The idea has been around for ages -- since before the Wright Brothers. But, lately, the military has gotten serious about trying to build airplanes that change shape in flight. The reason? To get "aircraft that loiter for a long time and that also fly very fast," a Darpa program manager told New Scientist in '03. "The type of wings that you design for each of those things are different... loitering wings are generally high span and a large surface area, whereas fast wings have a low wing span and a low area. We want to generate wings that drastically change their surface area and shape, meaning more than 150 per cent change in surface area."
Sounds cool. But pulling it off has been really tough. Some Pentagon-funded engineers are trying to design wing superstructures that slide or fold. Another group, also with Defense Department backing is aiming for an even further-out solution: materials that actually bend and twist into new shapes.
Nature has already figured out how to do this, Darpa program managers noted, with plants. A plant bends toward the light, quickly furls its leaves when touched, or pushes a concrete sidewalk aloft with its roots is essentially moving fluids between cells.
MIT professor Yet-Ming Chiang "realized that the solid compounds used to store electrical energy in lithium rechargeable batteries could be made to work in a similar way. The movement of ions to and from these materials during charging and recharging, he thought, was analogous to the moving fluids in plants," an MIT press release notes. "Could this be a synthetic counterpart to nature's solution?"
To find out, Chiang and Hall began testing commercially available rechargeable batteries of a prismatic form, then designed their own devices composed of graphite posts surrounded by a lithium source. The results were promising.
Among other things, they found that the batteries continued to expand and contract under tremendous stresses, a must for devices that will be changing the shape of, say, a stiff helicopter rotor that's also exposed to aerodynamic forces...
The researchers have already demonstrated basic battery-based actuators that can pull and push with large force. Later this year, they hope to demonstrate the shape-morphing of a helicopter rotor blade. The morphing capability should allow for a more efficient design, ultimately making it possible for a vehicle to carry heavier loads.
