Radial Master Con Rod Design

[skmetal7]

I posted this in the Engineering forum, but I'm getting no replies :/

so I'm designing a 5 cylinder Radial, with a bore diameter of 1.1" and 1" stroke (might increase stroke length). I'm having trouble with the Master con rod. I'm unsure of where to put the 4 slave rod connecting points. It can't be the typical 72 degrees (360/5), since TDC and BDC isn't 72 Deg on the crank relative to the master con rod.

I've taken some pics of the assembly, and drew an approximate motion path of the 3rd con rod connection point. It's sort of an ellipse, on an angle. I've tried making the master con rod longer and shorter, and moved the connection point around, but still doesn't make a circular path.

Is this typical for this type of crank/ con rod arrangement on radial engines?

[txcncman]

I could be wrong, but maybe you are thinking about this wrong. First, to have even power pulses and to help maintain balance of rotating assembly, 72 degrees is actually the answer. If concerned about spark and valve timing, that would be adjusted on your camshaft and distributor.

Have you consulted Google for drawings to see what has come before? I think for something like this, there is no reason to "re-invent the wheel". I doubt you have in mind any major improvements for performance or reliability. Just copy what others have done.

http://www.enginehistory.org/ModelEn.../HydraRods.jpg

[asuratman]

skmetal7,
What material u plan to use for con rod and also what mat for crankshaft? What usually factory use for those materials? Thanks.

[skmetal7]

I could be wrong, but maybe you are thinking about this wrong. First, to have even power pulses and to help maintain balance of rotating assembly, 72 degrees is actually the answer. If concerned about spark and valve timing, that would be adjusted on your camshaft and distributor.

Have you consulted Google for drawings to see what has come before? I think for something like this, there is no reason to "re-invent the wheel". I doubt you have in mind any major improvements for performance or reliability. Just copy what others have done.

http://www.enginehistory.org/ModelEn.../HydraRods.jpg

That's the Hydra engine. It's a rare example if radials, with an even number of cylinders, and therefore suffered from sever vibrations at certian RPMs, and was later scrapped. If you look at the master con rod closely, you can see that its not evenly spaced. This engine also used an overhead cam design, which is really interesting, and allows exact timing of each cylinder independently. I'm using the traditional cam around the crank design. So I can't adjust timing independently; each lobe on the cam has to be in time with the first cylinder.

asuratman: I haven't got that far in choosing a material, but I would probably use a type of aluminum for the con rod, like 7075 T6, or Fortal from an ebay reseller. The crank would be some form of steel like 1144 stressproof

[txcncman]

Ok. Why can't you incorporate 5 cam points into your design? Might make for a slightly longer crankcase, but I do not see that as an issue since you are probably doing this as a hobby project anyway. Otherwise, go and buy a 5 cylinder radial engine of appropriate size.

[stevehuckss396]

If you don't get an answer here, try the Home Model Engine Machinest website. I have seen the subject discussed at length but can't remember where.

I know what your talking about and I have seen the fix. Don't remember where.

Sorry

[bluejets]

A bit confused with your drawings as I can't seem to follow what you are trying to achieve here. If the yellow rod is the master then you have it all wrong. Master is a fixed rod as in photo below.

[skmetal7]

Ok. Why can't you incorporate 5 cam points into your design? Might make for a slightly longer crankcase, but I do not see that as an issue since you are probably doing this as a hobby project anyway.

Interesting suggestion, i might try that if all else fails.

Otherwise, go and buy a 5 cylinder radial engine of appropriate size.

but what's the fun in that?


stevehuckss396: YES someone knows what I'm talking about!! :P I took a quick look over at that site and didn't find much, just a mention of it. I'll keep looking. Oh and your V8 is awesome, along with cforcht's.

[dynosor]

You may be over complicating it. See the 7 cylinder radial master connecting rod below. The angular spacing is even. It is just that one of the slave big-end bearings appears to be missing because the master rod has a solid bridge in that location.

The principle would be the same for a 5, 7 or 9 cylinder radial.


Also see animated GIF: http://en.wikipedia.org/wiki/File:Radial_engine.gif

.

[skmetal7]

You may be over complicating it. See the 7 cylinder radial master connecting rod below. The angular spacing is even. It is just that one of the slave big-end bearings appears to be missing because the master rod has a solid bridge in that location.

The principle would be the same for a 5, 7 or 9 cylinder radial.


Also see animated GIF: File:Radial engine.gif - Wikipedia, the free encyclopedia

.

Hmm, do you know what engine that's from? Maybe they didn't bother with correcting it. I remember reading on the 5Bears website: Crankshaft and Connecting Rods and he shows the master con rod, and explains that it has been corrected for TDC.

[dynosor]

Hmm, do you know what engine that's from? Maybe they didn't bother with correcting it. I remember reading on the 5Bears website: Crankshaft and Connecting Rods and he shows the master con rod, and explains that it has been corrected for TDC.

This rod in my picture is from an OS engine - all parts shown in this pic : http://www.osengines.com/engines/osmg1307-parts-b.jpg

http://www.osengines.com/engines/osmg1307-inset-1b.jpg

http://manuals.hobbico.com/osm/fr7-420-manual.pdf

I can see how packaging can force the slave rod bigend bearings to be placed on a pattern that is not circular and how the individual rods may have different lenghts (not desirable). On the other hand, the combined load from multiple connecting rods on the master rod would tend to take up the slack and rotate it slightly one way or the other - I can see how slave rod bearing placement may be used to correct such motion.

If I was building a model radial I would ensure that all cylinders had the same stroke length and all reach TDC. The goal would be to use the same part for the slave conrods and to keep things simple. If this resulted in a slightly lumpy firing order I am not sure it would bother me - I am still not convinced that special intervention is required, but will take a look at you link and see where it leads.

Meanwhile you can browse this full size modern radial manufacturer's pics for inspiration: ROTEC RADIAL phototour

[stevehuckss396]

I took a quick look over at that site and didn't find much, just a mention of it. I'll keep looking. Oh and your V8 is awesome, along with cforcht's.

I'll post the question and see what I can dig up


EDIT: I just asked the question, give it a day or two.