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The objectives of testing on PTERA include the development of tools and vetting of system integration, evaluation of vehicle control law, and analysis of SAW airworthiness to examine benefits to in-flight efficiency. (Source: NASA)

Shape memory alloys at the core of future aircraft systems

The ability to have aircraft wings fold isn’t new, and, as Boeing is proving on the 777X, the desire to do so is not going away. As with the 777X and U.S Navy aircraft, the motive behind the fold has mainly to do with space management. Currently, for the Navy this type of wing folding, or actuation, happens only on deck and serves no aerodynamic purpose. Other aircraft, like the XB-70, have folded their wings in flight with success. However, these fold systems required bulky, multipart structures, including hydraulics, pneumatics, and electric motors, which weigh hundreds of pounds and take up valuable space.

Engineers at NASA and Boeing are amongst the believers who foresee folding wings in-flight using advanced materials and technologies being a potential game-changer for future aircraft. In fact, the two organizations have teamed to develop an actuation system that uses a shape memory alloy (SMA) that will accomplish this goal using less complex, lighter, and more compact hardware than conventional systems. The SMA is an engineered nickel-titanium alloy that can be trained to return to a desired shape after deformation by applying heat.

“By applying a temperature stimuli, you can trigger a physical change in the metal,” said Dr. Othmane Benafan, a materials research engineer at NASA Glenn Research Center. “It undergoes a reversible phase transformation much like ice melting and refreezing. 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.”

Much like folding a wing, SMA isn’t a new breakthrough. It is commercially available and its unique properties make it an attractive alternative to common actuators. However, current commercial SMAs have limited capabilities and can only be operated at or near room temperature.

The material NASA is developing is like these alloys, but with increased capabilities, higher operational loads, higher operating temperatures and energy density. The material has more predictable properties and can be accurately controlled, making it well-suited for aerospace applications. It is also unique in terms of memory or “training,” because the rare microstructural features produce a better, more stable material.  

Benafan also serves as the co-principle investigator of the Spanwise Adaptive Wing (SAW) project, which is focused on investigating the feasibility of bending or shaping portions of an aircraft’s wings in-flight. For the SAW project, NASA is using SMA materials as torque-tube actuators. In this configuration, a single or 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.

Electrically induced temperature change is only one possible stimuli. SMA can also be activated by using bleed air from the aircraft’s engines or simply through the ambient temperature changes experienced during flight. This compact, lightweight application, which is also said to be “extremely quiet,” allows the entire actuator package to be attached at the wing hinge point. Conventional actuation approaches typically cannot fit in this area, leading to heavy and complex linkages or transmissions to drive a wing fold or similar aerodynamic surface.

But going from the test bench to replacing proven systems with SMA will require the development of a complete actuation and control system, and this is where Boeing’s expertise and previous experience comes in.

“We’ve done a lot of work with NASA to look at how we develop the material: how we melt it, how we forge it, how we turn it into a component, how we train it, and how we integrate it into an aircraft,” said Jim Mabe, technical fellow at Boeing Research and Technology. “We’re not only developing the SMA technology, we’re developing everything around it to simplify integration. So, when requests do come in, we don’t have to do any of the science or research. We can quickly engineer an actuator according to application requirements. The way I look at it, putting SMA into an aircraft is becoming more of an engineering problem than a science problem because we’ve made huge strides maturing the science and processes.”

Being involved in NASA’s SAW project is just one of Boeing’s endeavors into wing shaping and SMA applications. Boeing previously used SMA as part of a flight test program in 2012, where it integrated a compact SMA torque tube actuator into a small trailing edge flap on a 737-800, one of Boeing’s ecoDemonstrator aircraft.

“We believe SMA could be something that will change the industry,” said Mabe. “This technology will eventually open the door to several different development approaches. And it’s not just limited to wings and flaps; there are other potential aircraft applications we can apply this material to as well.”

While NASA prepares to integrate SMA into SAW’s subscale flight-test of the Prototype-Technology Evaluation and Research Aircraft (PTERA) later this fall at NASA’s Armstrong Flight Research Center, the team at NASA Glenn wants to get a head start on ground testing SMA actuators on a large-scale wing.

“This testing is critical as we want to move quickly from subscale into full-size demonstration in the coming years,” said Benafan. “It will allow us to better understand how SMA will work in a real wing at different loads and operating conditions.”

To do this, NASA Armstrong removed a wing section from one of its F/A-18 scientific research aircraft for testing at NASA Glenn. The F/A-18 was selected not only because of NASA’s access to the Boeing-built aircraft, but also for the wing-fold system required by the U.S. Navy.

The wing section, which was delivered to NASA Glenn in July, will have all the factory fold mechanics removed, and it will be retrofitted with a 20,000 inžlb SMA torque-tube actuator.

“We are using the 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” said 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-lb section over a 180º sweep. That can be 90º from the flight-ready position to the vertical folded position, as well as moving 90º down. More importantly, NASA wants to demonstrate actuation to any position desired within that 180º sweep.

Through the full-scale ground tests and the upcoming subscale flight test, the SAW project team is working to transform aircraft design through SMA-enhanced wing shaping. Ultimately, this shaping capability could increase aircraft performance during all phases of flight, including ground, subsonic, and, possibly, supersonic.

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