In todays 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.
Benefit: The development of an effective riblet lifecycle, including fabrication, deployment, maintenance, and removal/reapplication processes, would have a large impact on the aircraft industry. The resulting drag reduction afforded by riblets can be used for decreasing fuel consumption and CO2 production as well as improving aircraft range or endurance. While increased range and decreased fuel burn have application to most transport aircraft, increasing aircraft endurance is of particular interest for time on-station activities such as those performed by the Navys P-8 Poseidon aircraft. While the military could benefit significantly from the drag reduction provided by riblets, it is the commercial sector that would be the larger market. Cargo and passenger aircraft alike would benefit from decreased fuel burn. With the number of commercial aircraft approaching 8000 in just the U.S. and with the size of the fleet expected to continually grow over the next several decades, the importance of drag reduction cannot be overstated. Other developments such as the potential electrification of aircraft would also hugely benefit from lower drag aircraft. As a result, it is believed that the demand for drag-reducing coating should be strong, and thus an entirely new discipline in the aircraft industry would develop should a suitable solution for the riblet lifecycle be developed.
Keywords: Drag Measurement, Drag Measurement, application, fabrication, surface metrology, Drag Reduction, Riblets
