  We have used nanoelectromechanical resonators to probe superfluid $^4$He at
different temperature regimes, spanning over four orders of magnitude in
damping. These regimes are characterized by the mechanisms which provide the
dominant contributions to damping and the shift of the resonance frequency:
tunneling two level systems at the lowest temperatures, ballistic phonons and
rotons at few hundred mK, and laminar drag in the two-fluid regime below the
superfluid transition temperature as well as in the normal fluid. Immersing the
nanoelectromechanical resonators in fluid increases their effective mass
substantially, decreasing their resonance frequency. Dissipationless superflow
gives rise to a unique possibility to dramatically change the mechanical
resonance frequency in situ, allowing rigorous tests on different damping
models in mechanical resonators. We apply this method to characterize tunneling
two-level system losses and magnetomotive damping in the devices.
