The open fan concept has been around for decades. Its high propulsive efficiency combined with the elimination of the nacelle drag and weight has been always appealing to replace high by-pass ratio ducted fans and reduce CO2 and NOx emissions. The CS1 and CS2 programs have made relevant efforts to pursue the contra-rotating open rotor (CROR) concept as well. Though CROR has not made it to market, progress has been done reducing noise levels to that of ducted fans.
Open fans exhibit several differences with respect to ducted fans which by today are highly sophisticated components accumulating decades of research. The chasm between the OP concept and its product is too big to be covered by a single demonstrator since a wrong materialization of the idea can give rise to misleading conclusions.
Turbomachinery simulations have been perfected for decades and are essential to close the gap between the concept and the detailed implementation of the product. However, open rotors exacerbate existing problems (e.g., blade-to-blade variations even for small angles of attack, strong coupling between CO2 and noise emissions, etc.). Moreover, open fans lack publicly available data or test cases preventing researchers from validating their ideas.
The first global assessment of CS2 reported an expected noise reduction of -9dB in the innovative TP 130 pax project with respect to the last generation of ducted fans though at a lower flight Mach number. This project aims to obtain relevant noise and performance experimental data of an unducted single fan (USF) for the short/medium-range aircraft with two objectives. Firstly, confirm that about 5-10 dB noise reduction is achievable at the expense of a slight penalty in fan efficiency, and secondly, validate and expand the scope of numerical tools. An experimental database with the key results of the projects will be built to unlock the application of the USF for SAF, Hydrogen, and Hybrid-electric engine and aircraft configurations.