What is a Ducted Fan?

Lori Kilchermann

A ducted fan is a device used to propel boats, aircraft and hovercraft along with other types of vehicles. Consisting of a fan blade surrounded by a shroud, the ducted fan can be powered by electric motors as well as gas-turbines, internal combustion engines and rotatory-type engines. Popular for producing greater thrust at reduced sound levels, the ducted fan is also a popular propulsion device in model airplane building and is often used to simulate a jet engine aircraft. When used in conjunction with a gas-turbine engine, the resulting turbo-fan engine is a common source of power for military and civilian aircraft worldwide. The normal blade arrangement within the duct will consist of an odd number of blades in order to reduce any resonance inside the duct.

Hovercraft use ducted fans for propulsion.
Hovercraft use ducted fans for propulsion.

An unshrouded propeller blade is not capable of generating the same amount of speed as the shorter blades used on a ducted fan motor. This is due to the tips of the propeller reaching the speed of sound at much slower speeds than the blades of the fan. The ducted fan is also capable of directing all of its thrust towards the rear of the shroud, creating more power than a similarly-sized propeller. The high thrust-to-weight ratio of the ducted fan motor makes it a popular and effective unit to power hovercraft.

The propellers the engines used on small, non-rigid airships are in ducted fan assemblies that can be rotated during landing procedures.
The propellers the engines used on small, non-rigid airships are in ducted fan assemblies that can be rotated during landing procedures.

The shrouded blades of the ducted fan make it a much safer engine while on the ground as compared to an open propeller alternative. It is also possible to modify the duct work to produce enhanced operation at high speeds while also creating a more efficient motor. A downside to using the ducted fan is the reduction in tolerances when assembling the motor. The fan tips must operate within a very close proximity to the duct work and the shroud of the motor in order to achieve its efficiency. This requires more time in assembly as well as more capable workers completing the assembly, which typically means higher labor costs.

The complex assembly of duct work and motor shrouds accounts for increased weight for the ducted fan compared to a propeller. The fan is also operated at increased engine revolutions, which mandate increased control of any vibration during the engine assembly. This is typically accomplished through closer tolerances in the machining and balancing of the engine's rotating assembly, as well as balancing the fan itself.

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