This work focuses on the nonlinear control of helicopters. Our global interest is a general model (7-DOF) to be used on the autonomous forward-flight of helicopters. However, in this paper we present a reduced-order model (3-DOF) representing a scale model helicopter mounted on an experimental platform. In this system the vertical flight of the helicopter can be studied. Although simplified, this 3-DOF Lagrangian model presents quite interesting control challenges due to nonlinearities, aerodynamical forces and underactuation. Due to the very particular dynamical and control properties of the 3-DOF model we propose a specific nonlinear controller using passivity properties to control the rotational part of the system. An I/O linearizing controller is used for the helicopter altitude control. Numerical simulations are presented to illustrate the performance of these controllers.