This alticle describes the evolution of sinusoidal waves propagating on a wafer sheet surrounded by two high-velocity, co-flowing air streams. The analysis is based on linear instability theory and includes viscosity in both gas and liquid perturbations, while for the stationary velocity profiles the flows have been treated as inviscid. To discern the role of the gas dynamics, air and water velocities have been varied independently. Flow renditions have been selected to compare the numerical predictions with experimental measurements of oscillation frequencies, wavelengths, and growth rates. The dispersion diagrams show a broad viscosity-enhanced zone, a consequence of considering viscosity in the gas perturbation.