An asymmetric relativistic model for FRII radio sources is described which takes account of both relativistic effects and intrinsic/ environmental asymmetries to explain the observed structural asymmetry of their radio lobes .
A key feature of the model is jet-sidedness , which can now be determined for about 80 % of the FRII sources in the 3CRR complete sample .
It is shown that a simple asymmetric relativistic model can account for a wide range of observational data , and that the relativistic and intrinsic asymmetry effects are of comparable importance .
The mean translational speed of the lobes is \overline { v } _ { lobe } = ( 0.12 \pm 0.04 ) c .
The results are in agreement with an orientation-based unified scheme in which the critical angle separating the radio galaxies from the radio quasars is about 50 \deg .