A duct (shroud) around a propeller changes how air enters and exits the disk — and significantly changes efficiency, thrust-to-size ratio, and handling. Tiny whoops (Mobula, BetaFPV Meteor — 1S, 65–75 mm) and larger ducted cinewhoops (BetaFPV's 2.2" Pavo20, 3S) use ducts primarily for prop protection, but the …

A propeller is a spinning wing, and like any wing it only makes clean thrust while air flows into it from the front — for a rotor, that means air coming from above and going down through the disk. Feed it clean air and it flies; stop feeding it clean air and it stalls, exactly like a wing yanked to too high an angle of …