Active Flow Control X-Planed

The Defense Advanced Research Projects Agency (DARPA), a secretive research and development division of the United States Department of Defense tasked with weaponizing inchoate technologies, has awarded Boeing subsidiary Aurora Flight Sciences a contract to build a full-scale demonstration aircraft under the auspices of the agency’s Control of Revolutionary Aircraft with Novel Effectors (CRANE) project. Subject program aims to design, build, and flight-test a novel X-plane that incorporates Active Flow Control (AFC) as a primary design consideration.

Active flow control is an aerodynamic technique that eliminates or mitigates the need for conventional flight-control surfaces by utilizing actuators and effectors to alter airflow over an aircraft for purpose of changing its course and speed. Unlike passive control features the likes of fences, vortex generators, and chevrons, which function in perpetuity in the presence of relative wind, AFC devices—such as sweeping jet actuators and plasma emitters—function on demand, adding energy to boundary layers to maintain, improve, or recover aircraft performance.

Since Prandtl’s 1903 discovery of the boundary layer, AFC actuation methods have included suction, blowing, mass flux actuators, plasma actuators, and fluidic oscillators. Wind tunnel and in-flight testing have demonstrated such technologies are capable of improving aircraft performance by reducing and/or eliminating boundary layer separation and increasing airflow circulation. Integrated into conventional aircraft designs, AFC would theoretically decrease engine fuel consumption and increase airframe efficiency by occasioning smaller, lighter, simpler, lower drag control surfaces—e.g., ailerons, elevators, rudders, slats, flaps, etc.

A preliminary design of Aurora’s X-plane was presented in the first phase of the CRANE project. The contraption’s architecture was vetted by Aurora and Boeing engineers who spent four-weeks gathering 14,000 data points from a quarter-scale model containing 11 movable conventional control surfaces complemented by 14 AFC banks with eight fully independent controllable AFC air supply channels. Over 8,800 of the data points were collected and used to develop the plane's control laws.

To characterize active flight control performance across the X-plane’s operational envelope, Aurora engineers then incorporated the aforementioned test data into a computer model of the vehicle.

The CRANE project’s second phase saw Aurora personnel create a more detailed X-plane design, complete with flight software and controls. Phase two culminated in a critical design review of a full-scale 7,000-pound, thirty-foot wingspan demonstration aircraft.

In exchange for its expertise and efforts, Aurora Flight Sciences earned $42.4-million—a modest sum by the standards of the American Industrial Defense Complex, but money enough for several long weekends in Tijuana.  

An optional third phase will see the building of a functional aircraft, control of which (pitch, roll, and yaw, up to a speed of Mach 0.7) will be predicated upon AFC mechanisms. If green-lit, flight testing of such an aircraft would commence in 2025.

Aurora Flight Sciences vice president Graham Drozeski remarked: "Given all that we have learned about AFC and its application to tactical aircraft in prior phases of CRANE, the next step is to prove out these learnings in flight.” Mr. Drozeski added that Aurora’s X-plane was specifically designed to explore AFC tech at scale, with real-world Reynolds and Mach numbers.

In fluid mechanics, the Reynolds number (Re) is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces.

DARPA set forth that the AFC X-plane would feature a modular configuration conducive to the integration of future technologies.

CRANE program manager Richard Wlezien—who contributed to the DARPA-funded AFC download reduction of Bell Boeing’s V-22 tilt-rotor—posited that as the wing and AFC effectors of Aurora’s X-plane are modular, the aircraft could endure as a national test asset long after the CRANE program concludes.

FMI: www.darpa.mil