Helicopter gearbox vibrations: cyclostationary analysis or bilinear approach?

We show that a nonstationary approach can be preferred to a nonlinear one for the study of some nonlinear phenomena. We prove that the spectral correlation could detect and determine precisely a nonlinear phenomenon. Moreover, in terms of diagnosis on helicopter gearboxes (known for their strong nonlinearity), this tool is much more efficient than the bispectrum. Finally, we introduce an algorithm determining the main nature of a nonlinear cyclic phenomenon. This process is based on the use of higher order cyclic statistics.