  When modeling the three-dimensional hydrodynamics of interstellar material
rotating in a galactic gravitational potential, it is useful to have an
analytic expression for gravitational perturbations due to stellar spiral arms.
We present such an expression for which changes in the assumed characteristics
of the arms can be made easily and the sensitivity of the hydrodynamics to
those characteristics examined. This analytic expression also makes it easy to
rotate the force field at the pattern angular velocity with little overhead on
the calculations.
