Taking Flight Lessons from Nature

In the earliest days of experimentation with heavier-than-air powered flight, inventors took inspiration from the best aviators of the natural world — birds. The result was the creation of a long string of often humorous contraptions meant to get a person airborne by flapping artificial wings, or by other methods that are laughable today. Today’s airplanes, in contrast, have very little in common with the mechanics that make bird flight possible. This is both good and bad; airplanes can fly much higher and faster, but birds are far more energy-efficient and nimble than even our very best designs.

Now aircraft design approaches may be coming full circle — at least to some extent — as a result of the findings of researchers at Stanford University and the University of Groningen. To help us better understand what makes bird flight tick, and give us some insights to make our aircraft more efficient, they have designed a robot that closely mimics a real bird in flight. The robot, called PigeonBot II, is covered in actual pigeon feathers and has a structure and actuation capabilities that are very close to its biological counterparts.

One particularly important question that the team wants to answer is how birds maintain stable flight without a vertical tailfin. Without this tailfin, most airplanes would roll out of control. But eliminating it would be a big win for fuel efficiency, because they produce a lot of drag. To answer questions like this, the robot’s design needed to be as close as possible to that of a real bird.

PigeonBot II's design incorporates feathers sourced from king pigeons, with primary and secondary feathers assembled from different individuals to ensure anatomical accuracy. The wing structure uses high-torque Dymond D47 servo motors for precise control and features 3D-printed ribs for strength and flexibility. The wings also include coupled wrist and finger motions to emulate bird wing articulation. For propulsion, counterrotating motors with 76-mm propellers are mounted near the wing joints, covered with 3D-printed nacelles to minimize aerodynamic disruption. A Teensy 4.0 microcontroller manages the servo operations, integrated with a PixRacer running ArduPilot firmware for flight control. The center of gravity (CG) was placed 24 mm behind the wing root leading edge, using anatomical reference points to mirror a stock dove's flight dynamics.

The tail of the robot uses five servo motors to actuate twelve pigeon tail feathers, enabling a range of movements such as elevation, lateral deviation, and spreading. The actuation system employs pushrods, Bowden cables, and a carbon fiber torque tube, ensuring lightweight yet precise control while maintaining the CG near the robot's center. Feathers are mounted on rotational pin joints and interconnected with tuned orthodontic elastics, which distribute force evenly to replicate natural feather spread and angles. The tail morphing system allows for postures like spreading, tucking, and intermediate positions, with exact angles calibrated to reflect those observed in pigeons.

It was found that accomplishing stable rudderless flight would take more than just replicating the physical structure of a bird. Their natural reflexes — which enable them to make constant, rapid adjustments to the position of their tail — would need to be incorporated into the robot. For this reason, a bird-inspired reflexive controller was developed to automatically tweak the tail position during flight for stability. When all of these pieces were put together, the researchers had a platform for exploring biomimetic flight dynamics that can perform bird-like aerodynamic maneuvers. It is hoped that by working with this platform, new breakthroughs in aircraft design will be made in the future.