The Defense Advanced Research Projects Agency (DARPA) is seeking proposals for the Small Business Innovation Research (SBIR) program focused on "Turbulent Boundary Layer Drag Reduction via Surface Actuators." The objective is to develop and flight test innovative solutions that actively modify turbulent flows to reduce drag on aerospace vehicles, thereby enhancing their performance in terms of range, endurance, speed, and payload. This initiative is particularly significant as even minor reductions in drag can lead to substantial improvements in operational capabilities, with potential applications in both military and commercial aviation. Interested parties should note that the solicitation is currently in the pre-release phase, with proposals due by June 25, 2025, and further details available at the official SBIR website.

## Description

DARPA seeks proposals for the purpose of demonstrating turbulent boundary layer drag reduction in flight using active modification of turbulent flows. Drag is a fundamental force that can limit the capabilities (e.g., range, endurance, speed, payload, etc.) of aerospace vehicles and weapon systems. Even a slight drag reduction can significantly improve system performance. Many drag reduction methods have a weight or energy penalty that offsets their benefit but approaches such as plasma-based drag reduction techniques show promise to simultaneously reduce drag and provide net power savings. Extensive research and experimentation have demonstrated that devices like plasma actuators can induce controlled perturbations in the boundary layer of simplified wind-tunnel configurations. We seek to extend these techniques to flight. Proposed approaches should seek to actively intervene in the autonomous cycle involving the lift-up and break-up of coherent streamwise vorticity that is associated with the wall &ldquo;streak structure&rdquo; first observed by Kline et al. (1967). Since this structure is linearly correlated with the wall skin friction, DARPA envisions innovative solutions that provide a net system performance benefit. Research has shown that the introduction of a mean spanwise velocity component in the near-wall region of the boundary layer can have a dramatic effect on drag. Flush, surface-mounted plasma actuators designed to produce either unidirectional spanwise near-wall velocity component or spanwise opposed wall jets have been shown to be effective. Proposals should demonstrate an understanding of boundary-layer-based scaling relations for the design of actuator array parameters to influence performance (e.g. spanwise spacing and induced velocity).

## Objective

Develop and flight test turbulent boundary layer drag reduction using surface actuators. The proposal must identify an actuator system that can be verified to reduce turbulent boundary layer drag in flight. The proposed approach to a flight demonstration must clearly outline an achievable path from actuator development to wind-tunnel and flight test validation at relevant conditions.

## Phase I

This topic is soliciting Direct to Phase II (DP2) proposals only. Phase I feasibility must be demonstrated through evidence of having demonstrated DBD drag reduction at laboratory scale. Proposers must establish their credibility in understanding the physics of drag reductions using such devices. They should be able to show that they can conduct such tests at scale in flight. They must make a convincing case that they understand how to transition laboratory research to a flight test environment.

## Phase II

DARPA envisions an approach which includes: <ol><li> Design optimization of an actuator array for a selected test article/aircraft </li><li> Development of closed-loop actuator control based on vehicle flight characteristics </li><li> Integration of actuators and drag measurement methodology on test article </li><li> Evaluate flight demonstration opportunities for candidate technology</li><li> Complete initial flight-testing validation</li><li> Conduct extended flight test campaign and fully document actuator performance</li></ol>Phase II fixed payable milestones for this program should include:<ul><li>Month 1: Identification of relevant target aircraft to enable a flight test of the actuator array.</li><li>Month 4: Design of actuator array to enable fabrication and flight test within the available program resources.</li><li>Month 9: Fabrication of candidate DBD array and document functionality in the lab.</li><li>Month 12: Install DBD array and provide initial documentation of ability to operate it on test aircraft.</li><li>Month 14: Complete ground test activities to verify functionality of the DBD array on the test aircraft.</li><li>Month 16: Complete of initial flight test campaign with sufficient flight test data to validate predicted performance. Flight tests should assess drag reduction throughout the selected flight envelope.</li><li>Month 18: Documentation of DBD performance with direct comparisons and assessment of relevant to aircraft without drag reduction technology.</li></ul>6 Month Option fixed payable milestones for this program should include:<ul><li>Month 22: Conduct flight test extension to explore DBD performance over an extended flight envelope.</li><li>Month 24; Fully document actuator performance across the extended flight envelope.</li></ul>

## Phase III: Dual Use Applications

Follow-on opportunities will include continued testing of the developed test article as well as implementing the process for designing and testing a system on a full-scale transport aircraft. It is anticipated that significant fuel saving can be achieved with an appropriately designed system. Once proven, the approach developed under this effort can be applied to a wide range of future commercial and military applications.

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