I would like to find out if soft engine mounts are effective is keeping the airplanes quieter.

Like the Hyde engine soft mounts which has a CF nose ring, this mount also requires only 2 bolts to fasten the engine.



I own the Dave mount products, but it does not have a nose ring.

The better the mount, the lower the idle rpm can be without the vibrations. I also understand that resonant at high rpm is more serious.

How to make sure that resonant frequency is slower than the lowest idle rpm ?

Soft mounts helps reduce noise mainly by reducing the vibrations transfered to the fuselage. Depending on the engine, the mount geometry and the dampening material used, a nose ring might be needed to avoid excessive engine vibrations at idle. It's very much a trial and error affair to determine the optimum set up. From my own experience with Dubro, Sullivan, Weston and Hyde mounts high speed engine vibrations is not a problem as long as the spinner and propeller is properly balanced.

What can you expect in terms of noise reduction:
Again this depends on a lot of factors but as a general rule one can expect a 1 - 2 db(A) noise reduction. Not very much one might argue, but one also have to take into account that the noise usually is perceived to be less harsh.

If noise reduction is the ultimate goal, consider doing the following things (in order of importance):
Invest in a good silencer or a muffled pipe
Keep the engine rpm down by selecting a proper propeller (high pitch)
Soft mount the engine
Enclose the engine in a cowl

For competition use the Hyde mount is probably the way to go but for general use (in sports and scale aircraft) I use Du-Bro soft mounts on everthing from my OS26-FS (totally unecessary but I do it anyway) up to my ST-3000 engine.

/Red B.

For models that are powered by four-stroke engines, these soft mounts are probably good.
For pipe equipped two-strokes, the soft mounts allow the engine to vibrate to a much larger amplitude, than would a rigid mount.

The cost could be exhaust headers that break, at the weld of the tube to the flange. In worse cases, it can also be the mounting lugs... If you use a regular mount, verify the engine is bolted very firmly to it. Also verify the mount is bolted very firmly to the firewall and the later is glued to the fuselage with plenty of 30-60 minute epoxy (I prefer Devcon 2-Ton), not with any CA trash.

I have never seen engine vibrations, with engine mounted rigidly enough, that have caused any airframe damage.

With a flimsy mounting, the large amplitude vibration could jar loose many parts, if they can pass to the airframe. The soft mount isolates a flimsily mounted engine from the airframe, so no damage is done.

DarZeelon wrote:
>For pipe equipped two-strokes, the soft mounts allow the engine to vibrate to a much larger amplitude, than would a rigid mount.

Most, if not all pilots that compete in F3A (FAI pattern competition) use soft mounts regardless if they are using 4-stroke or 2-stroke engines.

>The cost could be exhaust headers that break, at the weld of the tube to the flange.

Yes, but there are ways around this: Support the free end of the exhaust header from the soft mount itself instead of supporing it from the fuselage. I have seen bad examples of people soft mounting their engines only to rigidly attach the free end of the exhaust header to the fuselage, don't do this! Also use as short exhaust header as possible and select a longer pipe instead.

>I have never seen engine vibrations, with engine mounted rigidly enough, that have caused any airframe damage.

The point here was to reduce noise. And I can assure you that if switch harnesses, servos and Rx:s could speak thay would thank you for a soft mounted engine.

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

I do agree with Red Baron and Clair, about the use of soft mount, for engines that run at relatively low RPM, as today's pattern, F3A goes, for example. That is, less than 10,000 RPM.

I do understand that noise is a categorical problem and that these expensive soft mounts, do reduce it very significantly.

The break-ups I encountered were headers on TT, OS and MVVS .40-.50 engines, that were mounted on elastomeric, quiet mounts and were pipe equipped to spin at 14,000 to 16,500 RPM. The end of the header was only in the pipe coupler, in all cases. None were rigidly connected to the airframe.

The mounting lug broke (and the opposite one was cracked too) on an old TT .46.

The mounting frame that connects a Textron-Lycoming, or a TCM engine to the body of a full-size Cessna, or Piper is also equipped with elastomeric shock absorbers and grommets, to isolate the vibrations from the airframe.


Let us agree that the cheap soft mounts, when used with high RPM engines, could spell breakage.

Where do you buy the cheap ones? Where I live they are priced as if the were made of gold :-)

I like to use a "Well-Nut" (available in a variety of sizes) from GENUINE AIRCRAFT HARDWARE
in California. They look like a neoprene "top hat" with a threaded brass insert. The whole works installs
in place of a blind nut. Perfect.

Last time I ordered...they were about $1.00 (that's ONE dollar) each in small qty's.

They don't flop around, and they do provide some isolation between engine and airframe.

VERY handly little guys for soft mounting pipes as well.

'Race