Abstract: A method and apparatus for determining small magnitude fluid-dynamic drag resistance differentials between different structural configurations of a variable model mounted in an environmental test facility. The variable model has disposed therein a configuration conversion mechanism to convert the variable model from a baseline configuration to a modified baseline configuration, i.e., the baseline configuration with a minor structural modification superimposed thereon, and vice versa, a nulling force mechanism to apply a nulling force to the variable model to null out the total fluid-dynamic drag resistance acting thereon, i.e., return the variable model to an undeflected position, and a system for measuring the deflections of the variable model caused by the small magnitude fluid-dynamic drag resistance differentials acting thereon. The variable model is mounted in the environmental test facility, and a predetermined fluid flow is initialized and stabilized.