Everything You Need To Know About Propellers – A propeller is a special type of fan that converts rotational motion into pressure by producing a pressure difference in the surrounding fluid. Standard fans and propellers have the same physics, yet a fan is generally stationary, while a propeller causes the object to be in motion. Propellers are an important component in a number of industrial designs relating to rotating machinery. The key mission in designing a hydrodynamic or aerodynamic propeller is to ensure efficiency.
The efficiency of the propeller design is judged by the useful power it produces. For example, the useful power of a fan is how fast the fan can accelerate the surrounding airflow. Due to their dynamic nature, the efficiency of the propellers is measured rather than the thrust developed on the blades and how this drives the respective system, be it a boat, aircraft or other application. To calculate the true efficiency, the following equation is used:
Everything You Need To Know About Propellers
Design parameters can affect the performance of the propellers or fans. These variables may include the number of blades required, the size of the outer diameter, the pitch-affecting angle of attack, as well as the angle of the leading and trailing edges, along with many others.
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Increasing the number of blades will actually reduce the efficiency of the propeller, but with a higher number of blades there is a better distribution of pressure that helps keep the propeller balanced, therefore a trade-off must be established.
The diameter of the propeller has a significant influence on its efficiency. Larger propellers have the capacity to create more force and pressure on a larger volume of fluid. However, most designs face limitations when it comes to diameter, so optimization must occur elsewhere.
Instead of standard lift and drag coefficients, ensuring propeller design efficiency requires specific airfoils with prescribed angles of attack at each radius. The distribution of Cl (coefficient of lift) and Cd (coefficient of drag) along the radius can be investigated by performing analysis for the design point. For maximum efficiency, the airfoils must operate at maximum L/D. If the propeller is also to perform reasonably well in poor conditions, it is usually necessary to use a lower angle of attack for the design.
The assumed velocity of the fluid flow, whether air or water, is another important variable to consider. This force, along with the rotational speed (RPM), determines the pitch distribution of the system. Large propeller designs may become less efficient at axial speed. The most efficient designs are those that maintain a 1:1 pitch to diameter ratio.
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Although the actual density of the fluid has no effect on the efficiency of the system, it does play a role in defining the shape and size in the early stages of the design process. For example, an air propeller used for airplanes and drones will have a larger surface area than its aquatic counterparts since the fluid density is less.
Blades and shields can be optimized to maximize the unit’s power output while minimizing losses due to flow inefficiencies. CFD from online tools such as provide a great solution to perform rapid iterations to converge on an optimal design without the need for excessive physical prototyping. The following simulation project explores these concepts and overall propeller efficiency.
This project simulates a propeller design at multiple revolutions. Several factors including the airflow over the blades, the resulting turbulence and performance indicators such as torque, axial thrust and speed were evaluated.
The simulation measured the useful power, or thrust, resulting from the input power or torque acting on the propeller.
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Extract pressure and torque values for each operating mode using “forces and moments” result check (Source: )
At specific and fixed free flow rates, 5 different revolutions were tested to evaluate their respective effectiveness. As shown below, the simulation found that the propeller design was the least efficient at the highest RPM, running around 4000 RPM. minute for best results.
Using online simulation, it’s easier than ever to be able to use CAE technology to proactively test design iterations before prototyping. For fan or propeller designs, this is especially true, as testing different rotational speeds is essential to ensure a design’s overall efficiency.
Do you want to know more about optimizing your propeller design? Watch our webinar to see the project simulated in real time and become a CFD master now:
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Get instant access to CFD and FEA in the web browser and simulate your own design in minutes by creating a free account on the platform, no credit card required.
Dynamic Response and Dynamic Shock Analysis in FEA With Dynamic Analysis Finite Element Analysis (FEA) Harmonic Analysis Modal Analysis You know that propellers generate thrust, but how exactly does it happen? The answer is relatively simple and it all comes down to how lift is created and directed.
In the words of NASA: “A rotating propeller creates a pressure that is lower than the free stream ahead of the propeller and higher than the free stream behind the propeller. Downstream of the disc, the pressure eventually returns to free stream conditions. But at the exit, the velocity is greater than the free stream, because the propeller acts on the air flow. We can apply Bernoulli’s equation to the air in front of the propeller and to the air behind the propeller.”
Propellers are air blades, shaped in the same way as wings. But instead of producing lift in a vertical direction, propellers produce lift in a forward direction, which we call thrust. (We will dig deeper into this below)
How A Propeller Generates Thrust
Like wings, propellers have camber and chord lines, in addition to leading and trailing edges. If you look closely at a propeller, you will also notice that the angle varies from root to tip.
As the propeller rotates around the crankshaft, the speed of the propeller blades is highest at the tip and slowest at the root. During a full rotation, the tip of the blade must travel much further than the root of the blade, all at the same time. Therefore, the angle is largest at the root and smallest at the tip. By “turning” the blade, you get a relatively uniform angle of attack across the entire propeller blade.
If the angle were uniform across the support, thrust and thrust would have large variations from root to tip. There may be a negative angle of attack at the root and the blade stalls at the tip. This is why varying the angle plays such an important role, to prevent a large angle of attack and pressure differences across the blade.
The simple purpose of a propeller is to convert engine brake horsepower into thrust. Like wings, propellers accelerate the flow of air over their curved surfaces. The high speed of the air results in lower static pressure in front of the propeller, which pulls the airfoil forward.
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To accelerate an airplane, the thrust must be greater than the resistance. By increasing engine power and propeller revolutions (RPM), the air is increasingly accelerated across the blades, creating a stronger pressure difference that pulls the aircraft forward. This accelerates the aircraft but is limited by available thrust. When you fly faster than L/D max, more power is needed to compensate for increasing parasitic drag. As you accelerate, the tractive effort also increases. Because of this, more power is required at higher airspeed for acceleration.
Propeller efficiency also plays a large role in acceleration. At about the 80% efficiency point, any increase in forward airspeed results in a loss of propeller efficiency on fixed pitch propellers. This lack of efficiency at high airspeeds also reduces thrust and available power.
In an ideal world, a variable diameter propeller would be most efficient, with a large diameter for low airspeeds and a smaller diameter for high airspeeds. Due to structural, control and weight issues, variable diameter propellers are just not practical. Instead, the diameter of most propellers is sized to allow a “happy medium” between slow and fast airspeed operations.
Propellers convert engine horsepower into pressure by accelerating the air and creating a low pressure differential in front of the propeller. As air naturally moves from high to low pressure, you are pulled forward as your prop turns. Even if you own a boat, you may not know why propeller pitch matters or understand how it affects boat performance. Additionally, when it comes time to replace your propeller, consider the propeller pitch as part of your decision.
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For example, a propeller described as 14.5 x 19 has a diameter of 14.5 inches and a pitch of 19 inches. These dimensions are often stamped or cast right onto the propeller.
Recreational boat propellers are usually offered within a prop model line in two-inch pitch intervals. Still, some high-performance props are offered in one-inch increments to allow fine-tuning of the boat’s performance.
Propeller pitch is the distance the propeller would move forward in one revolution if it was moving through a soft solid – think of a screw turning into wood. The blades of a propeller are analogous to the threads of a screw. Some propellers have a constant pitch, meaning that the pitch is the same at all points from the leading edge to the trailing edge of
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