The effects of low air density (characteristic at high-altitude environments) on small-sized Unmanned Aerial Vehicles (UAVs) are studied in this paper. Flight data from a fixed-wing aircraft and a quadcopter (in representation of multicopters) is examined to substantiate the influence high altitude conditions have on small UAVs' flight. By using aeronautical theoretical background to construct a hypothesis concerning how altitude affects the energy consumption and, consequently, flight time of both types of UAV, a qualitative analysis of flight missions held at different altitudes will be performed to find a correlation between the energy consumption and target altitude to determine if low altitude flight missions could be used to predict consumption at high-altitude environments. Finally, guidelines abstracted from these flight experiences will be given to help in the future design of UAVs with high altitude applications.