity and performance of the propeller. The root faces the greatest stress in a blade since it has to withstand any forces transferred to it. Since the root of a blade is closer to the center of rotation than the rest of the blade, it witnesses lower speeds, and accordingly, less overall thrust. This means that the root can be thicker than the rest of the blade, allowing it to withstand greater forces without compromising on too much thrust.

Lastly, the hub. The hub attaches the rest of the propeller and plays a key role in transferring the rotational energy from the engine to the blades, allowing them to generate thrust.

Figure 1

Diagram of the different parts of a propeller blade

Source: McGraw-Hill

Alternative Propeller Designs

Likely the most famous alternative propeller design is the toroidal propeller. Originally invented in 1930 by Friedrich Honerkamp, a toroidal propeller is an innovative design featuring blades that form a loop or torus (donut-shaped) rather than the traditional flat, straight blades found on conventional propellers. Recently, these propellers have gained attention after a group of MIT students tested the propeller design on drones. These propellers were reported to show significant increases in thrust and a decrease in noise, increasing overall efficiency. The propellers see significant increases in efficiency past the 3000 rpm mark, which many boats, planes, and drones reach and surpass. 

Figure 2

MIT’s toroidal propeller drone

Source: Lincoln Laboratory

This efficiency gain is due to toroidal blades having connected blades largely due to the aforementioned tip vortices. Toroidal propellers’ connected blades allow them to reduce the strength and frequency of tip vortices by a large margin. This, in turn, increases the efficiency of the propellers by up to 105%. (Ferrell 2023)

Toroidal propellers aren’t the only other type of unique propeller. Another such design is being used by the delivery company Zipline, which operates largely in Rwanda. Compromising efficiency for noise reduction, Zipline’s propellers have two blades staggered by 45 degrees with a counterweight on the other end. The two blades on the propeller are also at different heights. While unintuitive, the propeller reduces both overall noise and also spreads the noise over various frequencies, which drastically reduces perceived noise. 

Figure 3

A 3D printed model of Zipline’s silent propeller

Source: Action Lab

Why are these propellers rarely used?

One major factor of both of the niche designs just described is the cost. Traditional propellers can be reduced to a little more than pieces of twisted metal welded to a hub with stronger material. While there are many small alterations to the general concept, these propellers have been mass-manufactured for decades and dominate the industry. Prices of these propellers are extremely low compared to other options since they haven’t been widely accepted yet. For example, a traditional propeller for a boat might cost $500, but its toroidal counterpart could cost up to ten times that. Zipline’s propellers are created custom and haven’t been seen in applications elsewhere. 

Further, these propellers always sacrifice something for their benefit. Toroidal propellers are generally heavier than traditional ones, making them less efficient at lower speeds. Zipline’s propellers offer their sound benefits at the compromise of efficiency. Traditional propellers have set the baseline “one size fits all” approach for a low cost, making them unbeatable in the current market. 

Conclusion

While traditional propellers are likely here to last, these innovative new designs have incredible potential for the future. As consumers, there’s not much that any single individual can do to change what’s been the standard for such an extended time. However, once designers and companies overcome the prices and manufacturing difficulties, it’ll be almost impossible to avoid the benefits that these propellers can bring. 

References and Sources

CJR Propulsion. (n.d.). Propellers: A complete history. CJR Propulsion. https://www.cjrprop.com/propellers-a-complete-history/#:~:text=The%20first%20iterations%20of%20what,upwards%20using%20a%20spiral%20movement.

Undecided. (2023, March 5). Why is this propeller getting so much attention? Undecided. https://undecidedmf.com/why-is-this-propeller-getting-so-much-attention/

Jones, D. R. (2023, May 23). What is a toroidal propeller, and could it change the future of drones? An expert explains. The Conversation. https://theconversation.com/what-is-a-toroidal-propeller-and-could-it-change-the-future-of-drones-an-expert-explains-206498

Lincoln Laboratory, MIT. (n.d.). Toroidal propeller. MIT Lincoln Laboratory. https://www.ll.mit.edu/partner-us/available-technologies/toroidal-propeller-0