Fast ground-to-air transition with avian-inspired multifunctional legs.

Most birds can navigate seamlessly between aerial and terrestrial environments. Whereas the forelimbs evolved into wings primarily for flight, the hindlimbs serve diverse functions such as walking, hopping and leaping, and jumping take-off for transitions into flight. These capabilities have inspired engineers to aim for similar multimodality in aerial robots, expanding their range of applications across diverse environments. However, challenges remain in reproducing multimodal locomotion, across gaits with distinct kinematics and propulsive characteristics, such as walking and jumping, while preserving lightweight mass for flight. This trade-off between mechanical complexity and versatility limits most existing aerial robots to only one additional locomotor mode. Here we overcome the complexity-versatility trade-off with RAVEN (Robotic Avian-inspired Vehicle for multiple ENvironments), which uses its bird-inspired multifunctional legs to jump rapidly into flight, walk on the ground, and hop over obstacles and gaps similar to the multimodal locomotion of birds. We show that jumping for take-off contributes substantially to the initial flight take-off speed and, remarkably, that it is more energy efficient than taking off without the jump. Our analysis suggests an important trade-off in mass distribution between legs and body among birds adapted for different locomotor strategies, with greater investment in leg mass among terrestrial birds with multimodal gait demands. Multifunctional robot legs expand the opportunities to deploy traditional fixed-wing aircraft in complex terrains through autonomous take-offs and multimodal gaits.