Definition
A type of gas turbine engine in which large fan blades are exposed to the open airstream rather than enclosed within a duct or cowling. The fan is driven by the engine's turbine section and produces most of the thrust by accelerating a large mass of air outside the engine core. Unducted fan engines combine the fuel efficiency of a turboprop with the higher cruise speeds of a turbofan.
Plain English
A jet engine with big, open fan blades on the outside instead of inside a covering. The exposed blades push a lot of air to create thrust, giving good fuel economy at fairly high speeds.
Context Anchor
Seen in discussions of aircraft engine design, especially advanced or high-efficiency turbine propulsion systems.
Derivation
Unducted' means 'not enclosed in a duct.' A duct is a tube or housing that channels airflow. So an unducted fan is simply a fan whose blades sit out in the open air rather than inside a surrounding case.
Why Pilots Care
Offers potential fuel savings and performance gains over traditional turbofans or propellers, relevant to future commercial aircraft designs.
Analogy
Like a large propeller mounted on a jet engine core but built to operate efficiently at higher speeds without any surrounding shroud.
Intuition Check
Do not read “fan” here as a small cooling fan. In this context, it means large aircraft propulsion blades driven by a turbine engine. Do not read “unducted” as damaged or missing parts. It means the design intentionally has no outer duct around the fan blades.
Example Sentence 1
The manufacturer demonstrated an unducted fan engine that promised significant fuel savings over conventional turbofans.
Example Sentence 2
Maintenance crews inspected the unducted fan blades for damage after each ground run.