Tech Briefs

A research team uses synthetic jets to manipulate the wake behind an external pod.

AFRL engineers, collaborating with aerospace manufacturers and other Air Force groups, recently demonstrated the first-ever airborne active flow control system when they manipulated the airflow behind an F-16 external pod. They significantly altered the turbulent wake using small, electrically controlled, piezoelectric synthetic jet (PESJ) actuators. This demonstration is just one part of AFRL's multiphase Aeroelastic Load Control program aimed at reducing the weight, complexity, and signature of air vehicles through the introduction of active control technologies.

The addition of active structural control reduces structural component weight because rather than adding structural mass to increase structural strength, it uses integrated, electrically controlled actuators to compensate for excessive stresses. To further reduce structural response, active flow control also modifies airflow before it impinges on the structure—preemptively reducing the load rather than altering structural components. The incorporation of active control technologies into air vehicles may eventually reduce, or even eliminate, control surfaces. These new capabilities will thus prompt a paradigm shift in air vehicle design.

ImageIn partnership with the US Air Force Test Pilot School, the Air Force Institute of Technology, General Electric, and Lockheed Martin, AFRL developed the concept of an "airborne wind tunnel" for the sole purpose of efficiently evaluating various active control technologies. The evaluation of the PESJ actuator system marked the first test of this concept. These firstgeneration actuators ingest boundary layer air and then expel it into the flow field at such a high frequency and velocity that the flow appears as a continuous jet. Each disc-shaped PESJ is approximately 3 in. in diameter and less than 0.5 in. thick. Although the researchers have not completely analyzed the voluminous amount of data collected, their initial results are very positive. Using only six PESJs in conjunction with available aircraft power, the PESJ system successfully reduced the buffeting behind the F-16's Low-Altitude Navigation and Targeting Infrared for Night (LANTIRN) pod (see figure on previous page).

In the next phase of the effort, researchers will evaluate a piezoelectrically actuated structural control system integrated into the F-16's ventral fin. In future efforts, they will assess other flow control actuator concepts and, based on the results of these nearterm endeavors, will develop synergistic design concepts employing active flow and structural control technologies to optimize system performance. Improved aircraft/stores integration is one possible application of this synergistic technology. Active flow and structural control promises to reduce the transonic buffeting that affects external stores, enabling engineers to expand aircraft operational envelopes and eliminate certain operational restrictions.

Due to the steadfast commitment of each participating team member, the use of the airborne wind tunnel concept to study active flow controls was an unqualified success. The dedication, tenacity, and collaborative spirit of the team overcame many obstacles in achieving this significant accomplishment.

Maj James Rogers and Mr. Leonard Shaw, of the Air Force Research Laboratory's Air Vehicles Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn/index.htm. Reference document VA-H-05-09.