CFD Study of an Over-Wing Nacelle Configuration

Engine bypass ratio (BPR) has grown significantly over the years, due to a desire for increased efficiency, and the large fan diameters that have resulted are forcing the engines so close to the wing that there is no room left for them to grow any larger due to ground clearance constraints. As BPR increases even further in the future, conventional Under-Wing Nacelle (UWN) installations will therefore no longer be possible without drastic modification of the wing and landing gear. Over-Wing nacelle concepts solve this problem by offering a convenient installation for high BPR turbofans and, additionally, offer the potential to mitigate community noise through engine noise shielding using the wing as a shield. However, OWN has historically warranted concern about unacceptably high drag levels at transonic speeds and the purpose of this research was to determine whether or not drag can be improved enough to take advantage of the aforementioned cross-disciplinary benefits. To do so, three studies were conducted: study 1 conducted a simple nacelle sweep in order to identify and visualize the physical mechanisms driving the configuration, study 2 then conducted a sensitivity analysis in order to understand important design variables and, finally, study 3 performed single point optimization for a trailing edge OWN concept. Overall, results suggests that OWN drag can be improved to levels commensurate with its Under-Wing Nacelle (UWN) counterpart. However, limitations of the analysis tools employed for this research (in the area of shape optimization) were insufficient to outperform the UWN baseline. Such limitations were successfully overcome by modern OWN concepts, such as the Honda Business Jet and the military Lockheed HWB for air mobility missions. Overall, it is therefore the authors' opinion that either leading-edge or trailing-edge mounted OWN configurations are concepts worth investigating further for civil transport applications.