Traditional aircraft wing designs create a shape and configuration with optimal performance for a single flight condition. When the vehicle moves away from that set condition, performance can decline dramatically. For example, short, swept wings work best for fast supersonic cruise, but are not efficient when an aircraft is performing high G-force maneuvers at large angles of attack. Similarly, wide, broad wings are great for high-altitude loiter, but can't handle high speeds at lower altitudes.

"The next level is a multipoint or even an infinite point design,"says Ernie Havens, Cornerstone Research Group Inc.'s (CRG, Dayton, Ohio) chief engineer. "A single morphing aircraft could do the job of several, because it would have large, reconfigurable areas that change shape in flight, to change the mission."That reality may not be far off - at least for unmanned aerial vehicles (UAVs). Research into shape-changing designs has been conducted for at least a decade by the Air Force Research Laboratory (AFRL, Wright-Patterson AFB, Ohio), the Defense Advanced Research Projects Agency (DARPA, Arlington, Va.), by military contractors Lockheed Martin (Palmdale, Calif.), Northrop Grumman (Los Angeles, Calif.), NextGen Aeronautics (Torrance, Calif.), Raytheon (Waltham, Mass.), Vought Aircraft Industries (Dallas, Texas) and by Cornerstone Research Group.

Shape-changing concepts aren't entirely new: Birds do it, naturally, achieving remarkable in-flight control. In imitation, the Wright brothers conceived of wing warping for roll stability in their 1903 Wright Flyer. "Swing-wing"(moveable wing) designs have been employed on the Grumman F10F Jaguar; the General Dynamics F-111, which emerged from the Tactical Fighter Experimental program in the 1960s; several Russian aircraft, such as the Sukhoi Su-17; and the B-1B Lancer bomber and F-14 Tomcat, which still fly today. While most had heavy and complex mechanical pivot systems, the F-14 swing-wing is 5,000 lb/2,268 kg lighter than its original fixed-wing design and provides a real aerodynamic advantage.

How morphing works

In contrast to a mechanically-hinged swing-wing, state-of-the-art morphing implies flexibility in the wing structure itself, says AFRL's Adaptive Structures team leader Brian Sanders: "We define it as shape control for aerodynamic performance - and it has to be a significant, unconventional shape change to the structure."DARPA's Dr. Terry Weisshaar, program manager for Morphing Aircraft Structures (MAS) adds, "Large-area change and a new shape are required - changes larger than 50 percent of total aircraft area.”

DARPA began a three-phase program in January 2003 to design, build and control active, variable-geometry wing structures that can change substantially in flight, says Weisshaar. AFRL's Multi-Disciplinary Technologies Center of Excellence has been investigating the technology even longer. Both groups agree that a morphing wing requires three specific elements: 1) A movable substructure with an array of linkages, 2) flexible, compliant wing skins to cover the joints, and 3) actuators to create movement. Reliable computer control of the various elements is key for rapid and accurate change, notes Weisshaar. Some type of "activator"- in current concepts, normally heat, but potentially light or some other stimulus - starts the process; small "actuators"that push, pull or rotate in response provide the force to move the substructure; sensors detect when the correct shape has been achieved; and locking mechanisms hold the wing in the new shape. Concepts include wings that morph in chord, sweep, span and thickness.

Moving the substructure without resorting to heavy, bulky hydraulic systems is a huge challenge. Numerous research programs at AFRL, DARPA and NASA Langley Research Center (Hampton, Va.) are zeroing in on small, lightweight piezoelectric or electro-active polymer (EAP) actuators, which have the ability to transform electrical energy into motion. EAPs expand when heated with an electrical current, a phenomenon that can be exploited to create small movements throughout a structure. Sanders is careful to point out that while EAPs and other "smart"materials, such as lead zirconium titanate (PZT), can act as actuators, "we're not just going to distribute PZT patches over a wing - the idea has been tried and does not work. Our design solutions for distributed actuation have progressed, and we are now using actuators that have appropriate force and stroke.”

"There's definitely a need for new materials,"says AFRL's Jeff Baur, senior material engineer in the Materials and Manufacturing Directorate, particularly for wing skins. Stretchy elastomers like rubber, while flexible enough, are too soft to handle high-pressure air loads without some type of reinforcement to prevent puckering. "Ideally, the reinforcement could be 'turned off' during shape change so that less actuator power would be needed to deform the skin,"notes Baur. "It's a key challenge in this research.”

One promising material for flexible skins is reinforced shape memory polymer (SMP). SMPs consist of "multiphase"thermoset polymer networks that can be elongated up to 200 percent into a new shape when subjected to heat. If restrained while cooling, SMP retains the elongated shape. When reheated above a specific, tailorable "trigger temperature,"the material relaxes to original shape, due to the elastic energy stored during the temporary deformation.

The process is possible because of the controlled mix of proprietary monomers and reactive modifiers. A polymer's crosslinking density can be varied from a dense, rigid thermoset to something approaching a linear thermoplastic. The degree of crosslinking determines the strain recovery properties and the Tg of the material. CRG has, over the past several years, developed families of SMPs, trademarked Veriflex, for a number of applications and offers the material for sale. Reconfigurable wing skins have been a major focus since 2001, says Havens (see HPC July 2006, p. 40, regarding SMPs in reconfigurable tooling mandrels).

Currently working on DARPA's MAS project as a partner with Lockheed Martin, CRG has developed a reinforced SMP wing skin that can soften within seconds of heating, thanks to small, stretchy electrode heaters embedded within the carbon fiber laminate. The heated, softened skin moves in concert with the underlying substructure joint as it morphs to the desired position and shape. Once cooled in the new, stretched configuration, the polymer "rigidizes"in the new shape. Havens reports that this SMP, used in a wing designed to morph in chord (that is, elongating the distance between leading edge and trailing edge) weighs less than DARPA's baseline wing design. Recent wind tunnel tests showed that it can deliver 80 percent increased lift. With an SMP-based gel coat, the wing maintains a smooth aerodynamic surface at strains up to 100 percent.

"We're now working on a new material system that is an order of magnitude faster,"reports Havens. The new system uses light rather than heat as the activating trigger, and softening occurs in micro-seconds.