Importance of flywheel size & material ?

[David Morrow]

I'm building Terry Coss's Horizontal Stirling from Home Shop Machinist Nov/Dec 2005. The plan calls for a flywheel made of steel .600" x 4.750". I don't normally cut steel other than very small pieces. I do have a piece of aluminum of the correct thickness but only 4.250" wide.

I assume that the larger diameter flywheel would have more momentum once it got moving and that would be accentuated by using steel rather than aluminum. The plan also calls for cutting a .200" pocket out of each side which would certainly lighten it. I could reduce that pocket thickness a bit to keep the weight up.

I also wonder just how much serious engineering really goes into the dimensions & material selection of flywheels on some model engines. I'm sure this depends upon the designer and perhaps there are just rules of thumb that may be used in many cases.

Any input would be most appreciated.

 

[mklotz]

I doubt that there's much engineering thought behind the average model engine flywheel unless it's a scaled copy of a real engine in which case the prototype engineering is incorporated.

If one were really concerned about design changes, one could do the calculations to match the moment of inertia of the substitute with that of the one called out in the plans.

Personally, I'd go ahead and make the aluminum flywheel and try it. If you decide later that it needs more inertia, add a steel "tire" or plugs of a dense material.

Another option is to purchase a flywheel. PMR sells them in cast iron and bronze. Other folks may be able to suggest other sources.

 

[joeby]

I would believe that Marv is correct.

One problem with Stirlings is that there is very little surplus power available in the model engines. It really shows in an LTD Stirling I built some time ago. The original plan was for a Plexiglass flywheel; but I decided on brass instead. The engine runs ok at higher speeds, but if it slows too much the engine doesn't have enough power to keep the flywheel moving. There are probably other little details that could be worked on to improve the low speed running, but I think a different flywheel with less mass or a heavier rim and thinner spokes would be all it really needs.

Kevin