Yes , this is a problem - to scale a newtonian fluid or worse still a thixotropic one! Normally air is substituted in initial small scale experiments but inevitably the real thing has to be employed (like at Teddington). often only the viscosity needs scaling. You are spot-on though regarding the eye's point of view (sic), the efforts to reproduce faithfully to scale are often spoilt by the results - I find that GWR tapered boilers and funnels rarely look 'right', in particular on the small praries as the cab falsely appears to be too large.
Peter A Montarlot