  We study effects of a gate-controlled Rashba spin-orbit coupling to quantum
spin-Hall edge states in HgTe quantum wells. A uniform Rashba coupling can be
employed in tuning the spin orientation of the edge states while preserving the
time-reversal symmetry. We introduce a sample geometry where the Rashba
coupling can be used in probing helicity by purely electrical means without
requiring spin detection, application of magnetic materials or magnetic fields.
In the considered setup a tilt of the spin orientation with respect to the
normal of the sample leads to a reduction in the two-terminal conductance with
current-voltage characteristics and temperature dependence typical of Luttinger
liquid constrictions.
