This paper presents a technique for assessing the influence of configuration geometry, size, and mass on the dynamics of hovering vehicles. First, the equations of motion for small perturbations from hover are discussed and certain derivatives removed or simplified from considerations of symmetry. The remaining derivatives are then related to configuration geometry, size, and mass by simple momentum theory. The accuracy of this theory is examined by comparing test results on ducted fan and free propeller configurations. The rather limited experimental data agree with the momentum theory for ducted fan configurations, provided flow separations are not important. The effects of mass and size are separated from the effects of geometry by introducing a nondimensional system of stability derivatives. It is shown how these nondimensional derivatives can be used to predict the effects of changing the mass or size of a configuration of given geometry.