Humanity may be divided on a great many issues, but most would agree that it would be very cool to have airplane wings made of shapeshifting metal. The geometry of those fabulous foils affects virtually every aspect of flight, and making them from metal that can change its shape in midair could make your journey smoother, safer, and more efficient.
Shapeshifting wings aren’t new. The Wright brothers flew over the North Carolina sand dunes using a hip brace and wires to warp the Flyer’s cloth-and-wood wings. Modern aircraft achieve the same results—OK, much better results—with mechanically driven flaps, slats, ailerons, spoilers, elevators, and rudders.
“If you look at conventional aircraft technology, you have so many moving parts,” says Othmane Benafan, an engineer at NASA’s Glenn Research Center. Those moving parts are essential—they are how pilots steer, reduce turbulence, take off, land, and basically do everything else besides glide aimlessly. But the actuators, cables, motors, lubricant, hydraulic gear, and other bits needed move those parts around take up weight and space—precious resources on any aircraft.
The alternative is to move those wing parts using shapeshifting metals. Or, as they’re known to engineers, shape memory alloys. “Parts made from shape memory alloys are typically 10 to 20 percent the size and weight of a conventional part,” says Jim Mabe, a shape memory alloy guru at Boeing. For an industry that spent $133 billion on fuel last year, anything smaller and lighter is exciting news.
Shape memory alloys are essentially reversible Shrinky Dinks. When heated to certain temperatures, they shrink, twist, and bend. Cool them off, and they return to their original shape. Hot, cold, hot—shape memory alloys can cycle back and forth millions of times without wearing out. All you need is the ability to generate heat or pull it from some other, already spicy hot part of the plane, like the engine.
Aircraft makers, researchers, and government agencies like NASA can use these metals to do more than just reduce fuel bills. Shape memory alloys can also be used to add moving parts to a plane, doing things that would cost too much in size and weight using conventional mechanics. For instance, quieting a jet engine’s roar. Temperature activated fold-up wings would allow aircraft carriers to cram more fighters on deck. This tech might even quell sonic booms, opening the door to the revival of supersonic passenger jets like the Concorde.
Down Shape Memory Lane
Shape memory alloys were developed in the aerospace field, though not for flight, per se. In 1959, at the Naval Ordnance1 Laboratory, a researcher named William Buehler was developing materials for intercontinental ballistic missile nose cones that could endure the extreme temperatures and pressures of doing missile stuff like flying to the edge of space, then reentering the atmosphere. Buehler came up with an alloy of nickel and titanium that was not only strong and fatigue resistant but also super malleable at high temperatures. He discovered the alloy’s most surprising feature by purposely dropping bars of the stuff on his shop floor and listening to the thuds. (I know, these are the kinds of stories people should be telling STEM-shy kids.)
The cool bars made a very different noise than the bars still warm from the furnace, indicating to Buehler that the molecules could be in different orientations at different temperatures—not a common property of metals. Later one of his colleagues held his lighter under an accordion-like strip of the alloy. To everyone’s surprise it completely unfolded, indicating that the heated molecules were doing more than just expanding in response to the heat; they were completely changing the orientation of their bonds. These alloys change phase, but not from, say, solid to liquid. They’re changing from one solid phase to another—like ice turning into a different kind of ice.
Since that discovery, engineers have gotten rather good at training the shape memory structures to produce predictable movements at precise temperatures. And they’ve come up with all sorts of shape memory materials—even some plastics. However, Buehler’s original shape memory alloy of nickel and titanium (called nitinol) is still quite popular. Most of the materials innovation these days is making the alloy more fatigue resistant and tweaking the nickel to titanium ratio to better control the alloy’s range of temperature response.
For example, Benafan makes long, hollow nitinol tubes, which he uses to replace the hinges for various movable wing parts. He is testing his tubes on winglets, those upturned bits you see at the end of many plane wings. Eventually Benafan says, these actuators should be able to move those 300-pound sections of wing up or down 180 degrees, giving pilots another tool to stay stable when turbulence hits. In October Benafan started installing his shape memory tubes inside a hollowed out F/A-18 fighter jet wing, part of longer-term tests on how nitinol-powered wings perform in various flight conditions.
The possibilities go beyond the wings. Several years back Jim Mabe and his Boeing colleagues invented a system to reduce turbine engine noise. Called the variable geometry chevron, it was essentially a huge donut of additional fairing that fit around the outer exhaust of a turbine. The aft end of that fairing zig-zagged—think of the V pattern on Charlie Brown’s shirt. Each V came bolted with thick strips of shape memory alloy. When heated, the shape memory alloy would bend, and the tips of those Vs would dip into the exhaust stream, adding just the right amount of turbulence to the hot air to reduce noise. “The more you heated the shape memory alloy, the more it would bend, so we could put more of an angle in the V when we were in different air conditions,” says Mabe.
Noise is a particularly vexing aviation problem, especially for supersonic flight. Despite what you’ve heard from Kenny Loggins, most trips to the danger zone involve little MiG dodging and lots of paying for costly noise violations called in by the curmudgeons living under your flight path. “Aircraft in supersonic flight experience shockwaves all along their surfaces,” says Darren Hartl, an aerospace engineer at Texas A&M University. These begin at the nose, and happen again at every discontinuous surface of the aircraft where air comes together sharply. These numerous shockwaves are separate at first, but as they head towards the ground they coalesce. The combined energy of all those shockwaves is how you get the boom.
Aircraft engineers try to design supersonic planes to minimize the boom, but shock waves behave differently depending on variables like temperature, humidity, and barometric pressure, so there’s no one-shape-silences-all solution. Shape memory alloys would allow engineers to design a plane with a profile that changed in response to these variables, adding a few extra milliseconds between the shockwaves. “Then you don’t get a boom, you just get a few soft little bumps,” Hartl says.
Yes, conventional motors could do the same work, but they are just too bulky to make the effort worthwhile. “The one bold thing to remember about shape memory alloys is they have the highest work density per volume of any actuator,” Hartl says. Nothing else packs more power per pound.
Hot and Cold
So what’s holding these miracle materials back from revolutionizing aerospace or reviving the Concorde? You guessed it: Government conspiracy.
Juuuust kidding. It’s your old pal bureaucracy. The Federal Aviation Administration tightly governs how planes are built—thus aviation’s near impeccable safety record, thus a very slow rate of change. Until recently, the FAA didn’t even have a certification standard for evaluating shape memory alloys. But a few years ago, Mabe and Hartl organized a group of shape memory material enthusiasts from NASA, the military, academia, and industry. The standard they developed, which outlines testing criteria for every conceivable facet of shape memory alloys, was only approved this year.
Technical challenges remain, of course. The major one is temperature. Shape memory alloys change phase in relatively restrained ranges, and planes don’t do restraint. “They go from -50˚ C at cruising altitude to upwards of 40˚ C on the ground,” says Hartl. (That’s roughly -58 to 104 Fahrenheit.) So he, and many others, are working on ways to keep these metals at just the right temperature, no matter what’s going on outside.
Another temperature-related issue is controlling how long it takes for shape memory alloys to heat and cool. Think about the time it takes to heat up or cool down your metal pots and pans. “That ought to give you some intuition how difficult this problem is to solve,” Hartl says. Unless engineers figure out some end run around these thermodynamics, shape memory alloys won’t be be used for primary control systems—the ones pilots rely on to have instantaneous feedback during complex flight ops like takeoff and landing.
Still, that leaves plenty of applications for noise reduction, tuning wing shape in flight due to air conditions, reducing drag, and other secondary controls. Hartl and other experts believe the first shape memory alloy controls will pass through the FAA’s compliance regulations and onto real world planes within a decade. And sure, this stuff might never lead to aircraft that transform into robots and battle over the fate of humankind. But then again, humanity is already facing plenty of nonrobot challenges, so just be happy with lighter, smaller flaps, OK?
1 No, we didn’t accidentally lose an ‘i’ between the ‘d’ and the ‘n’. Per language site Grammarist: “Although ordinance and ordnance now share no definitions, they both come from the Middle English ordinaunce, meaning to set in order. A third word, ordonnance, which still appears occasionally, began as a variant of ordinance but has since taken a meaning of its own—namely, the arrangement of parts in a building, picture, or literary work.” Though some military historians might argue that the Navy’s mid-century R&D efforts were in the service of setting certain things to order, the proper spelling of the Naval Ordnance Laboratory implies they were simply building things that go boom.