Smart materials have been incorporated into all manner of devices, from shock absorbers to steel beams, but not yet into airplanes-- for which they seem tailor-made. A smart material changes its shape in response to a stimulus, like an electric current or a magnetic field, and an airplane wing that could do that, bending or twisting itself like a bird’s wing, would be cheaper to produce and less prone to malfunction than one with hinged ailerons and flaps. Engineers at university and industrial labs across the country have spent the past few years racing to be the first to fly a plane, any kind of plane, with smart control surfaces. The race now seems to be over--won by an aerospace engineer who usually designs missiles, his team of graduate students, and a little glider named Mothra.
Mothra, named after a mothlike monster featured in Japanese horror movies, is the brainchild of Ron Barrett of Auburn University. Built of a graphite-epoxy frame with an outer shell of light plastic, it has a four-foot wingspan yet weighs a scant 5.5 ounces. One of the major criticisms of adaptive structures is that they are generally heavy, says Barrett. So I’ve built a very lightweight aircraft to prove a point--that if you do the proper design, you can make it very, very light.
Like many small gliders, Mothra is steered by its tail; the wings, unlike those of passenger planes, are immobile, with no flaps or ailerons on the trailing edge. The tail consists of two winglike horizontal stabilators, which move up or down to control the plane’s pitch, and a vertical stabilator that bends from side to side, turning the plane left or right. The backbone of each stabilator is a main spar running along the front edge. In most gliders, the stabilators pivot around their main spars, and that motion is controlled by a system of motors, gears, and wires. In Mothra the stabilators still pivot, but all that machinery has been replaced by a rear spar made of a smart material: a piezoelectric ceramic that expands or contracts when zapped with an electric current.
The rear spar is fixed to the main spar at its base (along the axis of the plane) and runs diagonally toward the tip of each stabilator. Just .014 inch thick, it is constructed of two layers of piezoceramic sandwiched around a piece of brass. If you put two of these ceramic layers together, one on top and one on the bottom of a sheet, Barrett says, you can expand the top one and contract the bottom one, and get the sheet to bend down. Currents from a battery in Mothra’s nose, controlled by radio signals from the ground, bend the piezoelectric spars. Each time a spar bends, the whole stabilator does, too, curling at the tip. Curling the vertical stabilator to the right, for instance, causes the plane to turn right; curling both horizontal stabilators downward causes the plane to lose altitude.
Losing altitude was easy for Mothra; coming in for a soft landing was trickier. After trials in wind tunnels, Barrett and his team put their creation to the test, launching it slingshot-style with a bungee cord. We crashed a lot, Barrett says. The airplane, like most lightly loaded aircraft, was susceptible to gusts. And some of the worst gusts were indoors. In some buildings, we couldn’t get the air-conditioning turned off, and as we would fly along, the plane would hit a stream of air and the gust would turn the plane upside down.
They fared better outdoors, at twilight, when the air was still. On 20 of those flights, Mothra’s operators succeeded in maintaining control over the plane’s motions and were able to bring it in for a gentle landing. The longest flight, during which Mothra flew a circular path of about 100 yards, lasted 41 seconds. Barrett points out that the first flight of the Wright brothers at Kitty Hawk lasted a mere 12 seconds.
To get Mothra to stay aloft longer, Barrett has since started experimenting with a configuration that includes two ounces’ worth of motor and propeller. And his next plane (named Rodan, after another horror movie monster) will be even heavier and more powerful, which should eliminate some of Mothra’s problems with wind gusts. Rodan will also be equipped with ailerons on the wings to give it more maneuverability. Mothra was built more simply for a reason, Barrett says. Since it was the very first airplane to fly with smart structures, we didn’t want to go very fancy, he explains. We just wanted to get it into the air and prove that it could fly.