scaling Steam path dimensions

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Well-Known Member
May 21, 2016
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I am planning on building a 4" scale compound steam engine. Changing the linear scale by 1/3 affects area by a factor of 1/9 and volume scale by 1/27. Physics doesn't scale so I am concerned that changing the linear scale may require modifications to the steam path dimensions, especially on a compound engine.

Does the scaling require any non-scale modifications to the steam path?
eg. steam pipe bore dimensions, piston/cylinder dimensions, stroke etc.
Scale passages will be fine, but models are arguably at an advantage here because of the lower Reynolds Numbers involved. This is why small engines can get away with a drilled passage instead of a big rectangular section one. At 4", the bore and stroke can be scale or pretty close.
I have copies of the original factory drawings for the 1925 Yorkshire Patent Steam Wagon that I plan to model. I have figured out that the easiest way to build a model is to simply scale and follow the factory drawings. The manufacture was highly optimised based on contemporary methods and technology. If I make a non-scale change from the factory drawings, it is likely to have a domino effect that will take more time and effort to model than just following the drawings. Avoiding changes to the steam path is an important goal.



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