Investigating the bubble bridge formed during an amphibious UAV’s water-to-air transition*

Amphibious unmanned aerial vehicles (UAVs) provide great versatility in surveying, exploration, and even next generation delivery missions by eliminating the need for multiple vehicles to traverse the two media separately. Previous studies have shown that quadrotors can navigate in both air and water and cross the air-water boundary, but produce unsteady, turbulent waves at the water surface during the transition. Specifically, while exiting the water, a "bubble bridge" forms due to air entrainment and corresponds to a sharp change in the rotor's rotational frequency, thrust, drag, and power consumption. We present here a system that simulates one UAV rotor dynamically traversing the air-water interface in a highly repeatable manner. We explored how the bubble bridge's timing and depth are affected by exit velocity, rotor diameter, number of rotor blades, and input throttle. The rotor's re-entry to the water differs significantly from its exit, requiring two alternate approaches to the transition. Better understanding the dynamics of air-water rotor transitions will help to 1) analyze the amphibious capability of commercial UAV rotors, 2) shed light on the best exit and re-entry strategies, and 3) offer design ideas for rotors designed specifically for amphibious operation.