7 cylinder Radial engine, optimised main rod

thingiverse

Not only must you have an odd number of pistons, but it's also crucial, or it's time to learn being a mech engineer, I'm proud to say that on the master rod, known as the hub here, the attachments of the rods are not evenly distributed around the part. They tend to be closer as you move away from the main rod. This is clearly explained at http://patrice.courtade.pagesperso-orange.fr/etudesw/etoile/etoile.html. It's in French, but to summarize, I'd say that the three pivoting points must be aligned for every rod when the corresponding piston reaches its peak position. This isn't the case when the distribution is even around the circle. Taking a look at my screen captures might help clarify things. Values should depend on the ratio between the lengths of the rod and the rotor. I created a model using plusalphaDesigns software and found the following values: 64.99°; 119.86°; 161.76°. I built the top and bottom models of the new hub, and everything works great except that the two bottom rods need to be filled in by hand a little bit since their linking points are closer together. Enjoy!