Why Ducting A Propeller Makes It More Efficient.

Technical Thursdays:

Why Ducting A Propeller Makes It More Efficient.

It's been a while since I last made an article and in the gap i've been thinking of the next articles I'd like to write. Thusly, I present to you Technical Thursdays; a series of articles realeased on Thursdays that cover any techical aspect of aviation.

In this article I will be explaining to you how ducting a propeller increases its efficiency by talking about how a propeller works and how the duct affects these characteristics.

The Principles

Let’s begin to understand how ducting works firstly by explaining what a propeller is. As some of you know, a propeller is a small wing fitted at an angle in a radial configuration around a central hub shown in the cross-sectional image below [1]. If you’ve read my ‘How Do Aircraft Fly?’ article you’ll have an understanding about how wings create lift, but for those of you who haven’t, I’ll briefly cover this now.

According to Newton’s third law, “every action has an opposite and equal reaction” which can be seen everywhere in our universe. It applies to wings and flight because as a wing travels through the air it splits it into two [2]. The wind, as it is being bent, is hitting into the wing and being pushed downwards. As a consequence, an equal and opposite force of the wind being pushed down by the wing is that the wind pushes the wing upwards [2] (shown by red arrows); creating lift.

On the other side of the wing a different process is happening. For this, Bernoulli’s principle must be explained. As a whole, one of his theory states that, as a fluid’s speed increases (including air); it’s pressure decreases. This can be linked to the airflow over the wing because on top, the airflow is travelling faster due to the fact it has further to go around the curved surface; creating an area of relatively low pressure shown below  [3]. Due to this, two things are happening. 1, the wing is being sucked into the area of low pressure and 2, the greater difference between the high and low pressures above and below the wing increases the lift generated.

All very well, however we wouldn’t need to duct propellers if it wasn’t for this next phenomenon. Imagine a ball on the end of a string hanging vertically. Now swing it around fast; what happens? The ball and the string lie horizontal when spun fast enough. This is because the ball is experiencing centrifugal force, the force that throws things outwards to the edge of their limiting circumference, for example, the length of the string tied to the ball. This force is important because it causes the area of low pressure on top of the wing to be flung outwards shown in the illustration above [4]. Again I must shamelessly plug another of my articles ‘How Do Winglets Work?’  because I’m only briefly going to talk about the next section and if you would like to know more please go read that article.

Due to a process called diffusion, an area of higher pressure or concentration will want to move towards an area of lower pressure or concentration to balance the volumes, and here on the wing we find two areas; one of high pressure and one of low pressure. As mentioned before the areas of pressure above and below the wing are flung outwards by centrifugal force until they meet with no wing in between them [5]. With no wing in the way, the high pressure if free to move towards the area of low pressure [5]. As a consequence a spiralling effect happens and vortices are made (see image below). 

These spiralling vortices reduce efficiency because they convert useful energy being used for lift into the spiralling movement of air at the tips of the wings. How do we stop this? Simply, with a fence. Winglets on airplanes are the tiny fences; they get in the way and stop the areas of pressure from getting to each other and reduce the formation of vortices dramatically. In our case the duct is the fence (see below) [6]. The area of high pressure can no longer move around the outside of the wing because the duct is stopping it from doing so [6].

Nevertheless, this is not the only way ducts increase the efficiency of propellers. If you look at the design of a duct you’ll see it’s normally a wide entrance to allow as much airflow in as possible, but on the large scale it can be seen as a narrow pipe. Confused? Well Bernoulli’s principal shows that as the radius of a pipe decreases the liquid flowing through it speeds up and it’s pressure drops. The duct is like a small pipe.  As air is forced into the duct it accelerates [7]. This means that a prop with a greater pitch to it can be fitted to keep up with the speed of the airflow and not require and more power from the motor, or a less powerful motor could be used to swing a larger propeller. In both instances gains are made from the improved efficiency.

The Practicality

The discovery of ducting propellers in around the 1930s has led to the development of turbo fan engines used on many jetliners today. The large fans of these engines are ducted because the duct can be designed as the housing of the engine. Whereas on propellers of Cessna 172s for example, designing a duct around them would add extra weight, complexity and wouldn’t look as nice, negating the benefits of the efficiency. 

Conclusion

Firstly I’d like to thank you for reading my article about how ducting a propeller makes it more efficient.

Covered in this article was;

• The Principles - A brief overlook on how wings and winglets work and so how those effects contribute to how ducts work.
• Bernoulli’s Principle - How liquids flowing have a reduced pressure.
• The Practicality - Why ducts can’t be used for every propeller.

I know this article may not be extensive enough for some and too confusing for others but I do try to find a balance where the things I say can be understood and learnt by the maximum number of people possible. I hope you enjoyed reading this article and be sure to check out my others linked below.

Disclaimer - Some of the images used in my articles may be copyrighted. I do not earn money from them; they are there for your educational purposes.