The lift on an airfoil in a velocity field of perfect superfluid flow within liquid helium ii has been found to vanish for sufficiently low velocity. Thus the flow is pure potential flow with no circulation, and the viscosity boundary condition of zero velocity at the trailing edge (Kutta condition) which is normally introduced does not apply. We believe this to be the first illustration of absolutely perfect hydrodynamic flow, i.e., one in which the viscosity vanishes identically and therefore asserts no influence on the flow field.

• Received 9 August 1957

DOI:https://doi.org/10.1103/PhysRev.108.1109