# Why Ducting A Propeller Makes It More Efficient.

| July 29, 2015 | (8) Posted in Just Fun

### 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.

• 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.

rick_harriss on July 31, 2015
Just to clear up something. The ball on the string experiences a force pulling it in towards your hand. (caused in your case by the string) there isn't any centrifugal force sending it outwards. The ball is merely trying to keep going in a strait line because of the acceleration you have given it (Newton again).
Ultimate_Red on July 31, 2015
Of course there is a force pulling it outwards otherwise the string would cease being taught..
rick_harriss on August 1, 2015
Ah, I didn't say there was no force. Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.

You provide an acceleration to the ball and the force that stops it moving in a straight line is the tension on the string. There isn't, as such, a force acting along the string away from your hand. The ball is trying to go in a straight line

Centrifugal force is often referred to as an imaginary force.

https://www.grc.nasa.gov/www/K-12/airplane/newton.html
https://en.wikipedia.org/wiki/Centrifugal_force

rick_harriss on August 1, 2015

"For almost all aircraft designers, their drawbacks have weighed more heavily:

Less efficiency, because a smaller mass of air is used for propulsion
Additional friction drag and weight from the shroud

The highest propulsion efficiency is possible with a large, slow-spinning prop. Once flight speeds approach the speed of sound, the propeller has to become smaller to avoid supersonic propeller tips, and then the shrouded design becomes more attractive. Electric airplanes so far are not fast enough to profit from this effect. I would be very skeptical of the increased thrust claim - both efficiency and thrust of the E-Fan ducted fans are lower than what a well-designed propeller can offer."

from:

1959cutter on August 4, 2015
I noticed a ring/ducting on an electric boat outboard motor on the lake, is this the reason?
Ultimate_Red on August 9, 2015
More than likely, on boat's it increases power at lower speeds.
Paulshort on July 31, 2015
Are you in New Zealand? I saw that the plane in the picture said New Zealand air force.
Ultimate_Red on August 9, 2015
oh ho, just an image to illustrate the vortices on a propeller..
Crash and Burn RC on August 14, 2015
Hi, great article! I really do enjoy reading these articles, as they really help when designing. I just have one thing to ask. RCTestFlight did a motor config. test. If you watch to the end he compares open props to ducted ones. The open prop is much more efficient. How is that explained. My guess is that the whole thing must be moving forward to have the efficiencies of the duct. Tell me what you think.
Keep doing the great articles,
Colin
Ultimate_Red on August 14, 2015
How odd? Thank you for enjoying my article nevertheless. The science says that the ducting should in theory work in static format and whilst moving.. I'm watching RC Model Reviews currently and he said he'll be doing a test video soon.. when that is i have no idea so keep a look out for it :)