in order to eliminate or at least to reduce a possible unbalance of the propulsive system , counterweights are brought in locations that are provided on the hub envelope , near the ends of the latter and away from the planes of the propellers . more particularly , the counterweights are provided as discrete elements located in two planes spaced from the propellers and orthogonal to an outer surface of the hub envelopes , or are provided as a non - uniform mass ring .