Winch assemblies for unmanned aerial vehicle delivery systems

An unmanned aerial vehicle, comprising:

• a chassis;
• a winch assembly comprising:
• a mounting assembly secured to the chassis;
• a spool rotatably mounted to the mounting assembly;
• a line including a wound portion wound about the spool and a free portion;
• an attachment device attached to the free portion of the line and configured to facilitate attachment of a parcel to the line, the attachment device having a raised position and a lowered position;
• a motor operable to rotate the spool in each of a raising direction and a lowering direction, wherein rotation of the spool in the raising direction raises the attachment device, and wherein rotation of the spool in the lowering direction lowers the attachment device;
• and
• an attachment device sensor operable to be actuated by the attachment device when the attachment device is in the raised position and configured to transmit a signal when actuated by the attachment device;
• a control system mounted to the chassis and in communication with the attachment device sensor;
• at least one rotor operable to generate lift under control of the control system;
• a spring positioned between the chassis and the attachment device such that the spring is engaged between the chassis and the attachment device when the attachment device is in the raised position;
• and
• a weight positioned on the line between the attachment device and the spring;
• wherein the spring is engaged with the attachment device via the weight when the attachment device is in the raised position.