drafting

[Peter DicksonParticipant @peterdickson]

I am trialing a number of drafting measures on a loco built to the ashford design.

I would be interested in comments with regards the performance of lococ built to the Evans drawings and any modifications applied

[Peter DicksonParticipant @peterdickson]

effects of changes in petticote pipe and blast pipe diameters

[julian atkinsParticipant @julianatkins58923]

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

[Peter DicksonParticipant @peterdickson]

Thanks Julian,

I have opted for 2 deg on chimney , I have made a range of blast nozzles, I am still not sure of any benifit in having a small divergent taper at the end of the blast nozzle..

this appears in many greenly designs but nowhere else

[julian atkinsParticipant @julianatkins58923]

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

[RRMBKParticipant @rrmbk]

Might be worth looking at locomotive Draughting, M.E. July/ August 1989. Contains some interesting data and drawings.

[Peter DicksonParticipant @peterdickson]

Brian,

found it brilliant many thanks answers all the questions

best regards Peter

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