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#6
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Prop Indexing
nrp wrote: I was in a 172M club that had a bad first order shake appear after an engine overhaul. It turned out the prop had to be indexed to 45 degrees @ TDC to get rid of it. The orientation requirements are in the Cessna manual, but there is no explanation. As an ME with some background in dynamics, this is the only reason I can see for this: All 4 cylinder horizontally opposed engines with offset (in contrast to directly opposite) cylinders will have a second harmonic yaw motion about their CG due to the short connecting rods. Without a double crank speed counterbalance shaft system, there is no way to eliminate this. This causes a yaw vibration or swinging of the engine at 80 Hz at 2400 rpm for example. I think it is also the reason the vibration environment on the front of a Lycoming engine is so severe. I think this is called a "rocking couple". It's an issue with aftermarket Harley engines designed to use offset cylinders so as not to need female (fork) rods. When Colombo laid out the "short block" Ferrari V12 he wanted to have the bores on centers and fork/blade rods but Enzo said since Packard did not to so with the Twin Six it was unnecessary. A little bit off topic, as always. Certified a/c are stuck with LyCon but homebuilders who use 'em are idiots. They belong with the OXX-6, Pobjoy, Menasco, Hirth, Siemens-Halske, Guiberson and Caminez on a museum wall. |
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