A robot that flies like a bird

Gliding birds do not have a vertical tail that can act as a rudder. However, their flight remains stable despite the turbulence. And this, without even needing to flap their wings to adjust their direction. On the contrary, current planes (almost) all have a vertical surface at the rear of the cabin: the fin. Its presence serves to limit the risk of lurching and to cushion the so-called “Dutch roll” phenomenon. The latter takes the form of an oscillatory yaw movement making the operation of the device unstable. Scientists from Stanford University (Stanford, United States) and the University of Groningen (Groningen, Netherlands) therefore sought to understand how birds proceed to maintain their posture in all circumstances, with the aim of that their work could lead to aircraft better suited to flight.

A robot at the crossroads of an airplane and a pigeon

The first author of the research published on November 20, 2024 in the journal Science Robotics is Eric Chang of Stanford University. During this work, he was able to count on the help of Diana Chin, from the same structure, and David Lentink, from the University of Groningen. The team did not start from scratch since it largely relied on a hypothesis put forward by the German biologist Franz Groebbels in 1929. He estimated that gliding birds must fly like “automatic airplanes”, using neuromuscular reflexes in the wings and tail. A solution that is inapplicable to current aircraft, whose wings and tail cannot deform like those of birds. Eric Chang and his two colleagues therefore set out to make their own machine inspired by birds: the PigeonBot II.

The bio-hybrid robot developed by the scientists consists of a biomimetic skeleton provided with elastic ligaments associated with wing and tail feathers. PigeonBot II thus has twenty flight feathers – large rigid feathers – on each of its wings, and twelve other feathers on the tail. All are true and come from the species Columba livia (the Rock Pigeon). The forty wing feathers are operated with four servomotors, and the tail feathers with five more of these servomotors. The total mass of the robot is close to that of a real pigeon (around 300 grams) and relies on two small thrusters – one on each wrist – to fly. The critical point of the study was to develop an adaptive reflex controller, capable of mimicking the instinctive reaction of birds. To do this, the research team placed their PigeonBot II in a 1*0.82*1.73 meter wind tunnel, where they subjected it to a wind speed of 10 m/s. Once the controller was trained to reduce turbulent disturbances, the robot could be released outdoors, flying autonomously while taking poses typical of a real pigeon. The results therefore corroborate Franz Groebbels’ almost century-old idea, and could inspire in the future the design of rudderless aircraft with greatly improved efficiency and maneuverability.