The key to Orville and Wilbur Wright’s historic 1903 flight was wing warping. Today, NASA engineers are developing a similar technique to increase the performance and efficiency of fixed-wing aircraft.
The goal is to be able to bend and fold wings in-flight without using complex hydraulic systems or mechanical linkages. In the future, subsonic and supersonic aircraft equipped with the technology may weigh up to 80 percent less than aircraft that rely on traditional wing flaps.
The Spanwise Adaptive Wing (SAW) project aims to validate the use of a cutting-edge, lightweight material to fold the outer portions of aircraft wings and their control surfaces to optimal angles in flight.
Working with Boeing Research and Technology, NASA engineers are experimenting with shape memory alloys (SMAs). The nickel-titanium alloy can be trained to return to a desired shape after deformation by applying heat.
“SMAs are functional materials that can produce ‘work energy’ via a reversible, solid-state phase transformation activated by thermal or mechanical stimuli,” says Othmane Benafan, Ph.D., a materials engineer at NASA Glenn Research Center. “Reversibility gives SMAs their unique ability to be used in places where structures need to morph, reconfigure or adapt to new shapes as temperatures or loads vary.
“In doing so, SMAs can provide large reversible deformation against large forces that can be used to move things like actuators,” explains Benafan. “By applying a temperature stimulus, you can trigger a physical change in the metal.
“It undergoes a reversible phase transformation much like ice melting and refreezing,” adds Benafan. “The difference is, it transitions from one solid state to another. The changes that happen at the atomic level are reversible, meaning the SMA is designed to bend and then return to its original shape once heat is applied.”
However, SMAs currently have limited capabilities. For instance, they can only be operated at or near room temperature. The material NASA is developing is similar to these alloys, but with increased capabilities, higher operational loads, higher operating temperatures and energy density.
“[Our] material has more predictable properties and can be accurately controlled, making it well-suited for aerospace applications,” claims Benafan. “It is also unique in regards to memory or training, because the rare microstructural features produce a better, more stable material.”
According to Benafan, SMAs can be bonded, bolted and, in some cases, welded to conventional materials like aluminum, titanium and carbon-fiber composites. “Just like any other design, [it] requires knowing the material properties to account for system level assembly such as yielding, thermal mismatches, stress concentrations and compatibility,” he points out.
For the SAW project, Benafan and his colleagues are using SMA materials as torque-tube actuators. In this configuration, a group of trained SMA tubes are heated via internal heaters or external electrical coils, triggering them to twist and perform the desired actuation to drive a folding wing.
“We are now using a F/A-18 wing as a test article to demonstrate the actuation concept at a much larger scale compared to what we have now, which is close to a few hundred inch-pounds,” says Benafan. “We need to understand if scaling up is feasible from all aspects, including material performance, work densities and control of the actuators.”
When activated, the wing actuators will heat up and twist to move the 300-pound section over a 180-degree sweep. That can be 90 degrees from the flight-ready position to the vertical folded position, as well as moving 90 degrees down. The NASA engineers also want to demonstrate actuation to any position desired within that 180-degree sweep.
To see a video of the SAW project, click here.