Drone with bird-bat-like wings can recover from mid-air collision

Scientists have created a drone that has the capacity to recover from mid-air collisions, after examining the wings of bats and birds and applying their flying mechanisms to their flying machine, Stanford University in California reported on Wednesday.

The scientists, who wrote about their study in the academic journal Bioinspiration and Biomimetics (citation below), said the mechanism they have developed will allow future drones to squeeze easily between branches of a tree, slip through gaps, and handle other obstacles, as well as recovering completely after accidental collisions.

We know that flying animals can weave through complex environments with amazing agility. So far, nobody has been able to design mad-made robots that can manoeuvre anywhere near as well.

Wings like a bird

 Comparison of the robot and bird wing morphology. (Image: iopscience.iop.org/)

Several species of birds such as swallows and pigeons can morph their wings until they are well tucked close into their body, allowing them to slip through gaps.

In this latest study, the scientists 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 considerably lighter and more reliable.


The scientific team modelled the robotic wing on those of birds and bats. It was made of carbon fibre and Mylar film (a form of polyester resin). The two robotic wings had a wrist joint, like bird wings do, which were custom-built using a 3-D printer.

The arm wing and the hand wing were connected with a pin joint. The arm wing was attached to the robot’s body at the shoulder joint, which initiated the flapping. The whole wing set had a length (chord) of 80 mm (3.15 inches) and a wingspan of 400 mm (15.75 inches).

Flapping like an origami bird

The wing’s wrist joint was hinged, which allowed the hand to freely fold and unfold over the arm during flapping, as one would see in an origami bird folding, without any actuation.

The team performed theoretical, physical and numerical simulations on the robotic wing. They successfully demonstrated that when the wing flapped, the folded hand wing could unfold back to the full wingspan configuration passively.

Co-author 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.”

Drone wingspan

The drone in fully folded (A) and extended (C) configuration showing the definition of the fold ratio. (Image: iopscience.iop.org)

The researchers made the robotic wing crash into a 7mm steel rod. When it collided, the hinged wrist joint allowed the robotic wing to temporarily morph its hand. “The joint allowed the robotic wing to comply with the object at impact and, after impact, the flapping motion caused the wing to automatically re-extend,” the authors wrote.

This is how the flexible feathers of a bird allow for impact with obstacles without the structural integrity of the wing being affected.

Lead author Amanda Stowers said:

“While birds are capable of responding to unexpected disturbances to their wings, these same disturbances would break the wings of most drones. By adding a passive wrist joint, the flapping wing we have produced can withstand an impact and recover automatically back to its original position.”

“Furthermore, the flapping wing can resist impact with minimal added weight and without any computer intelligence or power.”

Dr. Lentink added:

“After her discovery of passive wing morphing in the lab, I advised Amanda to hit her robot wing hard with a stick to see how well it could handle hard impact. We were both impressed that she could basically use a rod like a baseball bat and hit the hand wing of the robot, and it would still recover just fine.”

“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.”

Citation: “Folding in and out: passive morphing in flapping wings,” Amanda K Stowers and David Lentink. Bioinspiration & Biomimetics. Published 25 March, 2015. 10 025001 DOI:10.1088/1748-3190/10/2/025001.