Ducted fans are propellers that are enclosed around the circumference with a duct that has a wing-like section.
They can be used to obtain higher efficiency and higher levels of lift for the same input power.
The essential principle of operation is the expansion of the rotor flow downstream of the propeller. By expanding the exiting slipstream, additional thrust is recovered by the duct. At the same thrust level, a ducted fan will have a higher mass through the rotor than an open propeller (which implies a higher efficiency).
Ducted fans also provide protection from collision with spinning blades, and lower noise.
So why aren't ducted propellers more widely used? Several reasons:
-Complexity of duct manufacturing. The clearance from the blade tip to the duct must remain small in order for the duct to work. Expansion is limited by stall within the duct, and insufficient clearance exacerbates this.
-When experiencing side wind, the duct imparts a strong and problematic pitch-up moment. Open propellers do not produce this, though they may produce a much less severe roll moment.
-Additional weight of the duct (and its structural supports).
They can be used to obtain higher efficiency and higher levels of lift for the same input power.
The essential principle of operation is the expansion of the rotor flow downstream of the propeller. By expanding the exiting slipstream, additional thrust is recovered by the duct. At the same thrust level, a ducted fan will have a higher mass through the rotor than an open propeller (which implies a higher efficiency).
Ducted fans also provide protection from collision with spinning blades, and lower noise.
So why aren't ducted propellers more widely used? Several reasons:
-Complexity of duct manufacturing. The clearance from the blade tip to the duct must remain small in order for the duct to work. Expansion is limited by stall within the duct, and insufficient clearance exacerbates this.
-When experiencing side wind, the duct imparts a strong and problematic pitch-up moment. Open propellers do not produce this, though they may produce a much less severe roll moment.
-Additional weight of the duct (and its structural supports).