The purpose of this study is to realize long and stable flight of a forestry-use drone for highly efficient and safe forest survey. First, its a model for control design is derived to present the behavior of the drone. Second, in order to obtain the precise dynamics, the behavior of variable pitch propeller is modeled. Finally, we perform the advanced altitude-hold flight experiment with the laser sensor. As the result, the hexacopter model has been derived, the lift force and torque with respect to both rotational speed and pitch angle have been modeled. We confirm the effectiveness of the altitude-hold flight control based on the model by the laser sensor.