  The dynamics of the truncated Euler equations with helical initial conditions
are studied. Transient energy and helicity cascades leading to Kraichnan
helical absolute equilibrium at small scales are obtained for the first time.
The results of [Cichowlas et al. Phys. Rev. Lett. 95, 264502 (2005)] are
extended to helical flows. Similarities between the turbulent transient
evolution of the ideal (time-reversible) system and viscous helical flows are
found. The observed differences in the behavior of truncated Euler and
(constant viscosity) Navier-Stokes are qualitatively understood using the
concept of eddy viscosity. The large scales of truncated Euler equations are
then shown to follow quantitatively an effective Navier-Stokes dynamics based
on a variable (scale dependent) eddy viscosity.
