Linear size distribution of double radio sources in the constant speed symmetric expansion model

The distribution of projected linear sizes of double radio sources is derived in an intrinsically-symmetric model in which the hotspots move away from the parent object at speedv before fading at timeT after their simultaneous origin. This is applied to a sample of doubles, separating the two asymmetry types closer-hotspot-brighter (yes-type) and closer-hotspot-fainter (no-type). Along with the arm-ratio distributions for the two classes, no-doubles very roughly indicatevsim0.3c, but no consistent value can be found for yes-doubles. A possible generalization of the model is suggested to allow different expansion speeds for different sources (though still retaining the intrinsic bilateral symmetry). Other effects that could be included in the model are also indicated.