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  • 1
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    Wavelet Applications for Flight Flutter Testing (1999)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: Wavelets present a method for signal processing that may be useful for analyzing responses of dynamical systems. This paper describes several wavelet-based tools that have been developed to improve the efficiency of flight flutter testing. One of the tools uses correlation filtering to identify properties of several modes throughout a flight test for envelope expansion. Another tool uses features in time-frequency representations of responses to characterize nonlinearities in the system dynamics. A third tool uses modulus and phase information from a wavelet transform to estimate modal parameters that can be used to update a linear model and reduce conservatism in robust stability margins.
    Keywords: Aircraft Stability and Control
    Type: CEAS/AIAA/ICASE/NASA Langley International Forum on Aeroelasticity and Structural Dynamics 1999; Pt. 1; 393-402; NASA/CP-1999-209136/PT1
    Format: application/pdf
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  • 2
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    Development of an Efficient Binaural Simulation for the Analysis of Structural Acoustic Data (2003)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Applying binaural simulation techniques to structural acoustic data can be very computationally intensive as the number of discrete noise sources can be very large. Typically, Head Related Transfer Functions (HRTFs) are used to individually filter the signals from each of the sources in the acoustic field. Therefore, creating a binaural simulation implies the use of potentially hundreds of real time filters. This paper details two methods of reducing the number of real-time computations required by: (i) using the singular value decomposition (SVD) to reduce the complexity of the HRTFs by breaking them into dominant singular values and vectors and (ii) by using equivalent source reduction (ESR) to reduce the number of sources to be analyzed in real-time by replacing sources on the scale of a structural wavelength with sources on the scale of an acoustic wavelength. The ESR and SVD reduction methods can be combined to provide an estimated computation time reduction of 99.4% for the structural acoustic data tested. In addition, preliminary tests have shown that there is a 97% correlation between the results of the combined reduction methods and the results found with the current binaural simulation techniques
    Keywords: Acoustics
    Type: Paper 77 , 8th International Conference on Recent Advances in Structural Dynamics; Jul 14, 2003 - Jul 16, 2003; Southampton; United Kingdom
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  • 3
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    Worst-Case Flutter Margins from F/A-18 Aircraft Aeroelastic Data (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An approach for computing worst-case flutter margins has been formulated in a robust stability framework. Uncertainty operators are included with a linear model to describe modeling errors and flight variations. The structured singular value, micron, computes a stability margin which directly accounts for these uncertainties. This approach introduces a new method of computing flutter margins and an associated new parameter for describing these margins. The micron margins are robust margins which indicate worst-case stability estimates with respect to the defined uncertainty. Worst-case flutter margins are computed for the F/A-18 SRA using uncertainty sets generated by flight data analysis. The robust margins demonstrate flight conditions for flutter may lie closer to the flight envelope than previously estimated by p-k analysis.
    Keywords: Aircraft Stability and Control
    Type: NASA/TM-97-207564 , NAS 1.15:207564 , AIAA Paper 97-1266 , Structures, Structural Dynamics and Materials Conference; Apr 01, 1997; Orlando, FL; United States
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  • 4
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    Wavelet Filtering to Reduce Conservatism in Aeroservoelastic Robust Stability Margins (1998)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Wavelet analysis for filtering and system identification was used to improve the estimation of aeroservoelastic stability margins. The conservatism of the robust stability margins was reduced with parametric and nonparametric time-frequency analysis of flight data in the model validation process. Nonparametric wavelet processing of data was used to reduce the effects of external desirableness and unmodeled dynamics. Parametric estimates of modal stability were also extracted using the wavelet transform. Computation of robust stability margins for stability boundary prediction depends on uncertainty descriptions derived from the data for model validation. F-18 high Alpha Research Vehicle aeroservoelastic flight test data demonstrated improved robust stability prediction by extension of the stability boundary beyond the flight regime.
    Keywords: Aircraft Stability and Control
    Type: NASA/TM-1998-206545 , NAS 1.15:206545 , H-2222 , AIAA Paper 98-1896 , Structures, Structural Dynamics and Materials Conference; Apr 20, 1998 - Apr 23, 1998; Long Beach, CA; United States
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  • 5
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    Estimation of Modal Parameters Using a Wavelet-Based Approach (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Modal stability parameters are extracted directly from aeroservoelastic flight test data by decomposition of accelerometer response signals into time-frequency atoms. Logarithmic sweeps and sinusoidal pulses are used to generate DAST closed loop excitation data. Novel wavelets constructed to extract modal damping and frequency explicitly from the data are introduced. The so-called Haley and Laplace wavelets are used to track time-varying modal damping and frequency in a matching pursuit algorithm. Estimation of the trend to aeroservoelastic instability is demonstrated successfully from analysis of the DAST data.
    Keywords: Aircraft Stability and Control
    Type: NASA/TM-97-206300 , NAS 1.15:206300 , H-2214 , Atmospheric Flight Mechanics Conference; Aug 08, 1997 - Aug 11, 1997; New Orleans, LA; United States
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  • 6
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    Utilizing Flight Data to Update Aeroelastic Stability Estimates (1997)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Stability analysis of high performance aircraft must account for errors in the system model. A method for computing flutter margins that incorporates flight data has been developed using robust stability theory. This paper considers applying this method to update flutter margins during a post-flight or on-line analysis. Areas of modeling uncertainty that arise when using flight data with this method are investigated. The amount of conservatism in the resulting flutter margins depends on the flight data sets used to update the model. Post-flight updates of flutter margins for an F/A-18 are presented along with a simulation of on-line updates during a flight test.
    Keywords: Aircraft Stability and Control
    Type: Atmospheric Flight Mechanics Conference; Aug 01, 1997; New Orleans, LA; United States
    Format: text
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  • 7
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    Wavelet Applications for Flight Flutter Testing (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Wavelets present a method for signal processing that may be useful for analyzing responses of dynamical systems. This paper describes several wavelet-based tools that have been developed to improve the efficiency of flight flutter testing. One of the tools uses correlation filtering to identify properties of several modes throughout a flight test for envelope expansion. Another tool uses features in time-frequency representations of responses to characterize nonlinearities in the system dynamics. A third tool uses modulus and phase information from a wavelet transform to estimate modal parameters that can be used to update a linear model and reduce conservatism in robust stability margins.
    Keywords: Aircraft Stability and Control
    Type: H-2364 , International Forum on Aeroelasticity and Structural Dynamics; Jun 22, 1999 - Jun 25, 1999; Williamsburg, VA; United States
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  • 8
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    Development of an Efficient Binaural Simulation for the Analysis of Structural Acoustic Data (2002)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: Binaural or "virtual acoustic" representation has been proposed as a method of analyzing acoustic and vibroacoustic data. Unfortunately, this binaural representation can require extensive computer power to apply the Head Related Transfer Functions (HRTFs) to a large number of sources, as with a vibrating structure. This work focuses on reducing the number of real-time computations required in this binaural analysis through the use of Singular Value Decomposition (SVD) and Equivalent Source Reduction (ESR). The SVD method reduces the complexity of the HRTF computations by breaking the HRTFs into dominant singular values (and vectors). The ESR method reduces the number of sources to be analyzed in real-time computation by replacing sources on the scale of a structural wavelength with sources on the scale of an acoustic wavelength. It is shown that the effectiveness of the SVD and ESR methods improves as the complexity of the source increases. In addition, preliminary auralization tests have shown that the results from both the SVD and ESR methods are indistinguishable from the results found with the exhaustive method.
    Keywords: Acoustics
    Type: NASA/CR-2002-211753 , NAS 1.26:211753
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  • 9
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    Development and Testing of a High Level Axial Array Duct Sound Source for the NASA Flow Impedance Test Facility (2000)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: In this report both a frequency domain method for creating high level harmonic excitation and a time domain inverse method for creating large pulses in a duct are developed. To create controllable, high level sound an axial array of six JBL-2485 compression drivers was used. The pressure downstream is considered as input voltages to the sources filtered by the natural dynamics of the sources and the duct. It is shown that this dynamic behavior can be compensated for by filtering the inputs such that both time delays and phase changes are taken into account. The methods developed maximize the sound output while (i) keeping within the power constraints of the sources and (ii) maintaining a suitable level of reproduction accuracy. Harmonic excitation pressure levels of over 155dB were created experimentally over a wide frequency range (1000-4000Hz). For pulse excitation there is a tradeoff between accuracy of reproduction and sound level achieved. However, the accurate reproduction of a pulse with a maximum pressure level over 6500Pa was achieved experimentally. It was also shown that the throat connecting the driver to the duct makes it difficult to inject sound just below the cut-on of each acoustic mode (pre cut-on loading effect).
    Keywords: Acoustics
    Type: NASA/CR-2000-210645 , NAS 1.26:210645
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