Ceiling Fan Blade Design
Quality ceiling fans are much more today than a motor and a series of paddles to push air around. Modern ceiling fan designers call on the skills of aeronautical engineers and fluid dynamics experts to refine their designs. This is important because by using aerodynamics we can significantly improve the efficiency of ceiling fans as well as making quiet ceiling fans. This simple guide runs through the key points relating to a ceiling fan’s blades to be mindful of when purchasing a ceiling fan.
Number of blades
We are often asked whether more blades are better and the simple answer is NO! The principle being observed here is that we want to maximise the downward flow of air while minimising local turbulence around the fan blades. Each blade wants to pass through uninterrupted air. With too many blades a trailing blade risks moving through the turbulence of the blade in front, generating noise and reducing efficiency.
This point is illustrated by wind turbines which are designed for a combination of efficient movement and noiseless operation (a bigger issue in Europe than Australia). Contemporary wind turbines typically only have three narrow blades.
Look for fans with 3 to 5 blades. More is too much and should only be considered if you are after a particular look or you are using a very slow moving fan.
Length of blades
Bigger is not better! The bigger the blades the faster the tip of the blades will rotate and the noisier it becomes. Also large blades are more likely to interrupt your lighting plan. You should never mount a light behind a ceiling fan blade because it will cause a strobing effect as the fan spins which becomes very annoying.
A good rule of thumb is to look for ceiling fans with a total diameter of between 100cm and 160cm.
Width of blades
Narrower blades typically work as well or better than wider blades, particularly as the speed of revolution increases. This is largely for the same reasons as discussed under ‘Length of blades’. Plus wider blades increase the encroachment of the fan into your headspace and can resulting in a feeling of a lowered ceiling, which is particularly undesirable where ceiling heights are already low.
Blade textures, features and holes
Keep it simple! The more points of interest on a ceiling fan blade the more there is to cause noisy turbulence and inefficient operation. Textures like rattan mesh and palm leaf patterns will add to this, as will holes in the blades and curious features. If it doesn’t look aerodynamic and smooth then it is to be avoided, especially in bedrooms where quiet operation is curial.
Some ceiling fans have novelty blades, like planets orbiting a sun, or look like toys to appeal to kids. These may add an interesting feature to a room but generally don’t perform well as ceiling fans. Probably best to save your money for a good piece of art for the walls.
How to test a ceiling fan blade’s performance
The easiest way to test a ceiling fan blade’s performance is to review the technical specifications of the ceiling fan versus the CSIRO benchmark for an efficient ceiling fan of 1.87m3 of air-flow per minute per watt of electricity consumed on high. It must be on high because this is when a blade encounters the most air resistance. It is too easy for a ceiling fan blade to look good at low speeds when poor aerodynamics is largely hidden.
What if the distributor doesn’t quote these statistics? All manufacturers will provide these statistics to their distributors because quoting them is mandatory in the U.S. and all manufacturers produce fans to meet U.S. requirements (The U.S. is by far the largest market for quality ceiling fans). If these figures aren’t readily available chances are the fan didn’t perform well or performance isn’t important to the sellers of this type of ceiling fan.
Follow this link to review Spinifex’s ceiling fan technical data.