Experimental modal analysis of aircrafts on the basis of monophasic vibrations

Modal tests of aircrafts are carried out in order to determine eigenfrequencies, modes, generalized masses and damping characteristics of eigentones. These modal parameters are used to estimate the operational loading, strength, controllability and aeroelastic stability of the aircraft. The proposed test method refers to the classical experimental modal analysis of dynamic systems. To realize it, we separate the eigentones of structures in a given frequency range by the phase resonance method using a multichannel system of vibration excitation and measurement. The eigenfrequencies are determined by phase resonances. By the relations between the forced monophasic vibrations and eigenvibrations, the dissipative properties of the dynamic system are revealed. Thus, if monophasic vibrations on monophasic excitation coincide with eigenvibrations at frequencies that differ from eigenfrequencies, then the vibrations of the structure for each eigentone are described by the motion equation of the linear oscillator. The generalized mass, the generalized damping and the generalized stiffness of the corresponding tone are the oscillator parameters. The oscillator parameters are determined by the amplitude-frequency response of the structure in the neighborhood of the phase resonance frequency, from the condition of minimum differences of the generalized forces that realize the same structure and oscillator vibration amplitudes. If monophasic vibrations do not exist in monophasic excitation or do not coincide with the eigenvibrations, then nonmonophasic excitation is used to determine the modal parameters. Advantages of this structural modal characteristics identification method are the reasonableness of the adopted damping model, low sensitivity to vibration measurement errors during testing and to the mutual influence of the tones with close eigenfrequencies, and the possibility to estimate reliability of the generalized masses and stiffnesses determination through the phase resonance frequencies. The reliability of the eigentones generalized characteristics determined by the proposed method is confirmed by the modal test data on the Tu-334 airplane dynamically similar model and the full-scale products Su-30, Yak-152 and the spacecraft unit.