UL - 39 Albi is a prototype of an ultralight jet weighing 320 kg with its propeller placed inside of the airplane, so that radars are unable to detect the thermal radiation. The project has been carried out by the Faculty of Engineering, Department of Aerospace Engineering, Czech Technical Univerity, Prague in cooperation with LA COMPOSITE s.r.o. and JIHLAVAN Airplanes s.r.o. aviation businesses. Moreover, 150 uinversity students have been involved in the past 17 years of work on the project. 

Read more here and here (in Czech).