Ducted fan

The high-bypass turbofan engines used on many modern airliners is an example of a very successful and popular use of ducted fan design.

The duct increases thrust efficiency by up to 90% in most cases [citation needed], in comparison to a similar-sized propeller in free air.

The duct performs several functions: Principally, it reduces the vortices created by air flowing round the ends of the blades.

Because of this, the fan can either be used to provide increased thrust and aircraft performance, or be made smaller than the equivalent free propeller.

With careful design, the heated discharge from the engine cooling system can be injected into the low-turbulence fan wake to increase thrust.

Ducted fans are favored in VTOL aircraft such as the Lockheed Martin F-35 Lightning II, and other low-speed designs such as hovercraft for their higher thrust-to-weight ratio.

Glow plug engines combined with ducted-fan units were the first achievable means of modeling a scaled-size jet aircraft.

Despite the introduction of model-scale turbojet engines, electric-powered ducted fans remain popular on smaller, lower-cost model aircraft.