Birds exhibit controlled gliding flight without a vertical tail, unlike the necessary rudders on airplanes that damp Dutch roll oscillations.

To accomplish rudderless flight, Chang et al. developed a bioinspired aerial robot with morphing wings and tail named PigeonBot II.

The robot consists of a biomimetic skeleton and real pigeon feathers that form wings that can spread and a tail that can spread, elevate, tilt, and deviate side to side like a bird.

Initial experiments in a turbulent wind tunnel showed that reflexive tail tilting and deviation combined with wing morphing enabled stable flight by damping Dutch roll.

Outdoor flight tests further demonstrated that the autonomous reflexive controller provided stability to the robot during take-off, cruise, and landing.