An actuating device to displace a part of an empennage of a helicopter, including a first part including a first casing fixed to a frame of the helicopter, a first element mounted in rotation with respect to the first casing and a first motor the first element. A second part includes a second casing connected to the part of the empennage, a second element mounted in rotation with respect to the second casing and a second motor driving the second element. A connecting element connects the first part with the second part, being mobile in translation with respect to the first element. Rotation of the first element causes a translation of the connecting element, and is mobile in translation with respect to the second element. Rotation of the second element causes a translation of the connecting element with respect to the second element.