Over the past years the interest in using small unmanned arial vehicles (UAV) has increased steadily. For instance, small imaging systems on UAV are employed in earth observation, reconnaissance, monitoring, and surveillance missions. The reason for using radar sensors for such applications can be found in their all-weather capability. Especially imaging radars possess several benefits over optical systems, as they are not affected by rain, snow, fog, or the time of day. The advantages of using small unmanned platforms come at the cost of having to deal with restrictions on weight and volume. Hence, these parameters are the predominant design drivers for UAV sensor developments. This paper presents the first flight trials of the 3D imaging radar system ARTINO (Airborne Radar for Three dimensional Imaging and Nadir Observation), which is designed to be hosted on a small UAV. This new type of radar combines a real aperture, realized by a linear array of nadir pointing antennas, and a synthetic aperture, which is spanned by the moving airplane.