In the design and development of new operating equipment, machinery and apparatuses for agriculture, the main criterion for their evaluation is their resistance in a working environment, or, their specific energy consumption, respectively. To define the resistance in progressive or rotary motion, the specific design features, the mode of impact and the environment parameters should be taken into, consideration. In progressive motion the resistance depends mainly on the speed. It begins to increase considerably at speeds over 5 km.h -1 when the second gear is of extreme importance. With apparatuses using rotary motion it is important to note that the resistance also depends on the square speed. At idle stroke particularly, the excessively high speeds increase significantly the power needed. With fans the main criterion for minimum specific energy consumption can be determined by the specific relations between the power needed, the rotation frequency, the discharge and pressure head.