OS 120 FS and 82 db

[Peter Jenkins]

Has anyone any experience of getting an OS 120 FS to meet the 82 db noise test? Is the standard silencer up to it or do you need to get a custom made silencer.

As I intend to use the OS in a Topflite Mustang (65 inch wingspan) does anyone have this set up and what sound reading do you get with either standard or custom silencer? What prop are you using - make, diameter, pitch?

Thanks.

[Jon H]

if its a 120e you are likely to struggle, if its the surpass you should be ok.

Its pretty ott for the mustang though. I usually recommend our 80 for that model so any 80-90 should be plenty. A friend is using an os 120 in a 72 inch spitfire and is not short of power.

[Bob Cotsford]

I used 16*8 APC 2 blade and 15*7/9 Master 3 blade props with Just Engines ASP180 baffled silencers to get down to 80dBa on 120E and Surpass II-P and III-P engines. I found the II-P the hardest to get within limits, messing with different engine mounts, wheels and even stiffening the cowl on my Ta152 to reduce resonance and odd rattles. I also found the OS silencers very noisy.

Edited By Bob Cotsford on 07/09/2018 11:59:35

[john stones 1 - Moderator]

Ran mine on 16x8 apc as well in a Seagull Yak, can't give you any numbers though.

[Peter Jenkins]

Thanks for your experiences guys.
Jon, I flew a similar size Spit with an OS 90 and that was marginal on power. Ask any fighter pilot whether he has enough power and he'll always want more!
Bob - interesting that you found the ASP silencer better and good to hear you got down to 80 dba.
John - thanks for the prop info. I've heard APC are the quietest props so your experiences would seem to support that view.
There is always the option of a custom silencer which may help by keeping the silencer inside the cowl but will clearly cost a significant premium. Something to ponder as the build progresses.

[Jon H]

Posted by Peter Jenkins on 07/09/2018 20:11:37:
Thanks for your experiences guys.
Jon, I flew a similar size Spit with an OS 90 and that was marginal on power. Ask any fighter pilot whether he has enough power and he'll always want more!

In full size that's true, not so much for models. Propeller selection and flying style can often influence the required power. In this case, a 9lb mustang of 64 inches, an 80 or 90 propped on 14x7 or 15x6 would be more than sufficient.

Models can be really awful to fly if overpowered.

[Cuban8]

I have a 16X8 APC on my 14lb 69" Spit with OS 120FS - E. (it does have  an ASP standard silencer so I can use a JE 90 degree exhaust manifold. I'm pretty sure that the OS silencer is a bit smaller, so maybe not so effective.) Our club is very noise conscious and although I don't have db figures, the model is quiet, have been told it sounds 'nice' and has bags of performance for scale type flying. As for Master Airscrews - I used them for years because they were cheap and don't break easily, but I learned my lesson and now only use APCs because I find they are both superior in terms of noise reduction and they give more thrust and so better performance with reduced fuel consumption as a bonus. If one is looking to reduce engine noise - the first thing is to get rid of the Master Airscrew if you have one fitted, IMHO.

 

 

Edited By Cuban8 on 08/09/2018 13:04:04

[Peter Jenkins]

Thanks for the info Cuban 8 - another case of using the ASP silencer rather than the OS one I see.

Jon, thanks for the warning but I am used to flying a 2 mtr F3A machine in competition and they would qualify in your terms as being over powered. I am well used to the need for continuous throttle control and having some additional power on the odd occasion when it's needed.

Having the ability to swing a larger prop at lower rpm is also another advantage with a larger engine as you well know. Finally, the kit itself recommends up to 120 Four stroke. So, I think I'll stick to my chosen powerplant and use the prop selection and throttle to maintain the desired performance.