ducted fan vs propeller

Ducted Fan vs Propeller – What’s the Current Preference?

The era of aviation started with propellers. After its long journey, someone thought of putting a fan inside a duct. And that completely changed how jet planes were viewed. 

But there still remains an unclear concept and dilemma to choose between ducted fan vs propeller for RC.

Ducted fans are high-speed fans enclosed in a shroud where propellers are long 2-3 bladed fans. The fan provides huge thrust with controlled power. Propeller provides high static thrust and hovers in the air. Fans are used in airplanes and VTOLs whereas propellers are used in ships and drones.

Before trying out any of these on your precious drone you must know some things in depth. That’s where we come in.

Let’s move on to the main discussion!

Ducted Fan vs Propeller: Quick Comparison 

Let’s have a quick glance at the table before we dive in. 

Features Ducted FanPropeller 
Functionality Displaces or moves a medium within its environmentCreates movement by using a medium to displace itself. 
Thrust ProductionHigherLower
Efficiency LowerHigher 
Torque RequiredLow torque motorHigh torque motor
Blade Diameter SmallerLarger
UsabilityAppropriate for RC PlaneAppropriate for Drone
Noise Level40-45 dB75-82 dB

This table is just a summary of all the things that’s needed to be discussed. Now we’ll discuss the detailed analysis for your needs.

Detailed Analysis between Ducted Fan vs Propeller

Here we have composed a head-to-head comparison between propeller vs ducted fan.

propeller vs ducted fan chart

Functionality:

Sometimes fans and propellers are confused with the same meaning. But there are a few key factors that make them different. 

A fan is a stationary object used to displace or move a medium within its environment. Fans are designed to move air at lower speeds but at higher pressure.

Functionality
Viper Jet with huge 90mm ducted fan Source: YouTube

The speed of the motor is controlled by the ESC. In a turbofan engine, the fan compresses the air for the jet engine. A propeller can’t do that. 

Propeller is used to create movement by using a medium to displace itself. When it’s rotated, it exerts linear thrust on fluid such as air or water. 

The specially shaped blades create a  pressure difference between the two surfaces of the blade in the fluid.

Summary: Propellers will help the drones to move in and controlled manner with proper stability.

Thrust Production: 

Ducted fans have 8-12 blades that produce immense speed and thus exert greater thrust. There is a simple formula to calculate it concerning your variables. 

Check out the following formula for thrust calculation for ducted-fanned drones.

The thrust of a rotor is = CT ⋅ρ⋅Area⋅tip speed2

  • ρ = air density ( 1.225 kg m-3 at sea level).
  • Area = area of rotor disc (m2)
  • tip speed = the linear speed at the tip of the rotor = radial speed * radius.
  • CT = coefficient of thrust. (is usually in the order of 0.02 to 0.05 for small rotors).

The exit area of a ducted fan aircraft is larger than that of an open rotor. This produces mass flow needed to give that thrust.  

On the contrary, propellers cannot produce as much thrust as ducted fans. Mainly because the open tip of the rotors whips out the air in all directions. In this way, it loses a lot of its generated thrust into the open air. 

You can find the thrust of your propeller using this simple formula:

Thrust, F = m.ΔV

where 

  • m= mass flow rate 
  • V= difference in velocity

If the propeller of a drone moves at supersonic speed, it can’t push enough chunks of air down the blades. Because while it’s pushing down one chunk, another chunk of air comes in. It’s what propellers at low speed are good at. 

Summary: To fly a drone there’s no need to have maximum thrust. Rather thrust in a controlled manner will provide proper balance thus it’s better to use a propeller.

Efficiency: 

Ducted fans are largely efficient for the added control of the airflow through the shroud. The shroud or duct proves a passive airflow control through the nozzle.

The diameters of the nozzles can be made adjustable to exit the desired amount of thrust. And thus it creates higher dynamic thrust and lifts. 

The small blades move at high rpm and produce a huge mass flow of air. The tip loss is prevented by the duct design. So very low aerodynamic loss occurs.

Although, a two-bladed large diameter propeller can produce maximum efficiency. But due to the limited available diameter, a three-bladed propeller shows much excellent performance. 

I have used O’woda Foldable Propellers with my DJI phantom 3. It’s a carbon fibre triblade propeller with great flight time. 

But if you wanna go for a dual bladed propeller then I would say always try to get yourself the DJI Original Self-tightening Propeller

However, if you are looking for a propellers under budget then go with the O-XOXO Propellers.

But it’s not really feasible to use such long blades in our regular aviation vehicles anymore. Moreover, at equal or less diameter a ducted fan yields greater efficiency at low speed. 

As propellers are large in size and so it has high angular momentum. Therefore it will take much time to increase or reduce its speed and be competent for emergencies. 

Summary: In order to provide split second control adjustment the ducted fans will provide better performance. 

Torque: 

Torque is the measure of a twisting force applied to a motor to make it rotate on its axis. The measurement depends on different types of motors such as 4 pole or 2 pole motors.

Due to the short blades and contained structure of duct fans, you don’t need much torque.

But the blades of propellers are pretty large. So to get it to rotate, a greater amount of force is needed to be applied.

Summary: The motor required to run a ducted fan will need to produce lower torque than the propeller.

Blade Diameter: 

Disc diameter is one of the most important parameters in the magnitude of thrust.

A rotor of smaller diameter can produce a large thrust in the ducted fan. A properly shaped duct can give an auxiliary thrust. 

Propellers with large diameters can push out a bigger chunk of air. But it might not always be possible to have such large-sized vehicles/ flying objects. 

Summary: If you need large amount of thrust usually for RC planes ducted fans are the best choice. But the propeller’s lower thrust is great for stable flight of drones.

Usability: 

Drones cannot use ducted fans. It’s because ducted fans are more suitable for giving dynamic thrust and traveling at a much higher speed. This is specifically better suited for RC planes

But propellers provide much static lift and hovering, something ducted fans can’t do that well. So they are the best suitor for drones.

Blade Diameter
Source: BSS

Summary: Use ducted fans for RC planes and for drones get yourself props.

Noise Level:

The duct prevents loss of thrust around the blade. So the air gushing out from tip loss is also contained within. 

This way, such loud grinding sound faces a barrier. And comparatively much lesser sound actually goes out. It generates approximately around 40-45 dB.

The wind is gushed out by the large blades of propellers and loud whoosh sound is produced. The noise level is almost twice than the ducted fan here at 75-82 dB.

It often makes people loose their shit or sleep.

So, Which One Performs Better?

Ducted fans definitely perform better in terms of dynamic thrust and controlled high-speed driving. 

It’s faster and easier to handle. Currently, all modern air vehicles use ducted fans. 

Propellers are suitable for only a handful of crafts like drones and ships. Though you can differentiate performances between 3 blade and 2 balde propeller quadcopter. 

If you are thinking of building an aerial tech of your own, consider the facts above.

Frequently Asked Questions (FAQs)

Are ducted fans better than propellers?

By reducing tip loss duct fans perform much better than propellers. But propellers have their own pros as well. It depends on what kind of fan you’re looking for, and for what kind of function.

Why aren’t ducted fans used in drones?

A ducted fan isn’t that good at maintaining static lift in drones. That’s why a propeller is a much better choice. 

What is a gimbal error?

If the gimbal mounting system is tilted in a gyrocompass, this will cause an error. This can happen due to horizontal acceleration caused by the motion of the vessel.

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