Scaling Plans up or down

[Jack]

I have a question about a set of plans that I have. The model appears to be a little big so I was wondering if I wanted to make the model smaller, say half size, can I just cut all the measurements in half or is there some magic way of doing this? Is there something that I am over looking? I realize the fasteners will take a little adjusting but how about everything else?

Inquiring minds want to know ???

Thanks

Jack

 

[stevehuckss396]

Fasteners as you said. Gears and bearings need to be checked to see if a half size can be had.

 

[ShedBoy]

Some things obviusly scale easily enough but the thing that gets me wondering is port timing on 2 strokes. If you go up by % does this mean the ports will still be in the same place?
Hopefully Ramon can answer this.
I think the other thing is the change of forces on parts especially in combustion engines, cooling areas. I have some engine I want to build but scaled up. Interested to see how this discussion goes.

Brock

 

[Mainer]

Scaling down is not a problem from a strength point of view. Scaled-down fasteners will be strong enough. (This is NOT true when scaling up.)

You don't say what the model is, but if it's an i.c. engine you may have to adjust a few things to get it to run. The carburetor jet size, for instance, will probably need some experimentation to get right.

 

[mzetati]

Volumes, areas and linear dimensions scale at different ratios.

Marcello

 

[gbritnell]

When an engine or any mechanical device is designed it usually has metal walls made for the most strength and thinnest cross-section that is practical. When you go up or down sometimes these features have to be taken into account because a cylinder wall for example might become too thin or thicker than needed. Other parts like valve stems would also need to be looked at. Depending on what the original fastener size was sometimes you can half or double with no problem but as with metal walls they need to be looked at.
gbritnell

 

[Ramon]

Some things obviusly scale easily enough but the thing that gets me wondering is port timing on 2 strokes. If you go up by % does this mean the ports will still be in the same place?
Hopefully Ramon can answer this.
Brock

Hi Brock, I must admit I've not considered this before as my approach is to scale the engine dimensions based on the ratio of the original bore/stroke to that required and use that factor for subsequent scaling. When it comes to the timing I have tried to ascertain the original engines timing or base it on known attributes and then place the porting to give the same. I always use the timing calculator provided on Ron Chernich's MEN site for this. Whether that actually scales in direct proportion to the rest I'm not sure.

Re Jacks original question - something that needs to be borne in mind is if going up, or down, by 'half size' ie half physical size the 'volume' is increased or decreased by four. It's quite amazing how small a bolt becomes if going down or how big if vice a versa and as already said due allowance needs to be taken

This factor is something readilly apparent on the '5cc' versions I've built so far. They have far more volume of metal in them and consequently are quite heavy than if the engine was built as a basic 5cc working engine.

Regards - Ramon

 

[Ramon]

Brock - I've just found my pencil sketches of taking the dimensions off the original ETA 15d. Despite what I thought It appears I did directly scale the porting dimensions - the drawings show that (but the timing was definitely checked as referred to before) so I guess the answer to your question is no the porting does not get affected - providing everything is in proportion .

I'm absolutey no mathametician but if the piston is moving in direct proportion to the original then surely the arc of movement will be the same ???

Arggh - all I know is so far it works :big:

Ramon

 

[Ken I]

Just remember the simple rules

Linear dimensions will be to scale

Areas will be to scale squared

Volumes (and masses) will be to scale cubed.

So a half scale engine will be 1/8 the displacement.

As mentioned scaleing down seldom presents problems as strength to mass and inertial forces is always improving. (The converse is when scaling up it will all go pear shaped.)

With one important exception - drive shafts are to the power 4 for torque transmisson - so that applies to crank journals and the like as well.

Assume your original engine had 1" journals and you scaled by half, the torque would be down to 1/8 but the torsional shaft strength to 1/16 so you would need to up the shaft by the fourth root of 2 to get back to design torque to torsional modulus ratio.

So instead of a 1/2" shaft you would need a 1/2 x 1.1892071 = 0.594" (19/32" ?) go for 9/16 or 5/8 (or 15mm whatever).

Regards,
Ken

 

[kvom]

I found that when trying to scale up a CO2 engine 3x that the laws of physics would likely make it impossible to run w/o an extremely heavy flywheel.

OTOH, locomotives are easily scaled down 16x.

 

[ShedBoy]

Brock - I've just found my pencil sketches of taking the dimensions off the original ETA 15d. Despite what I thought It appears I did directly scale the porting dimensions - the drawings show that (but the timing was definitely checked as referred to before) so I guess the answer to your question is no the porting does not get affected - providing everything is in proportion .

I'm absolutey no mathametician but if the piston is moving in direct proportion to the original then surely the arc of movement will be the same ???

Arggh - all I know is so far it works :big:

Ramon

Thanks for that Ramon. I have just recieved the Motor Boys plan book and intend to build something from it. Really want a sparkie. I seen a pic on the web somewhere of a monster Cox .049 and have been bugged by it since. Maybe a larger boll aero.

What are you building Jack?


Brock

 

[Tin Falcon]

Jack like others said you should be OK if you pay attention to the details/ Like fasteners and bearings etc.
like others have said the engine will produce roughly 1/8 of the power of the original. and the force exerted on the piston will be 1/4 of the original and cross sections of drive/crank will also be 1/4. if this is meant to power something these issues could be a problem but if it is just a running display model you should be fine.

I have heard and read that thermal dynamics do not scale well. and efficiency goes down when things are scaled down. but I have not studied the subjects and the power consumption on small table to display engines is not usually an issue.
Tin

 

[Ramon]

Thanks for that Ramon. I have just recieved the Motor Boys plan book and intend to build something from it. Really want a sparkie. I seen a pic on the web somewhere of a monster Cox .049 and have been bugged by it since. Maybe a larger boll aero.

Brock

Oh I bet that got the creative juices flowing Brock - there are certainly some great projects within those pages Thm:. I have the Dyno earmarked for scaling but the one I'd really like to have a go at - as a sparkie too - is a 10cc version of that Weaver 1cc - ah he says with a bloody great sigh - will I ever live long enough :big:

Ramon

 

[ShedBoy]

I keep going back to the M&M piston valve because it is different and quirky.

Brock

 

[ShedBoy]

This should get you thinking on the same line as me.
Brock

 

[Ramon]

Just caught up with this Brock - that is one seriously big '049' (one has to be careful with the phraseology on this one )
Theres 'scaling' and then some eh?

I wonder if that spring actually does the business as well as the small version - any idea what size prop it turns?

Ramon

 

[Jack]

Well the plans that I purchased have finally arrived and they are for a 1 cylinder vertical marine steam engine thats huge (by model engine standards) 2 1/4 inch bore and 3 inch stroke.

Since I will never have a boiler large enough to operate the engine on steam i plan on running it on compressed air but I am afraid that it will consume a lot more air than my shop compressor can produce.

Therefore, my plan is to build it at one half scale and use a lot less steel, cast iron and brass in the process.

This is what I attempting to build... http://hasbrouck.8m.com/eng10.htm

This project should be interesting and does present a few new challenges but it should be fun plus its been to hot in the midwest to go outside so I plan to spend a lot of time in the basement shop where it is cooler..

Jack ;D

 

[Tin Falcon]

There are a few guys hear that knew ray and have built his engines.
Seem like nice designs
Tin