Wing warping makes its return

Kevin Petersen, Director of NASA Dryden Flight Research Center, introduces the FIA-18 aircraft modified with the Active Aeroelastic Wing.

A high-tech derivative of wing warping, a control technology pioneered by the Wright brothers almost a century ago, is under development by The Boeing Co., NASA, and the U.S. Air Force Research Laboratory. Called the Active Aeroelastic Wing (AAW), the concept uses new highly sophisticated flight control software, in conjunction with a highly modified wing that actually bends and twists to maneuver and enhance performance. Because such wings would require fewer moving parts for controlling flight, thinner, higher aspect-ratio wings would be possible, resulting in reduced aerodynamic drag, which would allow for greater range or payload and improved fuel efficiency.

The Wright brothers developed what is known as wing warping to achieve flight control on their early aircraft. The twisting and warping of wing surfaces later gave way to ailerons, flaps, and leading-edge slats that evolved to their common form on aircraft today. With the subsequent development of computer software for precision flight-control techniques, it is now possible to re-examine the twisting and warping flight-control method on a high-performance aircraft.

The AAW F/A-18 underwent wing torsion testing at NASA Dryden's Flight Dynamics Laboratory last year.

Project goals for the $41 million AAW program include investigating the use of the lighter-weight flexible wings for high-performance military aircraft and demonstrating aircraft roll control through aerodynamically induced wing twist on a full-scale aircraft. The test aircraft—an F/A-18A obtained from the U.S. Navy—has been modified by Boeing Phantom Works in St. Louis, with additional actuators, a split leading-edge flap, and thinner wing skins that will allow the outer wing panels to twist up to 5°. The traditional wing control surfaces—trailing-edge ailerons and the outboard leading-edge flaps—are used to provide the aerodynamic force needed to twist or "warp" the wing. Project engineers hope to obtain roll performance at transonic and supersonic speeds close to that of production F/A-18s, without deflecting the horizontal tails and with smaller control surface movements. Following extensive systems tests and simulation, the AAW F/A-18A will begin first-phase flight-testing by mid-year. The flight-test results will provide guidance for future aircraft designs.

The F/A-18's modified wings underwent extensive loads testing at NASA Dryden's Flight Dynamics Laboratory last year. The six-month structural loads effort included wing twist or torsional testing and extensive loads calibration testing at up to 70% of the design limit load of the airplane, with load distribution over the wings a particularly critical item.

Prior to wing torsion testing, the AAW F/A-18 was subjected to structural loads testing.

Following ground vibration tests and various checkout procedures, the two-phase AAW flight-test program is slated to begin with parameter identification flights in July 2002. Boeing's Phantom Works will use data obtained from the first series of flights to refine wing effectiveness models and design AAW flight control laws. The second phase of research flights to demonstrate the AAW concept with effective control laws should take place in mid- 2003, almost 100 years after the Wright brothers' first powered flight on December 17, 1903.

- Frank Bokulich

Taking the strain

Aerospace fatigue testing usually requires a continuous cyclic application of forces over a considerable time span to replicate loads produced in flight. A problem may be that just as components under test suffer fatigue, so do the hydraulic actuators doing the testing. Thames Side-Maywood has developed a range of load cells for controlling forces in high frequency testing and other repetitive applications designed to provide long-term stability and to give consistent forces throughout a test program. The company explains that unlike conventional cells that have a low resistance to bending moments, its VC7600 multi-shear web design is resistant to off-axis forces. The company says that if there is initial misalignment during installation or if the hydraulics become misaligned, the load cell is still able to provide consistent and accurate results. The VC7600 can be fitted with a base plate, avoiding the need to mount the system on a flat floor or level surface. It can be produced in stainless steel or alloy steel to meet specific applications, and capacity range is 2 to 1000 kN.

- Stuart Birch