NASA, MIT and a team of researchers from Kaunas University of Technology in Lithuania, and the firm Qualified Technical Services have been experimenting with a new type of wing construction designed to flex in response to its flight requirements.
Instead of the usual solid metal or composite framework with an outer skin, the new wing is made up of hundreds of minute polymer pieces of a mixture of stiff and flexible parts that form a lattice. The whole wing can deform, or just a part of it for a more subtle geometry change.
The researchers claim this means that the wing is actually mostly empty space, with the tiny triangles of matchstick-like struts making a ‘metamaterial’ offering the low density of an aerogel with the stiffness of a rubber-like polymer.
The theory behind it is that the different stages of flight have different requirements from a wing, and even with careful aileron design there are compromises causing a loss of performance. By allowing the wing to deform it can be better tailored to the job at hand whether that be take-off, cruising at altitude or landing.
Team member Nicholas Cramer from NASA Ames in California confirms, “We’re able to gain efficiency by matching the shape to the loads at different angles of attack.” By avoiding the use of miles of cable and motors by creating a system that automatically responds to changes in its aerodynamic loading conditions by shifting its shape, lightness is retained. Cramer continues, “We’re able to produce the exact same behaviour you would do actively, but we did it passively.”
The painstaking process of assembly began with cutting the parts out with a waterjet, although this idea has been ditched for the next phase of the project. At several minutes per part, it was just too slow. The next iteration will see the use of an injection-moulded polyethylene resin that the team reckon will take just 17 seconds to create each piece. Likewise for any series production, rather than relying on the goodwill of the students who hand built this, some form of robot would be constructed to automate the process. The artist’s illustration on the previous page depicts a fleet of them.
The resulting lattice arrangement has a density of around of 5.6 kilograms per cubic meter, as compared for example to rubber at about 1,500 kilograms per cubic metre. The stiffness is around the same.
The project follows on from a 2013 research project by team member Kenneth Cheung, where he produced a cruder model around one fifth the size. He says, “You can make any geometry you want. The fact that most aircraft are the same shape is because of expense. It’s not always the most efficient shape. But massive investments in design, tooling and production processes make it easier to stay with long-established configurations.”
Speaking of size, the new wing is the size it is because that’s the largest they can accommodate at NASA’s high-speed wind tunnel at Langley Research Centre, where the initial testing took place.
The team hope further tests will confirm this flexible approach works, and that the design can then lend itself to other structures, such as wind turbines, bridges or even more ambitious constructions in space.
The work was supported by NASA ARMD Convergent Aeronautics Solutions Program (MADCAT Project), and the MIT Centre for Bits and Atoms