The droop is caused by the weight of each aileron acting downwards when the aircraft is stationary. In flight the aileron is subjected to an upward force caused by the airflow. Let's say this aerodynamic force is about 40 pounds, for a typical Cessna 172. If on the ground two people both lift the ailerons with a force of 40 pounds each, then the half inch droop should go to zero, and the ailerons become in line with the trailing edge. A Cessna 172 is set up to have a certain amount of strain in each cable, which is measured by finding the deflection of the cable over a given length, when subjected to a known sideways load.