Compensation; no not money but angularity adjustment of rods in radial engines. Last month at the model show I purchased a set of plans for a 3 cylinder radial model aircraft engine. When you get into the design of an engine you begin to realize the compromises that are required. There are four ways to connect radially arranged cylinders to a crankshaft but the conventional one is with a master rod which bears on the crankshaft with link rods for the other cylinders which are hinged on the master rod. Because the location of the pivot points of the link rods are not at the center of rotation (crank throw) their motion is distorted. In this 3 cylinder example, if you evenly space the link rods around the master rod (120 degrees), when the crankshaft is in the 120 degree position, the link rod isn't even close to pushing the piston to TDC. So the link rod is repositioned so that with the piston at TDC, the centerline of the link rod passes through the center of the crankpin. For this engine that is a 12 degree shift away from the top of the master rod. Thus the link rods are set 132 degrees away from the master rod centerline instead of 120 degrees. While this puts TDC of each cylinder at the right place, the motion of the piston up and down in the cylinder is affected. This affects ignition and valve timing. As the link rod straightens out approaching TDC it is analogous to closing a Vise-Grip (R) pliers, a lot of crankshaft movement with little piston movement. After TDC its like opening a Vise-Grip, the link rod pulls the piston down faster than it went up. Thus the piston motion is not symmetrical around TDC for a cylinder with link rods. The other cylinder of this engine is going to have the reverse situation, the piston will go up faster than it comes down. Because of this and the long space between power pulses, three cylinder engines have been considered vibrators with many of them constructed with slipper rods which bear evenly on the crankshaft to avoid this problem. All of these irregularities generate the characteristic rumbling sound that lets you tell a round engine from an inline one without seeing it.

Bob Kroll is my counterpart in his chapter and copies me his newsletters and notices. I will start forwarding them to our members. A bit far to travel to his local events, but it is nice to know what goes on in the rest of the world.

On the Hatz front, I still have someone other than me to blame for what I have to unbuild, but that will change in a day or so. I have removed the ailerons from the top wings. I don’t think a Swazall is a common tool of homebuilders, but it worked well. The extensions to the ribs weigh a fraction of what I removed. Splicing the wingtip bow to the section from the aileron was the biggest challenge, but was easier to do than I thought. I started sketching the new ailerons which will be hung off the rear spar rather than a false spar to make them deeper. They will not go out to the tip. The neatest way to move them will be a pushrod through the rear spar, which I think should be reinforced at that point. I will try to get some pix up on the site.

 Happy Flying