R.P. Roberts

The representation of frequency dependent unsteady aerodynamics in aeroelastic analysis has been the subject of considerable research. The objective has been to provide a convenient method giving a good approximation to the theoretical frequency dependence for time domain analysis and eigenvalue evaluation (for root locus and flutter). This cannot be accomplished without a state space or equivalent formulation. The work reported here uses one of the existing rational function approximation techniques and illustrates its application to an aircraft configuration. The frequency dependent aerodynamics have been approximated using the Minimum State method developed by Karpel. This relies on fitting the variation of the real and imaginary parts of the aerodynamics as calculated from the B and C matrices (U.K. notation) for discrete values of frequency parameter. Additional states are required in the model to reproduce this variation but, because of the form of solution, the increase in dimension need only be of the order of one fifth of the number of states in the structural dynamic model. This represents a considerable saving when compared to related techniques. The method of solution for the fits is presented and it is shown how the resulting matrices can be introduced into the state space form of simple aeroelastic systems. Approximations to aerodynamic data are discussed and an optimizer is used with varying degrees of success to reduce errors. Preliminary aeroelastic results have been obtained using these fits, and some assessment is made of the effect of varying the accuracy of the approximations. The investigations include roots evaluations, frequency and time responses, and where possible the results are compared with a "baseline" response from current procedures. The eigenvalues illustrate that it is occasionally possible to introduce regions of instability to the locus of previously stable roots even though the rational function approximations themselves contain only stable poles. The frequency responses show considerable differences when compared to those produced using single low frequency parameter data, but are quite similar to each other and that produced using circular interpolations for the aerodynamics. Time responses are compared with values synthesized using an interpolated transfer. function and the Fourier coefficients of an input signal and various problems are highlighted.

International Forum on Aeroelasticity and Structural Dynamics 1991, Aachen

Conference Paper

englisch

21,0 x 29,7 cm, 10 Seiten

DGLR-Bericht, 1991, 1991-06, International Forum on Aeroelasticity and Structural Dynamics 1991 Proceedings; S.305-314; 1991; Deutsche Gesellschaft für Luft- und Raumfahrt - Lilienthal-Oberth e.V., Bonn

NA

aeroelasticity

Bestellbar

1991