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Passive Coatings for Aircraft Drag Reduction



OBJECTIVE: Develop an advanced riblet system (ARS) to reduce viscous drag on medium altitude long endurance aircraft (MALE) to increase range/time-on-station (TOS). Riblet application should be fast (m^2/min), compatible with depot processes/timelines, and produce structures resistant to fouling. 

DESCRIPTION: A significant source of aircraft drag is skin friction drag; the drag caused by the friction of air against the surface of an aircraft in flight. For a typical MALE aircraft the skin friction drag is about 1/3 of the total aircraft drag. Reducing skin friction is an obvious target for increasing aircraft performance (range or TOS). MALE aircraft usually achieve their performance in part by having laminar flow boundary layers (BL) on some surfaces (e.g. wings); the skin friction of a laminar BL is an order of magnitude less than that of a turbulent BL. They are designed to have the maximum extent of laminar flow practical and achieving more would be extremely difficult. However there are technologies for reducing the skin friction of the remaining turbulent aircraft BLs. Riblets are one such technology. 2D riblets are sized (peak-to-peak and peak-to-valley) to exclude the turbulent BL flow structures (which have a characteristic spanwise dimension) from “scrubbing” a significant portion of the surface; resulting in the flight-proven 6 percent skin friction drag reduction. Since skin friction drag is about 1/3 of the total drag of a MALE aircraft a 6 percent skin friction drag is equivalent to 2 percent drag reduction at the system (aircraft) level. (Note that this is a theoretical maximum since it is not possible to apply riblets to 100 percent of the OML with a turbulent BL.) 2D riblets have been flight tested on an A320 transport aircraft with about 2/3 of the aircraft’s surface covered with riblets. The A320 is typical of most Mil and Civ transport aircraft flying today; skin friction is approximately 50 percent of the total aircraft drag. With the 6 percent reduction in skin friction drag due to the riblets the total aircraft drag reduction achieved was 2 percent (2/3 x 50 percent x 6 percent). Despite the flight-proven 2percent transport aircraft drag reduction 2D riblets they have not transitioned to Mil or Civ aircraft because 1) excessive application time (current state-of-the-art (SOTA) is an adhesive-backed applique), and 2) limited duration (1-2 years) of the drag reduction effect due to the microscopic riblet grooves becoming fouled (dirt, hydraulic fluid, etc). Both of these factors have a negative impact on ROI, preventing transition. The SBIR will address these transition hurdles. A direct contactless microfabrication method (DCM) will “print” the riblets into a photo-curable aircraft topcoat. Advanced (3D) riblets will be designed, yielding 2.5-times the skin friction drag reduction as current 2D riblets. Superhydrophobic coatings will be incorporated into the ARS to keep it clean and functional for an entire programmed depot maintenance (PDM) cycle. An ultra-precision drag balance will be developed to aid development and performance validation of the ARS. As such this SBIR will close the gap between the theoretical promise of riblet technology for skin friction drag reduction, and its practical application to the U.S. Air Force fleet. It is recognized the multifaceted aspect of this topic will make it challenging for a single small business. 

PHASE I: Investigate DCM scale-up (ref 2) by examining use of multiple LEDs and assessing robotic- or gantry-based systems for optical head movement. Assess design variables for 3D riblets (ref 3) numerically (CFD) and develop experimental validation plan. Examine coating chemistries and refine aircraft-compatible application processes to improve the durability of superhydrophobic coatings (ref 4). Design an ultra-precision balance with milli-Newton resolution. 

PHASE II: Design and fabricate a scaled-up prototype DCM system capable of applying 3D riblets to a major portion of an aircraft (wing or fuselage section) at speeds on the order of m^2/min; mature 3D riblet designs with continued CFD simulations; use the DCM method to produce and wind tunnel test the ARS, using the ultra-precision drag balance designed in Phase I; create the superhydrophobic coatings with the chemistries and application techniques identified in Phase I. Validate the durability of the coatings using ASTM tests and determine thickness to adjust riblet dimensions to compensate. 

PHASE III: Develop and commercialize a full scale DCM system capable of applying an ARS to military and commercial aircraft. The ARS will be comprised of optimized 3D riblets and Superhydrophobic coatings with effects lasting an entire PDM cycle (nominally 5 years). Commercialize the ultra-precision skin friction balance. 


1. Viscous Drag Reduction on Transport Aircraft, AIAA 91-0865.; 2. Microfabrication of Riblets for Drag Reduction, AIAA-2018-0321.; 3. Design and Testing of 3-D Riblets, AIAA-2018-0324.; 4. Designing Superhydrophobic Coatings for Aircraft Drag Avoidance, AIAA-2017-0282.

KEYWORDS: Aircraft, Drag, Reduction, Passive, Viscous, Skin Friction, Riblet, Superhydrophobic, Force, Measurement 

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