A propeller or multibladed fan inside a coaxial duct or cowling, also called a ducted propeller or a shrouded propeller, although in a shrouded propeller the ring is usually attached to the propeller tips and rotates. The duct serves to protect the fan blades from adjacent objects and to protect objects from the revolving blades, but more importantly, the duct prevents radial flow of the fluid at the blade tips. Fan efficiency remains high over a wider speed range with a properly shaped duct than without. However, fan efficiency is sensitive to duct shape at off-center design conditions. Without a well-rounded inlet lip and a variable-area exit, off-center performance may be worse than without the duct.