Flapping hydrofoils, similar to the oscillating fins of fish, can dwarf the efficiency of the propeller, making them a promising alternative in ship and AUV design. Furthermore, the “waste” component of a flapping foil wake can create high forces perpendicular to the direction of travel, making them a promising method for improving maneuverability.
Unfortunately, those same forces that improve maneuverability also oscillate, meaning that a flapping foil actuator makes the vehicle a recoil back and forth as it moves. Controlling this perpendicular force component is key to the adoption of flapping foil actuation.
Turtles solve this issue by flapping their flippers diagonally backwards (right top), essentially rotating all the foil lift towards the ideal forward direction. A turtle flipper can therefore control the amount and direction of resultant fluid force.
Birds (right bottom) instead use inline foil motion in the opposite direction to augment the lift perpendicular to the airflow.