Use of a sensitivity analysis of a performance index $ has shown that it may be difficult to determine aerodynamic coefficients accurately from experimental data because of the insensitivity of the solution to variations in these parameters. In addition, techniques which obtain coefficient values should be carefully evaluated to make certain that they do not use one insensitive parameter in the determination of another parameter. A perfect fit (<£ = 0) can never be achieved because of variation in the data from the assumed solution. These variations or errors come from data noise, modeling errors, and time-varying parameters. Experimental data should be obtained in regions of maximum coefficient sensitivity.