They studied how pigeons and swallows can morph their wings until they are tucked close into their body, allowing them to pass through gaps. They created one of the first mechanisms in which the morphing of the wing was completely passive, requiring no actuation to fold or unfold, making the wing much lighter and more reliable.
The robotic wing was made using carbon fibre and Mylar film. Similar to a bird wing, each of the two robotic wings had a wrist joint, custom built using a 3D printer.
The pin joint connected the arm wing and the hand wing. The arm wing attached to the body of the robot at the shoulder joint, which initiated the flapping. The complete wing set had a wingspan of 400mm and a length (chord) of 80mm.
The wrist joint of the wing was hinged so, as the rest of the wing flapped, the hand could freely fold and unfold over the arm without any actuation.
Theoretical, numerical and physical simulations on the robotic wing successfully demonstrated that when the wing flapped, the folded hand wing was able to unfold back to the full wingspan configuration passively.
Co-author of the study, Dr David Lentink said: "Both the math and simulations worked out, showing that both tiny and big flapping wings can all morph passively within a wing beat. We were surprised it worked so well."
The hinged wrist joint also allowed the robotic wing to temporarily morph its hand when it came into hard contact with a rigid object. The researchers used a 7mm steel rod which allowed the robotic wing to comply with the object at impact and, after impact, the flapping motion caused the wing to automatically re-extend, similar to the way the flexible feathers of a bird allow for impact with obstacles without affecting the structural integrity of the wing.
Lead author, Amanda Stowers, hit the robot wing hard with a stick to see how well it could handle hard impact and noted: "the flapping wing can resist impact with minimal added weight and without any computer intelligence or power".
"This finding will greatly help make flapping winged drones much more robust. This is essential if we ever want to safely fly through a forest or land in a tree like a bird. The Office of Naval Research wanted us to find solutions to enable drones to fly in such cluttered environments, and this is a promising step forward."
Folding in and out: passive morphing in flapping wings is published today in Bioinspiration Biomimetics.
