  A likely candidate mechanism to heat the solar corona and solar wind is
low-frequency "Alfv\'enic" turbulence sourced by magnetic fluctuations near the
solar surface. Depending on its properties, such turbulence can heat different
species via different mechanisms, and the comparison of theoretical predictions
to observed temperatures, wind speeds, anisotropies, and their variation with
heliocentric radius provides a sensitive test of this physics. Here we explore
the importance of normalized cross helicity, or imbalance, for controlling
solar-wind heating, since it is a key parameter of magnetized turbulence and
varies systematically with wind speed and radius. Based on a hybrid-kinetic
simulation in which the forcing's imbalance decreases with time -- a crude
model for a plasma parcel entrained in the outflowing wind -- we demonstrate
how significant changes to the turbulence and heating result from the "helicity
barrier" effect. Its dissolution at low imbalance causes its characteristic
features -- strong perpendicular ion heating with a steep "transition-range"
drop in electromagnetic fluctuation spectra -- to disappear, driving more
energy into electrons and parallel ion heat, and halting the emission of
ion-scale waves. These predictions seem to agree with a diverse array of
solar-wind observations, offering to explain a variety of complex correlations
and features within a single theoretical framework.
