Glen, A few comments regarding vibration: 1. I found that the mounting position of the prop relative to the crankshaft had a big effect on vibration. I would check that prior to balancing the prop. 2. After reorienting the prop on the crankshaft, I still had some vibration in my Alon, so I had the prop dynamically balanced. I would say that balancing the prop removed about 75% of the vibration, but there is still more there than I would like. One thing that I would do differently next time is to have the balancing equipment hooked up and secured so that the test data could be collected in the air. The reason for this is that even at full throttle on the ground, you can't get the RPM up to normal cruise RPM. Therefore, all the test data is collected, and the prop is balanced, at a significantly lower RPM than normal cruise RPM (say 2200 vs 2400 RPM). The A&P/IA that balanced my prop said he wouldn't have a problem collecting the data in flight, but it would cost a lot more, just because of the extra time required to solidly mount the sensors, secure the wiring, and the time required to taxi, take off, land, add weights, taxi takeoff land, etc. We did 4 ground test runs on mine to get it as good as we got it, and that took about 2.5 hours, so obviously, doing 4 tests in flight would take quite a while. 3. Before I had the prop balanced, I could look at the trailing edges of the rudders in flight and actually see them vibrating slightly. After balancing the prop, they don't vibrate. The reason that I mention this is that if the engine is really shaking, you may be able to see it in the rudders. You can then check the rudders after each "fix" to see if you are making any progress. I found trying to remember how the plane "feels" from one flight to the next to be less than an optimal way to check results. 4. I have long suspected that the straight Alon style stacks either cause the engine to run a bit rougher, or that they allow exhaust impulses to impact the bottom of the fuselage, which can be felt through the rudder pedals and/or floor. I don't have any evidence to back this up, but my friends C-85 powered 415C with the original exhaust and muffler on it feels like a sewing machine compared to my C-90 powered Alon. I know the C-90 has a different cam in it and that may have something to do with it too, but I have a feeling that the muffler and/or the 4-into-1 exhaust system on the earlier model planes dampens the exhaust impulses much more than the straight stacks do. 5. I don't know where you would put balancing weight on a skull cap type spinner/prop, unless it is legal to add a washer under the prop mounting bolt(s). Whoever balances your prop should know the answer to that question though. 6. Be sure to keep a close eye on those exhaust stacks. The original Alon stacks have traditionally been prone to cracking up around where the two down-pipes merge into one pipe. My old stacks had been welded so many times that the welds had welds. I sent them both off to Dawley Aviation and asked them to replace all of that metal, and they have not cracked since, but I still inspect them with a flashlight and a mirror at every oil change. 7. There have been a few different iterations of supporting and vibration dampening of the lower ends of the exhaust stacks. I think the latest iteration is the short rubber strap vibration dampers that go between the stack where it exits the cowling and the nearby engine mount tubing. These seem to work OK, but the rubber deteriorates over time and must be replaced every few years. A friend of mine got a 337 approved for a much improved method, using automotive vibration dampers, and sent me all of the parts and paperwork to install them on my plane, but I haven't had a chance to install them yet, so I don't know how well they work. Let us know what you find out. Maybe you already have the problem fixed by keeping the stack from touching the cowling. That will cause some vibration too. Best Regards,
Wayne DelRossi Alon N5618F Hours logged since restoration: 362.0 "Nobody has ever scientifically proven that life is supposed to be serious." **************The year's hottest artists on the red carpet at the Grammy Awards. Go to AOL Music. (http://music.aol.com/grammys?NCID=aolcmp00300000002565)
