Abstract: A test rig of the closed loop type for testing power transmission units of any geometry, in particular helicopter transmissions, comprising drives with friction belts, flat belts in particular. The belt drives have pulleys with diameters that compensate for the creep in the belts. In some embodiments, the test rig comprises ‘slave’ transmissions. In some embodiments the slave transmission is positioned and oriented in such a manner that all its input and output shafts, regardless their different space angles, become parallel and rotating in the same direction as their corresponding shafts on the tested transmission. The shafts can thus be connected by the flat belt drives to close the loops. In other embodiments, sets of gearboxes are run with their slave units; their orientation is also set so as to enable the parallelism and identity of the rotational direction of the shafts the belt drives are connected to.