Unmanned aerial vehicle delivery systems

A method of operating an unmanned aerial vehicle (UAV), the method comprising:

• placing an attachment mechanism in a raised position relative to the UAV, wherein the attachment mechanism is attached to a free portion of a line having a wound portion that is wound about a spool rotatably mounted to a chassis of the UAV, wherein rotation of the spool in a winding direction raises the attachment mechanism, and wherein rotation of the spool in an unwinding direction lowers the attachment mechanism;
• placing a brake mechanism in a locked state, wherein the brake mechanism in the locked state prevents rotation of a spool in the unwinding direction;
• and
• placing the brake mechanism in an unlocked state in response to an unlock condition, wherein the brake mechanism in the unlocked state does not prevent rotation of the spool in the unwinding direction, and wherein placing the brake mechanism in the unlocked state comprises:
• operating the motor to rotate the spool in the winding direction, thereby facilitating unlocking of the brake mechanism by disengaging a pawl from a ratchet wheel;
• and
• operating a brake actuator to move the brake mechanism to an unlocked state, thereby facilitating rotation of the spool in the unwinding direction;
• wherein disengaging the pawl from the ratchet wheel reduces a force required to be exerted by the brake actuator to move the brake mechanism to the unlocked state.

An unmanned aerial vehicle, comprising:

• a control system;
• at least one rotor operable to generate lift under control of the control system;
• a spool mounted for rotation in a winding direction and an unwinding direction opposite the winding direction;
• a motor operable to rotate the spool under control of the control system;
• and
• a brake mechanism operable to selectively prevent rotation of the spool in at least the unwinding direction, the brake mechanism comprising:
• a ratchet wheel mounted for rotation with the spool, the ratchet wheel comprising a plurality of teeth;
• a pawl operable to selectively engage the teeth when in a holding position, and configured to disengage from the teeth when in a release position;
• and
• a brake actuator operable to move the pawl from the holding position to the release position state to an unlocked state under control of the control system;
• wherein the brake mechanism has a locked state in which the pawl is in the holding position and engaged with at least one of the teeth, and an unlocked state in which the pawl is in the release position and disengaged from the teeth;
• wherein the control system is configured to initiate an unlocking operation in response to an unlock condition, the unlocking operation comprising:
• rotating the spool in the winding direction, thereby reducing forces acting between the at least one of the teeth and the pawl and facilitating movement of the pawl from the holding position to the release position;
• and
• after rotating the spool in the winding direction and reducing the forces, operating the brake actuator to move the pawl from the holding position to the release position, thereby unlocking the brake mechanism.