Problems of Scale with Scale Models

Another one for the brains trust after another wet weekend.

I was looking at the equations for flywheel weight for steam engines.
If the same equations hole for scale models then the flywheel would
have inadequate mass for smooth motion of a model running at original
speed and the model would have to run faster - e.g. 12 times faster
for a 12th scale model, or the motion would be jerky or stop.

But then I started to think about bearings. The mass of a 12th scale
model is reduced by 12 X 12 X 12, the bearing area is reduced by 12 X
12. So the bearing area on the model is 12 times larger than is
actually needed to support the mass (good), but that means 12 times
the viscous drag, and if the model is running 12 times faster too,
then 144 times the viscous drag.

And then I started to think about flyball governors ....er.... and
rapidly decided to give up and ask the newsgroup if there have been
any ME articles or books published on the problems of scale? Its
fairly fundamental stuff that must have effected everyone making scale
models - but I don't think I have ever seen a nice clear article on
the topic.

Steve

Cheshire Steve wrote:
....
> rapidly decided to give up and ask the newsgroup if there have been
> any ME articles or books published on the problems of scale? Its
> fairly fundamental stuff that must have effected everyone making scale
> models - but I don't think I have ever seen a nice clear article on
> the topic.
>
> Steve
>
I will echo that interest. It's something that makes modelling a
challenge - you can have a model that looks good or you can have a model
that works well, but how do you achieve the two together.
Peter

In message <1185220909.219828.326520@w3g2000hsg.googlegroups. com>,
Cheshire Steve <oldnoccer@btinternet.com> writes

[snip]

>What you say is encouraging. I was after making a beam engine model
>that would run close to normal rpm to allow you to see the Watts
>linkage working, and wondering if I could make a working governor
>using a variable cut-off.

The late Lillian Lawrence (a.k.a.LBSC) wrote, "You can't scale nature".
I believe that no one has yet proved him wrong by producing a true small
scale working centrifugal governor.

--
Mike Hopkins
CSME <http://www.cheltsme.org.uk>
5" gauge (2 1/2" scale) Alice class Hunslet

>I hoped this would have been discussed in one of the modelling
>magazines - it is so fundamental to everything to do with scale
>modelling.

Exactly - it would be hard for a mag to cover 'everything', while many
articles may have discussed particular cases where scale effects are
important. In any case there often won't be any 'correct' solution.

Consider a fire-tube boiler, for example. If you tried to make a model
with the same number of tubes as in full size, it would not work. Many
years ago Keiler did some research (measuring boilers that worked) and
came up with a formula that seems to work satisfactorily that relates
tube length to the area of its bore. That formula is completely
empirical.

If you are really interested in this then you have to get to grips with
the engineering science for yourself, and where the science doesn't help
you have to experiment. A good pre-war text book such as D A Low's
'Heat Engines' should contain a chapter on governors.

--
Charles Lamont

Cheshire Steve wrote:

> I hoped this would have been discussed in one of the modelling
> magazines - it is so fundamental to everything to do with scale
> modelling.

I remember that I've read something about it, but don't ask me where.

Re governor:
Increase its RPM, use lead weights, make a bit out of scale longer levers.

If you want a working model, you have to accept physics. Meaning bigger
tubes than scale, higher rpm, less power etc.

Nick

No, mine are not governed although two have scale governors that are not
used.

I would have thought a beam engine with its large scale flywheel would
easily rotate around 40 rpm (maybe less) and at less than one rev per sec
the Watts linkage would easily been seen. Should be controllable with the
steam throttle valve at that sort of speed without too much variation if a
decent boiler capacity is behind it.

On 24 Jul, 17:20, "Alan Marshall" <alan_marsh...@ldsl.pipex.com>
wrote:
> No, mine are not governed although two have scale governors that are not
> used.
>
> I would have thought a beam engine with its large scale flywheel would
> easily rotate around 40 rpm (maybe less) and at less than one rev per sec
> the Watts linkage would easily been seen. Should be controllable with the
> steam throttle valve at that sort of speed without too much variation if a
> decent boiler capacity is behind it.

Thanks guys, I'll proceed with my model, and run it before finalising
the governor design. Its not just the speed but the ability to get
enough force out of a tiny governor to operate a valve or linkage. I
thought the variable cut-off might take less force.

The technical problem of scale-up is the same as that of scale-down.
The only engineering texts I can locate relate to scale-up from pilot
plant to full scale production of chemical reactors. These days I
assume scale protypes of mechanical items are rare as its all done on
computer, but I still hear arguments about the ability to scale up
10kw wind turbines to 1 MW wind turbines, and that one design will
overtake another at large scale (horizontal axis versus vertical
axis).

Maybe I am asking too much of science/engineering - and maybe the
model engineering side has always done it in dribs and drabs - and no-
one has ever pulled it all together. I was trying to understand what
goes wrong so I can design around it in the first place (rather than
build something and be disappointed, and have to adapt it). But this
discussion has been helpful.

Thanks, Steve

	#1
Cheshire Steve	Problems of Scale with Scale Models Another one for the brains trust after another wet weekend. I was looking at the equations for flywheel weight for steam engines. If the same equations hole for scale models then the flywheel would have inadequate mass for smooth motion of a model running at original speed and the model would have to run faster - e.g. 12 times faster for a 12th scale model, or the motion would be jerky or stop. But then I started to think about bearings. The mass of a 12th scale model is reduced by 12 X 12 X 12, the bearing area is reduced by 12 X 12. So the bearing area on the model is 12 times larger than is actually needed to support the mass (good), but that means 12 times the viscous drag, and if the model is running 12 times faster too, then 144 times the viscous drag. And then I started to think about flyball governors ....er.... and rapidly decided to give up and ask the newsgroup if there have been any ME articles or books published on the problems of scale? Its fairly fundamental stuff that must have effected everyone making scale models - but I don't think I have ever seen a nice clear article on the topic. Steve

	#2
Peter J Seymour	Re: Problems of Scale with Scale Models Cheshire Steve wrote: .... > rapidly decided to give up and ask the newsgroup if there have been > any ME articles or books published on the problems of scale? Its > fairly fundamental stuff that must have effected everyone making scale > models - but I don't think I have ever seen a nice clear article on > the topic. > > Steve > I will echo that interest. It's something that makes modelling a challenge - you can have a model that looks good or you can have a model that works well, but how do you achieve the two together. Peter

	#3
Mike Hopkins	Re: Problems of Scale with Scale Models In message <1185220909.219828.326520@w3g2000hsg.googlegroups. com>, Cheshire Steve <oldnoccer@btinternet.com> writes [snip] >What you say is encouraging. I was after making a beam engine model >that would run close to normal rpm to allow you to see the Watts >linkage working, and wondering if I could make a working governor >using a variable cut-off. The late Lillian Lawrence (a.k.a.LBSC) wrote, "You can't scale nature". I believe that no one has yet proved him wrong by producing a true small scale working centrifugal governor. -- Mike Hopkins CSME <http://www.cheltsme.org.uk> 5" gauge (2 1/2" scale) Alice class Hunslet

	#4
Charles Lamont	Re: Problems of Scale with Scale Models >I hoped this would have been discussed in one of the modelling >magazines - it is so fundamental to everything to do with scale >modelling. Exactly - it would be hard for a mag to cover 'everything', while many articles may have discussed particular cases where scale effects are important. In any case there often won't be any 'correct' solution. Consider a fire-tube boiler, for example. If you tried to make a model with the same number of tubes as in full size, it would not work. Many years ago Keiler did some research (measuring boilers that worked) and came up with a formula that seems to work satisfactorily that relates tube length to the area of its bore. That formula is completely empirical. If you are really interested in this then you have to get to grips with the engineering science for yourself, and where the science doesn't help you have to experiment. A good pre-war text book such as D A Low's 'Heat Engines' should contain a chapter on governors. -- Charles Lamont

	#5
Nick Mueller	Re: Problems of Scale with Scale Models Cheshire Steve wrote: > I hoped this would have been discussed in one of the modelling > magazines - it is so fundamental to everything to do with scale > modelling. I remember that I've read something about it, but don't ask me where. Re governor: Increase its RPM, use lead weights, make a bit out of scale longer levers. If you want a working model, you have to accept physics. Meaning bigger tubes than scale, higher rpm, less power etc. Nick

	#6
Alan Marshall	Re: Problems of Scale with Scale Models No, mine are not governed although two have scale governors that are not used. I would have thought a beam engine with its large scale flywheel would easily rotate around 40 rpm (maybe less) and at less than one rev per sec the Watts linkage would easily been seen. Should be controllable with the steam throttle valve at that sort of speed without too much variation if a decent boiler capacity is behind it.