The Effect of Aerodynamic Pressure on the Natural Frequencies of Helicopter Windshield

Abstract:
Operational shifts in natural frequency constitute a critical phenomenon for structures exposed to multiple excitation sources over a broad frequency range. To investigate this behavior on the helicopter windshield, finite element models were correlated with ground modal test results and assessed using in flight measurement data. In rotorcraft, the large number of operational excitation sources makes it particularly difficult to distinguish structural responses associated with the inherent dynamic characteristics of the windshield from those induced by the aircraft's periodic excitations. To address this challenge, time synchronous averaging was employed to remove the dominant main rotor frequency components from the measured flight data. The residual response was then analyzed using joint time frequency analysis techniques, revealing that the windshield natural frequencies shift with increasing flight speed and the associated aerodynamic pressure variation. For further correlation of the finite element models, in flight impact hammer tests were performed. The resulting responses were processed through operational modal analysis methods to extract the exact natural frequencies more accurately under operational conditions. The validated methodology was subsequently applied to windshields of different thicknesses, demonstrating that windshield thickness has a pronounced influence on the observed frequency shift. These findings indicate that the problem should be treated as a coupled dynamic, static, and aerodynamic interaction. An illustrative mitigation study was also conducted to examine possible interventions for this phenomenon, and the influence of structural modification on the variable dynamic characteristics of the structure was evaluated.

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