“Morphing wings have been used since the beginning of aviation,” said Jose Rivas-Padilla, Ph.D. student in mechanical engineering. “The Wright Brothers used their own bodies to warp the control surfaces of their wings. Modern airplanes have ailerons, which use hydraulics to change the shape of the wing’s trailing edge, controlling the airplane’s roll angle.”

As the design of morphing wings continues to advance, engineers face three challenges: making structures that are light, strong, and flexible. If you create a structure that is too heavy, the plane won’t be cost-effective; compromise on strength, and the structure won’t survive flight; make the wing overly stiff, and the airplane won’t be controllable.

Enter Andres Arrieta and his programmable structures. Arrieta specializes in building structures that are bi-stable; like a slap bracelet, they snap back-and-forth between two stable states, with very little force required to actuate. He has used this technique to create origami that mimics the wing of an earwig; small-scale furniture that snaps open and closed like a Venus flytrap; and oddly satisfying metamaterials that store energy in their skin.

Jose, along with fellow graduate students Matt Boston and Karthik Bodapati, set upon the task of designing a morphing airplane wing using bi-stable structures. The ideal design would be a seamless, continuous surface with a lightweight construction. “The way we do this is by 3D printing it as a single structure, with bi-stable elements embedded,” said Jose. “This allows the structure to snap easily between two different states, without needing to apply continuous force, like the current hydraulic systems on airplanes. And it provides a method to address the challenge of overcoming the elastic energy when actuating compliant structures. So it’s potentially easier to manufacture, much lighter, and much less complex.”

The bi-stability also changes the global stiffness of the entire wing. The structure can become more flexible or more stiff, depending on the mission requirements. “If you’re flying at high speed, for example, you need your wing to be more stiff, with smaller movements,” said Jose. “At lower speeds -- like when loitering for reconnaissance, or doing a short takeoff and landing (STOL) -- a more flexible wing offers you more lift and greater control.”

They first presented the concept at an AIAA conference in 2019. At that point, the designs were still theoretical. As they began to optimize the geometries, they built prototypes out of balsa wood and plastic, until they finally optimized their design. In March 2021, they began building a proper small-scale wing section demonstrator using macro-fiber composite actuators, fine-tuning the actuation method for the 3D-printed structure to snap from one state to the other.

Finally, they tested their design in a wind tunnel. With the help of Sally Bane, associate professor of aeronautics and astronautics, they took their demonstrator into the Boeing subsonic wind tunnel, located at Purdue Airport. They wanted to see if their bi-stable wing structure would maintain its two distinct shapes under real aerodynamic loads. They actuated the snap using a simple servo with two nylon wires; a small tug snapped the structure into one shape, while a tug in the opposite direction snapped the structure into its original shape. Their test was successful: in up to 30 meters/second airspeeds, they actuated the structure multiple times, and observed no elastic instabilities in either state, even while rotating through +/- 10 degrees angle of attack.

In 2021, Jose presented the results at the International Conference for Adaptive Structures and Technologies (ICAST) in Zurich, for which the team received an award for Best Poster for the conference. The morphing wing also was highlighted by Aerospace America magazine in their 2021 Year in Review.

 “People are very excited about it,” said Jose, “which makes us very excited!”

For their next steps, Arrieta’s team has plans to expand the single-structure demonstrator to build an entire wing, with all control surfaces being bi-stable or multi-stable structures. They also want to go beyond the wind tunnel, and attach the wing to an actual unmanned aerial vehicle (UAV) to get some proper flight data.

“Our overarching goal is to reduce complexity,” said Arrieta. “Airplanes are incredibly complex machines. If we can use bi-stable structures to make them simpler, then eventually they’ll also become lighter, cheaper, more functional vehicles.”

This project has been supported by the US Air Force Office of Scientific Research (AFOSR) under the Grant FA9550-17-1-0074 “On-demand Stiffness Selectivity for Morphing Systems”, monitored by Dr. Byung Lip (Les) Lee.

Writer: Jared Pike, jaredpike@purdue.edu, 765-496-0374

Source: Andres Arrieta, aarrieta@purdue.edu

Topology Optimization and Experimental Validation of a Selectively Stiff Multistable Morphing Wing Section

Jose R. Rivas-Padilla, David M. Boston, Karthik Boddapati and Andres F. Arrieta
https://doi.org/10.2514/6.2022-1987

Morphing wings provide a potential solution to increase aerodynamic efficiency and performance for aircraft required to operate at multiple design conditions. Nevertheless, morphing wing design is constrained by the mutually exclusive goals of high load-carrying capacity, low weight, and sufficient aerodynamic control authority via conformal shape adaptation. We propose a potential avenue to alleviate this trade-off by storing strain energy in a morphing system and exploiting the stiffness adaptability of curved structures that exhibit two structurally stable configurations. This stored strain energy can be used to morph the camber of the airfoil and hold a stable, deflected configuration, thus eliminating the need for constant actuation when morphing a compliant structure. We first optimize the topology of a morphing rib to maximize structural compliance and lift-to-drag ratio in the low-speed flight condition while maintaining the desired maneuverability in the higher-speed condition. The optimal morphing rib is manufactured using a fused deposition modeling printer and the numerical model is validated with experimental structural tests. Using this optimal individual, three distinct actuation strategies are implemented to compare the elastic strain energy induced by the actuators to hold a target deflection, and it is observed that a single actuator which can both switch the bistable element and trim the trailing edge deflection can outperform MFCs bonded to the upper surface by a considerable margin. Furthermore, this single optimized rib topology was used to manufacture a morphing wing section which features a load-carrying skin made of a carbon reinforced laminate and MFC actuators. The power and energy requirements of actuating and holding a target deflection were experimentally measured, and it is shown that the energy penalty necessary to switch the structure into the deflected configuration outperforms the other actuation methods within seconds since these alternative strategies need to overcome the elastic energy of the compliant system to hold an equivalent deflection. Finally, we test the experimental demonstrator in a low-speed wind tunnel to characterize its aerodynamic performance. It is shown that the wing section is capable of varying the lift by only switching to the deflected state, and that, at high aerodynamic loads, the morphing wing section is observed to be dynamically stable, demonstrating the feasibility and potential benefits of the proposed morphing concept.