hi peter,

the greenly 1:3 and 1:6 cant be bettered IMHO.

a slight divergent taper on the chimney is supposed to be advantageous ie 1.5 or 2 degrees (though ASHFORD has a short chimney). the fullsize rules dont quite apply to miniature because the exhaust pressure is much lower as per the findings of the late jim ewins. jim didnt like the idea if applying sam ell/swindon draughting to miniature locos and decided the swindon 'bell' to the bottom of the petticoat pipe to be superfluous but there is no harm in adding a 'bell' if space permits.

have a close look at the correspondence at the time about the S15 valvegear and ASHFORD's in ME at the time from don ashton.

cheers,

julian

Edited By julian atkins on 06/06/2013 00:19:48

hi peter,

the divergent taper on some blast nozzle designs seems to be a throwback to what steam does at high pressure when existing a nozzle… as in the steam cone of an injector. i dont know enough about gas flow to be able to comment on whether it has any effect on a blast nozzle in miniature by way of reducing the 'spreading out' of the envelope of exhaust steam in the smokebox… it seems very doubtful to me due to the very low pressure of the exhaust in miniature and the fact that the bigger 'envelope' the greater the vacuum created.

i have always made the nozzles with a short converging taper finally a short parallel section. of crucial importance is getting the chimney to be concentric with the blastpipe. ive also made quite substantial alterations to a few cylinder blocks to avoid a 'bent pipe' blast pipe and instead have a straight blast pipe which is as rigid as possible.

cheers,

julian