How differential deflection of the inboard and outboard leading-edge flaps affect the handling qualities of this modified F/A-18 was evaluated during the first check flight in the Active Aeroelastic Wing AAW Program. NASA Photo / Carla Thomas

AAW project team meets goals, limits risks and challenges

Jay Levine
X-Press Editor

As the Active Aeroelastic Wing (AAW) project team explores new technologies on a highly modified F/A-18, completing their work safely is job one.

AAW is investigating a lighter-weight, more flexible wing for improved maneuverability of future high-performance aircraft. Researchers seek to validate that aerodynamically induced wing twist on the modified F/A-18 can be used for aircraft roll control. It revisits the primitive wing-warping flight control system devised by the Wright brothers for their 1903 Wright Flyer.

Before the project began proving itself in the air, the AAW project team mitigated every potential risk it could envision. The project went through a vigorous multi-stage design, development test and review before flights began so that project team members could under-stand the unique challenges and limit risks.

Dryden's AAW Project Manager Denis Bessette and AAW Chief Engineer David Voracek explained that the first effort is placed toward designing out problems. Testing of developed systems to confirm the design was met is the next step.

Simulation of the design in flight is performed to get increased confidence that the design will meet expectations. Reviews are conducted with engineers and technicians within the project and outside the project, so there is a wide input of ideas and experience applied to all our design and operating challenges.

"The wings aerodynamic control surfaces are used in an unusual manner. Their deflection at high speeds will create forces that will cause the wings to twist. We had to be sure that this would not cause a structural overload and that adequate safety features were available to recover from any failure," Bessette said.

A main component of the AAW modification is an independent drive system for the outboard leading edge flap. It allows the outboard leading edge flap to move independently of the inboard and at five times the original rate. This makes it a very effective tool for creating wing twist and controlling the twist, Voracek said.

The second modification element is the flight control system changes to control and operate the wing flight control surfaces in an unusual manner to induce wing twist.

"The flight controls system design has the same redundancy as a standard F/A-18. We tried to design out problems at the start. We made it so that no single failure could cause a loss of aircraft," Voracek said.

Designing out a problem requires an understanding of the dangerous failures up front, and designing the hardware and software to continue operating despite failures or fail in such a manner that the aircraft is still controllable and can be flown home.

Concerns about the forces the wings would endure in flight were mitigated through ground tests. These tests included some of the most extensive wing loads testing ever performed at Dryden. The wings loads tests included both bending loads and twisting loads. The forces to bend and twist the wings were measured and compared to prediction, to help prove that we had valid load prediction techniques for flight.

"The extensive loads calibrations ground tests demonstrated that the bending forces and the twist wings forces matched predictions. Over the next few months, we will build up flight forces and see how they correlate to the models," Voracek said.

Since this was a new team and included a large number of younger engineers that had little or no flight test experience, project managers also decided to perform extensive preflight ground support team training. Extensive control room training was used on the project and a pilot round table discussions and instruction was performed.

The control room was used to simulate a mission in conjunction with the AAW flight simulator. Both normal and emergency flight actions were performed. Pilots and engineers reviewed emergency procedures and processes at a roundtable. At these sessions the pilots also provided instruction on the F/A-18 normal and emergency performance.

Control room specialists also worked to meet the specific demands of engineers and project personnel who requested the 1,600 independent parameters during flight. The unique displays had to exhibit the most information with the least amount of clutter.

"During flight you can't watch 1,600 pieces of information," Voracek said, noting the hard work by the control room staff in determining how to display the safety of flight and mission parameter information in a straight forward manner.

"Simulating flights helped us build coordination and confidence and the first flight went without a hitch," Voracek said.

Preflight checks of the aircraft and its systems also ensure safe flights. Every flight control surface and system are checked. Again, the drive system that controls the leading edge flaps, including both the inboard and outboard flap drives, are checked as they are driven individually rather than as a single unit in regular production F/A-18 aircraft, Voracek said.

Dryden AAW partners include the Air Force Research Laboratories Air Vehicles Directorate, Wright-Patterson Air Force Base, Ohio, and NASA's Office of Aerospace Technology. Additional partners include the Boeing Phantom Works of St. Louis, which performed the AAW F/A-18 modifications; Moog in Torrance, Calif., who developed the leading edge actuators; and BAE Systems in Binghamton, N.Y., built the modified the flight control computer.

"Our basic philosophy is safety is integral to the project team. All engineers and technicians continuously review their activities with the concern about unintended adverse consequences. We conduct thorough tests of our work and review procedures to reduce risk and seek to eliminate errors." Bessette said.