Airframe noise is a significant part of the overall noise of typical transport aircraft during the approach and landing phases of flight. Airframe noise reduction is currently emphasized under the Environmentally Responsible Aviation (ERA) and Fixed Wing (FW) goals of NASA. A promising concept for trailing-edge-flap noise reduction is a flexible structural element or link that bridges the gap between the wing and the deployable flap side edges. The proposed solution is distinguished by minimization of the span-wise extent of the structural link, thereby minimizing the aerodynamic load on the link structure at the expense of increased deformation requirement. Development of such a flexible structural link necessitated application of hyperelastic materials, atypical structural configurations, and novel interface hardware. The resulting highly deformable structural concept was termed the FLEXible Side Edge Link (FLEXSEL) concept. Prediction of atypical elastomeric deformation responses from detailed structural analysis was essential for evaluating concepts that met legacy design constraints.
The approach taken in this work entailed a paradigm shift toward minimizing the span-wise extent of a structural link between the flap side edge and the main wing, e.g., with a span comparable to the flap thickness or ~2% of the flap span. Minimization of link span has the potential to relieve aerodynamic load and thereby reduce the complexity and weight of the noise-treatment structure. Many notional concepts for enabling a link of short span were considered, and ranged from purely mechanized to purely deforming, and included combinations of the two. A purely deforming approach utilizing hyperelastic (elastomeric) materials was selected for detailed study due to its simplicity and potential for reliability, fail-safety, and low weight.
The baseline FLEXSEL configuration has the aerodynamic shape of the flap section and is stress-free in the retracted configuration. The baseline structure must deform under the load of the flap movement to achieve the deployed configuration. The use of an elastomeric link structure presents a great stiffness and deformation discontinuity where the FLEXSEL structure meets the otherwise conventional airframe structure. Many integration options were considered to facilitate joining the two structure types and bridging this discontinuity. All concepts centered on integral “elastomeric attachments” of the FLEXSEL structure encased within fittings. Options considered were with regard to creating a strong and durable structural joint within the fittings. The approach that was selected leveraged the incompressibility of the elastomer to strengthen the joint. The embedded conical inserts push the elastomeric material away circumferentially as load tends to liberate the elastomer from the fitting. The incompressible, volume-conserving nature of the elastomer generates compressive stress between the inserts and fitting walls, where mating tongue-and-groove features also work to retain the elastomer.
Development of the concept and study of its feasibility required complex finite element analysis of evolving concepts. Analysis was very challenging because of material and geometric nonlinearities, very large deformations, material and deformation discontinuities, atypical material responses, and intricate geometric detail. Approaches to overcome these challenges were presented. The requirement that was found to be the most difficult to satisfy was the deformation-induced force, or the force required to reconfigure/deform the FLEXSEL, which has significant implications for the airframe and flap actuator (added weight, etc.).