An unmanned aerial vehicle according to certain embodiments generally includes a chassis, a power supply mounted to the chassis, a control system operable to receive power from the power supply, at least one rotor operable to generate lift under control of the control system, and a motor operable to lower a free end of a line. The free end of the line is operable to engage a parcel to be delivered by the unmanned aerial vehicle. The control system is configured to operate the motor to cause the free end of the line to accelerate toward a delivery surface as the free end of the line passes through a first portion of a distance between the unmanned aerial vehicle and the delivery surface, and to decelerate as the free end of the line passes through a lower portion of the distance.