Application to A Helicopter of A General Method For Modifying a Finite-Element Model to Correlate with Modal Test Data

A general method to systematically modify the element properties in a finite element model to improve its structural dynamics correlation with modal test results is reviewed. The method has been programmed in MSC/NASTRAN DMAP language and uses FORTRAN code for iterative control. In this program, called PAREDYM, the user specifies the modal frequencies and shapes that he wishes the finite element model to correlate with, as well as the element properties that he wishes to allow to vary. The program then solves for the changes required in the design variables to achieve the desired correlation. The correlation method has been applied to a production UH-60A BLACK HAWK helicopter, which had previously been finite-element modeled and shake tested. To keep the time required for solution within manageable bounds, use is made of a linking feature which allows properties of more than one element to be tied to a single design variable. The resulting element property changes, indicated by the program, are discussed as to their reasonableness and their help in the development of an improved model. The same program has also been used as a design tool to move vibrational modes out of an unwanted region for a new helicopter currently under development.