This paper studies the downward tip vortex trajectory of a small rotor in the near proximity of a wing, similar to the geometry of small unmanned-air-vehicles (UAVs) in hover (disk loading < 100 N/m^2 and Reynolds number < 100,000). Rotor height above the wing and rotor wake impingement area on the wing are varied. Empirical equations of the trajectory as a function of rotor height generalized from tip vortex trajectory results in smoke visualization experiments are then employed. Using the rotor wake model based on the tip vortex trajectories, image vortices, and momentum theory, a potential flow solution to predict rotor thrust and download force on the wing is developed and discussed, which links rotor wake, thrust and download force in rotor-wing interaction. The analytical results will be compared to the corresponding experimental force data in terms of magnitude and trend for accuracy assessment.