A propulsion unit includes a motor rotor that spins about a central rotational axis, propeller blades each having a proximal base and a distal tip, and pivot mounts each coupling the proximal base of a corresponding one of the propeller blades to the motor rotor. The propeller blades each freely pivot at the proximal base about a corresponding offset pivoting axis that is substantially parallel to but offset from the central rotational axis of the motor rotor.