  1156+295 is a flat-spectrum quasar which is loud at radio and gamma-ray.
Previous observations of the source revealed a radio morphology on pc to kpc
scales consistent with a helical jet model. In our present research, this
source was observed with the VLBA at 86, 43 and 15 GHz on four epochs from 10
May 2003 to 13 March 2005 aiming at studying the structure of the innermost jet
in order to understand the relation between the helical structure and the
astrophysical processes in the central engine. A core-jet structure with six
jet components is identified. The apparent transverse velocities of the six jet
components derived from proper motion measurements are in the range between 3.6
c and 11.6 c. The overall jet shape shows oscillatory morphology with multiple
curvatures on pc scales which might be indicative of a helical pattern. Models
of helical jet are discussed on the basis of both Kelvin-Helmholtz (K-H)
instability and jet precession. The K-H instability model shows better
agreement with the observed data. The overall radio structure on the scale from
sub-pc to kpc appears to be fitted with a hydrodynamic model with the
fundamental helical mode in Kelvin-Helmholtz (K-H) instability. This helical
mode with an initial characteristic wavelength of 0.2 pc is excited at the base
of the jet on the scale of 0.005 pc (or 1000R_s, the typical size of the broad
line region for a super massive black hole of $4.3\times10^8M_{\odot}$). A
presessing jet model can also fit the observed jet structure on the scale
between 10 pc and 300 pc. However, additional astrophysical processes may be
required for the presessing jet model in order to explain the bendings on the
inner jet structure (1 to 10 pc) and re-collimation of the large scale jet
outflow (>300 pc).
