On the Operational Modal Analysis Techniques for the estimate of modal parameters of aircraft structures during flying vibration tests

Operational Modal Analysis (OMA) for modal identification is currently expanding from the civil to the aerospace engineering field. This is due to the capability to extract modal parameters from operational conditions with a non-deterministic input, even though assumed random in space and time. Thus, no assumptions shall be made on the boundary conditions for the structure under test, since those are actual in service. Furthermore, the need for a random excitation source could be easily satisfied in flight, setting the aircraft in a straight and level, stationary condition. These peculiarities are fostering the interest in OMA for flutter testing applications. Nevertheless, when compared to classical Flight Vibration Testing techniques, OMA is more sensitive to measurement chain noise and sensors quantity. In this paper, frequency and time domain OMA techniques are applied, and their effectiveness evaluated on simulated random response data generated from the Finite Element Model of a typical high-performance aircraft wing, the AGARD 445.6. Optimal sensors positions are identified for an increasing number of measurement points. Natural frequencies and mode shapes are identified with different OMA techniques and compared to the true data source, even introducing output noise components

Study on a modified mode tracking technique in support of aeroelastic stability verification by flight testing

Mass and aerodynamic modifications often lead to significant changes in the aeroelastic behavior of an aircraft, requiring flight test evidence to demonstrate the aeroelastic stability within an intended flight envelope. This is particularly true in those cases where no aeroelastic modeling products are available. In this context, the growing interest in Operational Modal Analysis (OMA) poses new challenges when dealing with aeroelastic modes tracking since modal parameters estimation is done considering the aircraft as a linear time invariant system. This is not the case when dealing with mass changes due to fuel consumption. As result, modal parameters estimates could be significantly biased. Consequently, mode shapes tracking could become difficult through consecutive test points, particularly when using the pole-weighted version of the Modal Assurance Criterion (MAC), called MACXP. The purpose of this paper is to propose an improved tracking technique, based on the MACXP, to be applied with modal estimates obtained with the OMA method. The proposed tracking method is validated on a known multiple mass-spring-damper system. Modal parameters estimate accuracy is also compared with the current state of the art MACXP tracking method. An experimental application is also provided for modes tracking on a modified single seat aircraft to evaluate effectiveness with real data from testing