The low-altitude operation of rotorcraft is inevitably affected by the ground effect that poses challenging issues in designing altitude controller. This letter is concerned to the altitude tracking problem for rotorcraft unmanned aerial vehicles (UAVs) under the presence of the ground effect. In the light of existing in-ground-effect (IGE) models, we present adaptive altitude controllers to deal with unknown parameters in the IGE models. The controllers guarantee robust stability of the rotorcraft UAV in vertical movements, and improve the altitude track performances in close proximity to ground. Compared to the existing study, the improvement of the proposed controllers are then illustrated by several flight experiments.