59526 - On the Use of an Inflatable Rubber Lip to Improve Reverse Thrust Flow Field in a Variable Pitch Fan 

The use of Variable Pitch Fans (VPF) to generate reverse thrust can lead to significant mission fuel burn benefits of future aircraft by the elimination of the bulky nacelle based thrust reversers and aiding the implementation of low installation drag ‘slim-line’ nacelles. The feasibility of using VPF to generate reverse thrust in installed conditions was studied by the authors using a newly developed integrated airframe-engine reverse thrust research model which considers two future 40000lbf geared high bypass ratio engines installed on to a twin-engine airframe in landing configuration. A rolling ground plane to mimic the runway is included. In order to resolve the development of reverse flow within the engine, the internal engine model includes a complete representation of the bypass flow path, outlet guide vanes, core engine splitter and a newly designed VPF in reverse thrust mode. The installed reverse thrust flow field during the complete aircraft landing run is obtained from the 3D RANS solutions of the integrated model.

It is observed that from the aircraft touch down velocity of 140 knots to the typical thrust reverser shut-off velocity of 40 knots, the reverse flow out of the nacelle inlet is washed down by the free stream towards the engine exit regions. Consequently, throughout the active thrust reverser engagement regime, the reverse flow enters into the engine from a 180° degree turn of this washed down flow stream at the bypass nozzle exit. This results in a region of separated flow at the bypass nozzle exit lip that blocks the entry of the reverse flow into the engine. The presence of this separated region at the bypass nozzle exit plane, that varies around the annulus because of the differences in the external flow field, is undesirable because of the blockage it introduces, and the radial and circumferential variation of flow properties that impacts further reverse flow development in the engine.

To mitigate this issue, a smooth guidance of the reverse flow into the engine to avoid separation at the bypass nozzle exit can be achieved by using an inflatable rubber lip in the nacelle outer surface near the engine exit. During the nominal operation, the rubber lip is not inflated and is stowed flush with the optimized nacelle surface design. When the VPF is engaged in reverse thrust mode, the rubber lip is inflated to define a bell-mouth like geometric feature with a rounded radius. Design options that represent different geometric implementations are explored in this study to explore the design space of the inflatable rubber lip. Design variants that consider ranges of parameters like the rounding radii of the rubber lip (up to 10% of the nacelle length - limit based on nominal nacelle lip thickness) and the corresponding lengths of the rubber lip (up to 25% of the nacelle length - limit based on volume of inflating air, weight and thickness), location of the turn radius with respect to the nacelle trailing edge are implemented to the integrated reverse thrust research model to understand the effect of the design modifications on the separated flow region at the nozzle exit plane. Each of these design variants also include installation related annular coverage design restrictions to make it as representative to an actual aircraft implementation. It is observed that the rounding radius of 10% of nacelle length is effective in significantly reducing the separated region and to increase the ingested reverse stream mass flow. The improved flow distribution at the bypass nozzle exit also reduces the pressure drop of the reverse stream into the core engine. The detailed reverse thrust flow field changes in different design variants that lead to the improvements are discussed in the paper. Therefore, the inflatable rubber lip represents a design modification that can aid in the improvement of VPF reverse thrust operation, in cases where an augmentation of reverse thrust capability over the baseline is desired.