The next challenge for future engines is to increase the bypass ratio in order to improve the propulsive efficiency. These very high bypass ratio engines improve the fuel consumption but create new technical challenges. These challenges include the fact that larger engines require bigger nacelles, hence the external drag and weight increase; furthermore the installation of these engines on civil aircraft is more challenging. Therefore important work has been carried out to optimise the design of these new nacelles and their installation on the airframe.
The present work is based on a new engine, with a very high bypass ratio. For this engine Snecma has designed an advanced nacelle with a low diameter-low length fan cowl and a core cowl mounted thrust reverser. A very short secondary flow nozzle is required to improve accessibility to the core engine and its accessories. The fan cowl length is dramatically reduced.
Different solutions were investigated to design the inlet and the fan cowl. Based on 2D potential inverse codes and 3D Euler codes, CFD calculations were made to optimise the aerodynamic lines for cruise conditions while keeping good off-design performance. A highly challenging exhaust nozzle was designed to maintain good efficiency with very tough geometric constraints. For this Snecma used 2D Navier-Stokes CFD codes. New reverse systems were studied for this type of nacelle, for example core cowl mounted thrust reversers. The optimisation of these highly 3D geometries was performed using 2D and 3D Navier-Stokes calculations.