Red B is right. While I respect Dar's opinion on most things engine-wise, I was involved in pattern when the noise limits came into effect, and at that time, almost everyone was flying piped 60 two-cycle engines. Merle Hyde (Chip's Dad) was one of the pioneers in soft mounting, and he lived and flew here in Arizona, so I had a grandstand seat to the developments. Soft mounting the engines, with the pipe also soft mounted, reduced noise significantly, as did props of higher pitch, lower rpm, and cowled in engines and pipes. Airframes could be built more lightly without stress failures, and the lower vibration was much easier on servos and other radio components. Header failures did happen at times, but they also happened with hard mounted engines. Nose rings did calm the engine down at idle. And Merle and Chip commonly bolted the engine to the rails with only the two front bolts. Most of the movement in the engine as it fires is rotational, i.e. the prop goes to the left, and the cylinder goes to the right, at the power stroke.

I no longer fly pattern, but I still soft mount all of my larger two-cycle engines, and we can detect the difference in flight between my Extra 300/Webra 120, soft mounted, and the exact same set-up hard mounted.

Good luck with your noise control. It is pretty essential to keeping our fields in many cases.

Clair
AMA 15654