A simple integral relation is obtained for the distribution functions of jet speeds and jet-counterjet flux asymmetries of classical double radio sources on the basis of a simple model in which it is assumed that the jets are intrinsically symmetric and that the asymmetries are attributed to relativistic beaming .
Analytic expressions relating the mean jet speed and the mean jet flux asymmetry , and their variances are derived .
The results are considered in the light of orientation-based unified schemes , and an analytical equation for the critical angle separating quasars and radio galaxies is derived .
The mean jet speeds of classical double radio sources are estimated using the asymmetry of jet-counterjet flux densities taken from the 3CRR and B2 samples .
For FRI radio sources the mean jet speed is \sim ( 0.54 \pm 0.03 ) c , while for FRII low-redshift radio galaxies and intermediate redshift quasars the values found are \sim ( 0.4 \pm 0.06 ) c and \ga 0.6 c respectively .