  The spiral magnetic order in cubic MnSi-type crystals is considered using the
model of classical Heisenberg ferromagnetics with an extra interaction of the
Dzyaloshinskii-Moriya (DM) type between neighboring atoms. It is found how the
wave vector of magnetic helices depends on DM vector. The wave number k
determines both the sign and strength of global spiraling whereas locally,
within a unit cell, the helical order can be strongly frustrated so that the
twist angles between neighboring ferromagnetic layers may be even of different
signs. Conical deformations of helices caused by an arbitrary directed external
magnetic field is also considered within the same model. The critical field of
helix unwinding is found and it is shown that even in the unwound state there
remains a residual periodic splay of magnetic moments which can be measured by
diffraction methods. It is also demonstrated how the usually used continuous
picture of moment distribution can be obtained from the discrete one in a
coarse grain approximation.
