Bioinspired Main–Auxiliary Composite Airbag for Emergency Landing of Multirotor UAVs: Cushioning Performance and Attitude Stability

When small multirotor unmanned aerial vehicles (UAVs) undergo uncontrolled descent following low-altitude failures, inadequate impact protection can cause airframe damage and injuries to people on the ground. Inspired by the feline paw pad, whose central palm pad bears the primary load and the surrounding digital pads provide coordinated support, this study proposes a bioinspired main–auxiliary composite airbag configuration comprising a central main airbag and three concentric annular auxiliary airbags. Starting from a fully deployed airbag configuration, a two-stage approach combining the arbitrary Lagrangian–Eulerian (ALE) and control-volume (CV) methods was developed to model the entire process of free fall and touchdown cushioning. The ALE model was assessed against a theoretical estimate, whereas the CV model was validated against drop-test results. The cushioning results showed that the main airbag carried most of the impact load during initial touchdown. The annular auxiliary airbags then progressively shared part of the load and, together with the outer skin, restrained further airframe rotation, consistent with the cushioning mechanism of the feline paw pad. Even at large touchdown inclination angles, the composite configuration effectively suppressed airframe tip-over and promoted gradual attitude recovery. Parametric analysis showed that the peak center-of-mass acceleration first decreased and then increased as either the vent-opening pressure threshold or the initial pressure of the main airbag increased. Increasing the vent area generally reduced peak acceleration, although further increases provided limited additional improvement. These findings provide guidance for emergency protection and cushioning airbag design for small multirotor UAVs.