Conventional VTOL-UAVs (Vertical Take-Off and Landing Unmanned Aerial Vehicles), like multicopters or helicopters, are not very energy efficient in cruising flight. Convertible VTOL-UAVs, typically composed of a set of propellers and wings, have been developed to improve energy efficiency. This paper proposes a modeling approach for VTOL-UAVs composed of a set of coplanar propellers and wings. The ultimate goal is to optimize the wings inclination with respect to the propellers plane so as to achieve energy efficient flight. Standard momentum and blade element theories are the main ingredients for modeling of propellers aerodynamics. In order to obtain simple closed-form expressions, modeling sim-plifications are made and an eight-parameter-analytical model is proposed. The model parameters are identified from the experimental data reported in the literature. As for the wing(s), a classical symmetric NACA profile is selected and an approximate model of lift and drag coefficients over the entire flight domain is defined. Based on these mod-els, the calculation of energy consumption reduces to solving a minimization problem in two variables. As an application, we compare the energy consumption of different UAV structures in the horizontal-flight range of [0, 20]m/s. Concerning UAV design, the impact of various parameters (choice of propellers, wing(s) area, mass of UAV) is analyzed. This work provides useful guidelines for both conception and control of convertible UAVs.