Robot pigeon reveals how birds fly without vertical caudal fins

A pigeon-inspired robot has solved the mystery of how birds fly without the vertical tails that human-designed aircraft rely on. Manufacturers say the prototype could eventually lead to passenger planes that reduce drag and fuel consumption.

The tail fin, also known as the vertical stabilizer, allows the aircraft to turn from side to side and helps prevent it from changing direction unintentionally. Some military aircraft, such as the Northrop B-2 Spirit, are designed without tails to make them less visible to radar. Instead, it uses flaps that create extra resistance on only one side when needed, which is an inefficient solution.

Birds do not have vertical fins and do not seem to intentionally create asymmetric drag. David Lentink and colleagues at the University of Groningen in the Netherlands designed the PigeonBot II (pictured below) to investigate how birds can maintain control without such stabilizers.

The team’s previous model, built in 2020, flew by flapping its wings and changing shape like a bird, but still had a traditional aircraft tail. The latest design, which includes 52 real pigeon feathers, has been updated to include a bird-like tail, and test flights were successful.

Lentink says the secret to PigeonBot II’s success lies in its programmed, reflexive tail movements designed to mimic movements known to exist in birds. When you hold a pigeon and tilt it left and right or front and back, the tail automatically reacts and moves in a complex manner, as if to stabilize the pigeon’s flight. This has long been thought to be the key to the bird’s stability, and a robotic replica has now proven it.

The researchers controlled PigeonBot II’s nine servo motors, using propellers on each wing to not only steer the craft, but also automatically twist and fan the tail in response, normally programmed a computer to create the stability that comes from vertical fins. Lentink said these reflex movements are so complex that humans cannot directly control PigeonBot II. Instead, the operator issues high-level commands to the autopilot, telling it to turn left or right, and the onboard computer determines the appropriate control signal.

The control system was improved and, after many failed tests, was finally able to take off, cruise, and land safely.

“Now we have a recipe for how to fly without a vertical stabilizer. Vertical stabilizers are just a nuisance, even on airliners. They add weight, which increases fuel consumption, but… It also creates drag, which is just unnecessary drag,” says Lentink. “If you just copy our solution[for large-scale aircraft]it certainly works.[But]if you want to translate this into something a little more manufacturable, you need to do more research.”