A robot inspired by pigeons has solved the mystery of how birds fly without the vertical tail fins that human-designed planes rely on. Its makers say the prototype could eventually lead to passenger planes with less drag, thereby reducing fuel consumption.

Tail fins, also called vertical stabilizers, allow the plane to turn from side to side and help avoid changing direction unintentionally. Some military aircraft, like the Northrop B-2 Spirit, are designed without a fin because it makes them less visible on radar. Instead, they use flaps which create additional drag on only one side when needed, but this is an inefficient solution.

The birds do not have a vertical fin, nor do they appear to deliberately create asymmetrical drag. David Lentink at the University of Groningen in the Netherlands and colleagues designed PigeonBot II (pictured below) to study how birds maintain control without such a stabilizer.

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

Lentink says the secret to PigeonBot II’s success is the reflex tail movements programmed into it, designed to mimic those known to exist in birds. If you hold a pigeon and tilt it from side to side or back and forth, its tail responds automatically and moves in complex ways, as if to stabilize the animal in flight. This was long thought to be the key to the birds’ stability, but has now been proven by the robotic replica.

The researchers programmed a computer to control the Pigeonbot II’s nine servo motors to steer the craft using propellers on each wing, but also to automatically twist and fan the tail in response, to create the stability that would come from normally a vertical fin. Lentink says these reflexive movements are so complex that no human could directly pilot Pigeonbot II. Instead, the operator sends high-level commands to an autopilot, telling it to turn left or right, and an on-board computer determines the appropriate control signals.

After numerous unsuccessful tests during which the control systems were refined, it was finally able to take off, navigate and land safely.

“We now know the recipe for flying without a vertical tail. Vertical tails, even for a passenger plane, are just a nuisance. It costs weight, which means fuel consumption, but also drag – it’s just unnecessary drag,” says Lentink. “If you just copy our solution (for a large-scale aircraft), it will work, for sure. (But) if you want to translate that into something a little easier to make, then it takes an additional level of research.