N222-114 TITLE: Modern Integration/Application Techniques for Resilient Riblets
OUSD (R&E) MODERNIZATION PRIORITY: General Warfighting Requirements (GWR)
TECHNOLOGY AREA(S): Air Platforms; Materials / Processes; Weapons
OBJECTIVE: Develop methods to produce accurate riblet profiles in outer mold line (OML) surfaces that yield significant drag savings (> 5%), require little or no maintenance or cleaning, are inexpensive to apply or to include in production or normal maintenance, and achieve long useful life (> 5 years), yielding fuel cost savings and extended range for USN aircraft.
DESCRIPTION: Riblets are inverted V-shaped grooves that have been proven to reduce viscous (friction) drag approximately 5 to 8%. The inverted groove patterns have heights on the order of 50 microns with the width typically equal to or less than the height, and can be adjacent to one another or spaced laterally to maximize performance. Drag reduction is optimal when they are flow aligned, but performance is tolerant of misalignment up to 10 to 15 degrees. Moreover, riblet profiles may be constant or three-dimensional, with variable peak heights and/or groove direction.
Prior efforts to implement riblets on commercial aircraft focused mainly on plastic films and suffered from high initial cost and short lifetimes, thus negating economic benefits. This SBIR topic seeks development of a system for accurately producing a variety of riblet-like shapes into the OML of USN aircraft. It must be cost-effective so that the fuel saved due to drag reduction is not significantly offset by production cost. Likewise, the resulting OML should be maintainable and have long life (> 5 years). The prototype system can be a film but must be compatible with Navy requirements and durable in the maritime environment. A prototype may be developed that produces the final shape in the paint/topcoat. This can be done with photo-curable paint or rapid curing of shaped paint; alternate means of production are encouraged. Compatibility with Navy topcoat requirements must be considered.
Drag-reduction performance is sensitive to geometric features of the riblets. Height and spacing within 10% of the desired design are sufficient, but height and spacing should not vary rapidly in the streamwise direction from design specifications. The peak of the profile must be sharp. Radius values should not exceed 5% of the riblet height. The system should allow production of the riblet shapes in the local flow direction when the aircraft is flying at best range, cruise conditions. This could be accomplished through smooth changes in the riblet direction to match known or predicted local flow direction or step changes, so long as the profile alignment can be maintained with the nominal flow direction within 10 degrees.
PHASE I: Define and develop a concept for a system to produce riblet shapes in the OML of USN aircraft that can meet the performance requirements listed in the Description. Perform high level modeling that demonstrates the feasibility of the manufacturing concept and clearly defines a path to meeting the requirements outlined in the Description. Based on the modeling results or initial prototype testing, develop plans for a Phase II prototype that is expected to meet the requirements.
PHASE II: Produce prototype hardware based on experiments or modeling results and initial plans created in Phase I. Demonstrate production of riblets with the prototype system. Depending on technology maturity, perform riblet production demonstrations that could focus on both conventional and/or more complex three-dimensional geometries for improved performance. Production demonstration can be done on flat coupons as small as 12"x12", though scale-up issues should be considered. Validate that the riblet geometry produced by the prototype system meets the requirements in the Description. This could be done with laser profilometer or scanning electron microscope measurements. Conduct low-speed wind tunnel testing or other low-cost drag testing. Measure the aerodynamic drag reduction achieved with the completed coupon or multiple coupons. Complete larger panel testing and subsequent wind tunnel testing at flight conditions that match those of Navy aircraft flight profiles, focused on cruise conditions. Develop plans for integration of the prototype into a system for creating large areas of riblets on surfaces with complex curvature. Integration issues should include consideration of aircraft surface normals that may have any direction relative to gravity (e.g., upper surfaces, lower surfaces, and vertical surfaces).
PHASE III DUAL USE APPLICATIONS: Integrate the prototype from into a system for application to large surface areas with complex curvature. Maximum aircraft surface area coverage is a goal, but 100% coverage is not expected or required. The prototype system should be designed to cover sufficient area of a Navy aircraft to produce measurable drag reduction. Deliver a prototype to the Navy for production of riblets to use on a flight test aircraft.
Reynolds number and Mach number at cruise conditions for Navy aircraft and commercial airliners are very similar. As an example, the P-8 Poseidon operated by the USN is a derivative of the Boeing 737 commercial airliner, which is one of the workhorses of the current commercial aviation fleets worldwide. Benefits to the commercial sector would be similar, if not greater, to the benefits to the Navy. Commercial and military ships may also benefit as riblets can be applied to reduce the friction drag produced by a ship moving through the water, though maintenance issues are expected to be more difficult and OML requirements will be significantly different.
KEYWORDS: riblets; drag reduction; photo-curable paint; photo-curable film; increased range; tactical aircraft
** TOPIC NOTICE **
The Navy Topic above is an "unofficial" copy from the overall DoD 22.2 SBIR BAA. Please see the official DoD Topic website at rt.cto.mil/rtl-small-business-resources/sbir-sttr/ for any updates.
The DoD issued its 22.2 SBIR BAA pre-release on April 20, 2022, which opens to receive proposals on May 18, 2022, and closes June 15, 2022 (12:00pm est).
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