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Why Don’t Drones Use Ducted Fans?

If you are a drone enthusiast, you probably know that most drones come with a fairly similar design in the sense that they use propellers to achieve lift to get themselves up in the air similar to how helicopters work. With that said, you might be wondering why most drones are designed this way and are not using a different way to propel themselves, for example a design such as a ducted fan. So why don’t drones use ducted fans?

Ducted fans simply are not as efficient as propellers for static lift. They have smaller blade diameters and lower torque motors, and can compensate with higher RPM and more fan blades to provide the thrust needed, but this means that they aren’t as efficient as propeller blades when used on drones.

You may have seen ducted fans being used in a lot of different flying objects or even flying vehicles. However, you also have to understand that there is a difference between those objects and vehicles and a drone. Drones, as small and as light as they are, need to operate much more efficiently. So, let us try to understand more about why drones do not use ducted fans.

How Effective are Ducted Fans?

When you are looking at different flying objects that need a certain level of thrust to get up in the air, you have to consider the fact that these need engines that propel them up and towards the direction they want to go. One of the basic ways for you to do that is by using a classic propeller fan that spins at high speeds so that it can propel the aircraft or vehicle upwards. The classic propeller was what was used to fly the very first airplane.

However, another one of the more popular ways to propeller an aircraft or a flying object is the ducted fan. The ducted fan looks like your classic propeller fan but now has a duct encasing a series of shorter and more numerous fan blades. Basically, the ducted fan will still spin to produce the torque and lift that the aircraft needs to fly but it works at higher rotational speeds.

So, basically, even though ducted fans still look a lot like your usual propeller fans except for the fact that there are shorter and more blades and that they have a duct, they are actually advantageous to a certain extent in comparison to the ductless propeller blades. 

That’s because the duct is capable of reducing the losses in the thrust produced by the blades. Meanwhile, you can also change the cross-section of the duct so that you can change and vary the velocity and pressure of the airflow according to Bernoulli’s principle to your advantage. This is why ducted fans are more commonly used in the modern version of aircrafts such as the usual airships and hovercrafts.

Here are some of the other advantages that you can have when you are using ducted fans on a flying object:

  • The tip losses from the propeller blades can be reduced by the duct. When that happens, the ducted fan can operate at a more efficient level as it can produce thrust that propellers that are similar in diameter and size cannot. This is best seen when the aircraft or the object is flying at low speeds and high static thrust levels.
  • If you are able to vary the size of the duct the right way, you can adjust the size of the fan to allow it to work at a more efficient level at higher airspeeds compared to what a propeller can do.
  • Of course, ducted fans work a lot more quietly than propellers do because the ducts shield the noise that the blades produce while also reducing the tip speed that could very well contribute to more noise.
  • At the same thrust level, a ducted fan can be smaller than a propeller. This allows you to use smaller engines on a flying aircraft or object while retaining the same thrust level.
  • Ducted fans, because of how the ducts shield and cover the propeller blades, are safer when operating on the ground because the chances of the blades hitting someone or something are greatly reduced.

So, as you can see, there are certain advantages when you are using ducted fans. In other words, these propellers are pretty much effective for all sorts of different flying objects, and that is why hovercrafts and aircrafts prefer using ducted fans over propeller fans. And even after the turbojet engine was introduced, smaller and low-cost airplanes still make use of ducted fans because they are more affordable. Nowadays, plenty of different remote-controlled model aircrafts are propelled using ducted fans.

Why Don’t Drones Use Ducted Fans?

Some of the most popular modern model aircrafts that are being used by different hobbyists nowadays are drones. 

When you notice how most drones are constructed, one of the things that you will first see right off the bat is that drones make use of propeller blades instead of ducted fans. And at this point, you already know how effective ducted fans are when propelling aircrafts and model planes. So, this might make you wonder why drones aren’t even using ducted fans and are using traditional propellers?

To understand this, let us look at the disadvantages of ducted fans:

  • They don’t work as efficiently when the flying object is at cruise level or is operating at a low thrust level.
  • You need to be able to have very small clearances between the tips of the blades and the duct itself for the ducted fans to work efficiently.
  • It runs at higher RPMs and the vibration it needs tends to be quite minimal.
  • The duct can add weight to the entire flying vehicle or object even when the duct is made from light composite materials.

So, if you look at it, it becomes quite easy to understand why most drones don’t use ducted fans and why there are only a few of these drones that actually use them. Drones work at a low thrust level, and their engines need to be able to provide low-level static thrust all the time for the drones to maintain their flight. This makes the inefficiency of ducted fans apparent as they do not work very well at low thrust levels.

Of course, you should also factor in how difficult it would be for you to make use of ducted fans on drones that are supposed to be light and easy to carry. The ducts themselves add weight to the drones, which would now require the propellers to work harder just to provide the lift and the thrust that the drones need. This makes it harder for drone manufacturers to design drones that can be light enough when using ducted fans as their propellers.

Meanwhile, because more and more mini and light drones are becoming popular because you can basically fold them and their propeller blades in order to place them inside your bag, it would be quite difficult for drones with ducted fans to be portable. The ducts themselves would have to take up a ton of space, and would not have a readily foldable design. And while traditional propeller blades are usually longer than the blades on a ducted fan, the fact is that the propeller blades on most drones can be folded so that you can carry them easier. This is something you probably cannot do with ducted fans because of the ducts.

All in all, traditional propeller blades are simply much more efficient in terms of performance and design when compared to ducted fans when we’re considering their use in drones. 

How Much Thrust Does a Ducted Fan Produce?

The thrust that a ducted fan can produce is highly dependent on a lot of different factors such as the design of the ducts and the size of the ducted fan itself. As such, there really is no specific number when it comes to the thrust that a ducted fan can produce because it can be entirely dependent on how it was made and designed.

As an example, we can look at a 15 cm diameter ducted fan so that you can understand how much thrust a larger or a smaller ducted fan can produce. A 5.9 inches (15 cm) ducted fan can probably lift about 8.8 lb (4 kg) of mass. But you can increase its thrust capacity by increasing the solidity of the rotor. If that is the case, the ducted fan might be able to lift somewhere close to 6 kg. However, you would need more ducted fans that are similar in size if you want to be able to carry 10 or more kilograms.

Another good example is the Martin Jetpack, which makes use of two ducted fans that are probably close to about 80 cm in diameter. This jetpack is enough to lift a person off the ground but not to the point that it could actually provide enough verticality. The Martin Jetpack doesn’t say how much thrust it produces but it might be considerably good enough because it can lift an average-sized man off the ground.