The Campbell diagram represents the frequency versus the rotation speed of the shaft: the evolution of the natural frequencies corresponding to a mode are drawn in function of the rotation speed of the shaft.
A necessary condition for an Engine Order (EO) excitation to excite a bladeddisk is that the EO frequency coincides with the natural frequency of the structure. Þ Thus to detect the possible resonant points, it remains to superpose on the graph the Engine Order lines.


At the intersection between the nth Engine Order line and the line of natural frequencies of a mode characterised by n nodal diameters, a possible resonant condition can be found. The most dangerous EOs are generally the first ones (1 to 4).
