Propfan engines are sometimes identified in former Soviet Union countries as turbopropfan (like the D-27 engine) or unducted fans.
There are two categories:
- Single rotation
The single rotation has a single forward unducted fan that combines the advantages of the propeller of a turboprop and the fan of a turbofan engine.
The second type has two propellers, one behind the other that rotates in opposite directions (CR). Counterrotating designs offer the best performance. It has some similarities with the aft fan type, but it is composed here of two counterrotating fans.
Advanced Ducted Fan
These designs are essentially turbofans with large, swept fan blades that have pitch control and reduction gearing similar to propfans, but the fans are enclosed in ducts like the turbofan.
There are two basic designs: one with geared, variable pitch, single-propeller fan and the other with counter-rotating blades with pitch control. The need for such engines has been spurred by increasing airplane traffic, which raises noise, environmental, and fuel consumption issues.