· A ducted fan is a propulsion arrangement, whereby a propeller is mounted within a cylindrical shroud or duct.
· The duct prevents losses in thrust from the tips of the propeller and if the duct has an airfoil cross-section , it can provide additional thrust of its own
· Ducted fan propulsion is used in aircraft, airboats and hovercraft.
· In aircraft application, ducted fans normally have shorter and more number of blades than propellers, and thus, can operate at higher rotational speeds. blades than propellers, and thus, can operate at higher rotational speeds.
· The operating speed of an unshrouded propeller is limited since tip speeds approach the sound barrier at lower speeds than an equivalent ducted propeller.
Duct Shapes
decelerating shroud 
noise reduction.
accelerating shroud 
low speed heavily loaded propellers (improves efficiency)
Ducted fans are favoured in VTOL and other low-speed designs for their high thrust-to-weight ratio.
