[NSRCA-discussion] Metal Servo Arms

[rcmaster199 at aol.com]

Bob,


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If I were to take an educated guess, I'd say that the larger gassies, particularly two cylinder opposed, give the false impression that they don't vibrate. I had a 105 cc opposed twin and must admit that its vibration was low, perhaps less than a pattern size single cylinder (1/4 the displacement). Or larger gassie single cylinder. The uninitiated basically believe the hype that these vibrate less and don't do?anything to soften the blow. The thinking appears to be why bother because the "blows" cancel out. I believe that there were some Hyde soft mounts made for the large engines and these were a bit heavy....on the order of a pound or more


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I made a soft mount anyway and the difference in noise between the two conditions (with and without the soft mount) was considerable on standard muffler?so there are other elements to vibration than the obvious. The pattern community has known that for many years as you know. 


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The soft mount was essentially a set of large lord mounts, aluminum plate etc. It was fairly heavy compared to the ply box it was intended to replace. But it did perform its intended task adequately.


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Fast forward 5 years.....I am doing the same thing for the 40cc F3A engine from ZDZ, only this time?I am building a Hyde style mount for it. The weight is much lower (3.7 ozs) for?the smaller engine, and vibration/noise reduction should be even better. 


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Soft mounting the nose is another reason some won't bother due to interference of a nose ring device with the mag pick-up. The large model cowlings are generally not thought as being capable of carrying the load. This has proven to be tricky for my set-up also, but I think I have figured out what to do. Won't discuss it until I actually try it


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MattK










-----Original Message-----

From: Bob Richards 

To: NSRCA Mailing List 

Sent: Sat, Sep 1 9:19 AM

Subject: Re: [NSRCA-discussion] Metal Servo Arms







Earl,


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You are right about soft mounts. One thing I don't understand is why soft mounts have not made much progress in the larger 50cc+ size planes. 


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Bob R.






Earl Haury ejhaury at comcast.net> wrote:



Bob


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I agree - in reality servo arm / spline strength is pretty much of a mute point for pattern. I did the tests after a friend broke an 8411 shaft on a big gasser, and some heli friends claimed the Al arms stripped the spines from nylon servo shafts. Takes excessive force to do either. 


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Recall changing a clevis to a ball link on a Rossi carb arm - arm broke on the first flight, a doubled arm broke on the next - back to a clevis and never broke another. Vibratory forces aren't understood by most - but experience is a great teacher.


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Somewhere I've some DSA data gathered with mini-accelerometers mounted on aileron servo mounts, firewalls, etc. One look at those data with and without a soft mount will make one a believer in soft mounts!


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Earl

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