Funny enough Julius tends to do just the opposite as most of his drawings are enlarged versions of an original design. If you look along the bottom of most drawings you will see the source and can look that up to see what the original size was.

Within reason most parts will be OK just being halved etc though you will have to watch that some parts don't get too flimsy or make the call up/down to suit nominal sizes,

Also be aware that he has not built any of these engines so there are the odd errors, fits that are questionable or parts that just can't be assembled from time to time

Edited By JasonB on 18/08/2021 13:34:24

Not so, if linear dimensions are halved then areas are reduced to a quarter, and volumes are reduced to an eighth, of the original. Basically an inverse square law for areas and an inverse cube law for volumes.

Andrew

Plus if you " …simply divide all the given dimensions by, say, 50%…" you'll actually multiply them by 2!

But if you didn't already twig that, you'd surely find out on the way…

In one DO I worked in several decades back, they had an enormous G-clamp, allegedly made by an apprentice scaling from a 10:1 drawing. I still don't know whether I really believe that.

I would call these 3d sketches, artists illustrations, nowhere near regular engineering drawings for manufacture. Decide on some basic dimensions say bore and stroke and work outwards from there. After all you will have to interface with real world available components, nuts bolts, tubes, material thicknesses etc

Also work out how big your flywheel needs to be as scaling down half say wont work for example

But then I never went for model engineering and seeing old men sat on 5" gauge steam locos running round a back garden seemed a peculiar taste to me

Well plenty of models have been built from his drawings and although a little crowded for my taste the isometric views and assembly drawings are a big step up from the usual model engineering drawings. Particularly for the beginner who may have difficulty visualising a part from traditional plans & elevations and/or not have much idea of how all the parts fit together.

I have scaled down and up quite a few engines both steam and IC and never had a problem with the flywheels having to be scaled differently, what is the reason that it won't work?

Another batch of scaled down engines by the late Tony Boxall, mostly Anthony mount designs again every part scale by the same amount

Edited By JasonB on 18/08/2021 18:44:49

That's interesting. I'm in the process of re-designing the steam ports , slide valves and ultimately the valve gear for my one third scale traction engines. The port sizes are mostly copied from full size, scaled as per the model engine. I didn't think the ports needed enlarging. My reasoning went along the lines of the port areas have gone down by a factor of nine but the volumes are down by a factor of twenty seven. So the flows are reduced well below the reduction in area. Calculations for the model engines seem to indicate flow rates and Reynolds numbers well below what I would expect for a full size engine.

Of course I could have got my reasoning wrong – what does the team think?

Andrew

Andrew – I can't comment on the technical apsect from a personal view as I have never had need to do similar.

However I believe the reason is a result of your comment previously, that scaling by half reduces volume by one eigth. If the passages are small in the first instance of the model scaled then they become somewhat inefficient at a half reduction. The one thing that can't be scaled of course is the medium passing through them.

In the frequent discussions I have with Chris he has referred to doing this – not on all engines he has scaled – but on quite a few to get a reasonable running engine without recourse to high inlet pressures

Flow rates and Reynolds numbers wil be relevant on all but Chris is much like myself – he wants to make an engine that just works reasonably well – the enjoyment being in the making far more than the performance derived.

Tug