Flexible Wings Are Hard to Make!

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How hard can it be to make a flexible wing flap for an airplane? Almost all aircraft today use rigid wings with rigid landing flaps. They work, but they waste fuel. German engineers embarked on a mission to reduce kerosene consumption by 6%: "integrating flexible landing devices into aircraft wings is one step towards that target," a news item from Fraunhofer says. They've named the project SARISTU, for Smart Intelligent Aircraft Structures.

Birds are way ahead of them:

While birds are able to position their feathers to suit the airflow, aircraft wing components have so far only been rigid. As the name suggests, landing flaps at the trailing edge of the wing are extended for landing. This flap, too, is rigid, its movement being limited to rotation around an axis. This is set to change in the SARISTU project. "Landing flaps should one day be able to adjust to the air flowand so enhance the aerodynamics of the aircraft," explains Martin Schüller, researcher at the Fraunhofer Institute for Electronic Nano Systems ENAS in Chemnitz. (Emphasis added.)

What are some of the challenges in building a flexible wing?

1. Knowing where to flex: The flap can't be flexible all over, or it would be hard to control. The designers made "five hard and three soft zones, enclosed within a silicon skin cover extending over the top."
2. Finding stretchy skin: When the soft zone moves, the skin of the aircraft has to stretch with it. "The mechanism that allows the landing flap to change shape can only function if the skin of the landing flap can be stretched as it moves, aproblem tackled by researchers from the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in Bremen."
3. Covering the air gap: "Any gap between the flap and the fixed aircraft wingwould cancel out any positive effect," the article notes. "This led us to develop an elastic connecting element, and this work already covers everything from thechemical makeup to the process technology and manufacture of the component," an engineer says.
4. Designing the material to tolerances: "The mechanism sits underneath the soft zones, the areas that are most distended. While the novel design is noteworthy, it is the material itself that stands out, since the flexible parts are made of elastomeric foam that retain their elasticity even at temperatures ranging from minus 55 to 80 degrees Celsius."

No feathers, but it's a start. The team showed off their prototype at the ILA Berlin Air Show in May. Apparently it was not quite ready for takeoff:

When the prototype takes off for the first time it will benefit from a development known as SARISTU, a deformable wing which is currently the subject of intensive research by Fraunhofer Gesellschaft. In future the landing flaps will be designed to adapt in flight to the air flow conditions, thereby always ensuring the best possible aerodynamics.

We celebrate this advance, but you know where we're going. Birds had it all figured out long ago: the right shape, the right material, the control of airflow, and much more. As Dr. Timothy Standish says in the film Flight: The Genius of Birds, "Feathers do a number of jobs remarkably well." They are individually controllable, they flex, they insulate, they save on weight, and they can handle the temperature requirements of avian flight. That's just a partial list achievements in powered flight that surpass anything man has yet designed.

If the world's top engineers are struggling to get three soft zones to flex on a landing flap, why would anyone think birds achieved far better performance by blind, unguided processes? "I believe intelligent design is the best explanation for avian flight," Standish states at the film's conclusion, "because it's the best explanation for every other kind of flight we see."

He draws the logical inference: "They're engineering marvels. They're works of art. We know where engineered things come from. We know where works of art come from. So why would we attribute a bird to anything other than intelligence or mind?"