Development, Evaluation, and Testing of Functional Riblet Fabrication Approaches

In today’s aviation world, decreased fuel and emissions are desired, while increased aircraft performance is also a priority.  Since decreasing aircraft drag can address both of these issues, it has received increasing interest.  Whereas reducing pressure drag through streamlining the vehicle has long been understood, reducing the viscous drag that accompanies the flow over any aircraft has been less studied.  In the 1980s, riblet surfaces were identified as a possible means of reducing viscous drag, but the practical implementation of these fine micro-structured surfaces was not considered feasible.  With the advances in materials and manufacturing processes as well as a renewed interest in drag reduction, riblets have received renewed interest over the past decade. Here, we propose to address one of the critical issues still challenging the use of riblets today - the ability to quickly and inexpensively fabricate and apply riblets to aircraft without adversely impacting their performance.  To accomplish this, we will model and demonstrate three separate manufacturing/application processes.  The first process is a roll-to-roll process where material from a first roll is imprinted with riblets that are stored on a second roll.  The roll of riblets is then applied to an aircraft surface in a separate process.  The second process is similar to the first, but instead of storing the imprinted riblets on a second roll, the material is directly applied to the surface.  The third process is a transfer coating process, where the top coating of the surface is directly imprinted with riblets using a transfer film. While developing these processes is important, it is our experience that assuring that the riblets produced have achieved the intended geometry is critical as is testing of the final product to ensure that the desired drag reducing characteristics have been obtained.  We propose to evaluate the manufactured riblets using a confocal microscope/profilometer that can accurately measure the riblet geometry to assess its accuracy and quality.  Similarly, we plan to test the riblets in a Taylor Couette cell as well as on a flat plate in a low-speed wind tunnel to assess the riblets’ drag-reduction capability. Demonstrating the riblet fabrication/application, the riblet geometry measurement, and the riblet drag reduction measurement approaches will put us in the position to extend the work to more relevant shapes and conditions similar to those in flight.  This provides a methodical approach to mature the technology for application to commercial, transport, and military aircraft.