  The existence of a second quadratic inviscid invariant, the helicity, in a
turbulent flow leads to coexisting cascades of energy and helicity. An
equivalent of the four-fifth law for the longitudinal third order structure
function, which is derived from energy conservation, is easily derived from
helicity conservation cite{Procaccia,russian}. The ratio of dissipation of
helicity to dissipation of energy is proportional to the wave-number leading to
a different Kolmogorov scale for helicity than for energy. The Kolmogorov scale
for helicity is always larger than the Kolmogorov scale for energy so in the
high Reynolds number limit the flow will always be helicity free in the small
scales, much in the same way as the flow will be isotropic and homogeneous in
the small scales. A consequence is that a pure helicity cascade is not
possible. The idea is illustrated in a shell model of turbulence.
