Vibration absorbing engine mounting

[Phil G]

I am planning on putting a Saito 40 four stroke into a Super 60 4 channel. In order to reduce vibration into the airframe, I am considering using rubber backed washers and /or 'o' rings in the engine mounting.

Is there any issue with doing this, and is it a good idea or a waste of time?

[Denis Watkins]

Just washers will not isolate the motor Phil

But your motor mount attached to rubber bobbins will, just one in each corner M3 or M4

 

Edited By Denis Watkins on 29/03/2020 21:03:44

[Peter Miller]

Yes,I used touse four of those.They really do make the model much quieter.

There used to be some special mounts which were expensive .Yhey were like normal plastic mounts but made out of aluminium.

They had a separate aluminium plate embeddd in rubber in the arms.They worked well too.

[Gary Manuel]

It's worth doing, but you need to completely isolate the engine from the firewall. Try Rawlnuts rather than plain rubber washers.

THESE are better though, but quite expensive.

Edited By Gary Manuel on 29/03/2020 21:09:49

[Wilco Wingco]

I bought 2 pairs of these at the Blackbush Model show a couple of years back. £5 for the lot and they work perfectly

[Jon H]

be very careful with that sort of thing as they can cause damage. If they arent man enough the engine will thrash about all over the place which is really bad for it.

Its not always easy to see due to the frequency of the vibration but due to the way the camera captures the moving images this video highlights the problem you might face.

all that waving around puts massive strain on the exhaust and in the case of a saito with a heavy cast muffler you could easily break something in the harmonics are right. The rubber will also degrade over time so that is another thing for the maintenance list.

Given the size of the engine and the task its intended to perform i would just use a standard glass/nylon mount and not over complicate things.

[Bob Cotsford]

Posted by Gary Manuel on 29/03/2020 21:07:18:

It's worth doing, but you need to completely isolate the engine from the firewall. Try Rawlnuts rather than plain rubber washers.

THESE are better though, but quite expensive.

Edited By Gary Manuel on 29/03/2020 21:09:49

I used the rawlnuts on my Jungmeister with a 61 two stroke on a grp mount, they definitely took the edge of the noise and may help with a four stroke though I'd be surprised if noise would be an issue with a 40. How is the engine mounted in the S60? I would have thought it would use wooden beam mounts in view of the design's age.

[John Stainforth]

As far as I can remember, Saito do not recommend these kind of "soft" mounts at all.

[Nigel R]

We have bushed mounts on our cars. Always arranged so that the only motion to speak of is a small rotation along the axis of the crank.

There will be excess gyroscopic precession type forces in a model, if we make a poor install that does not control the movement.

Look at the 'hyde' mounts used in f3a models for an example if controlling the vibration reasonably well. Is it worth this effort for a sport model with a 40 size four stroke?

For my money, i would stick with a standard glass mount or wooden beams. 

Edited By Nigel R on 30/03/2020 06:30:10

[Chris Walby]

IMO rubber bobbins would need great care and additional frequent checking as from experience as years ago their performance was poor. We used them for exhaust and silencer mounts on 2 stroke motorcycles. The mounts did not like the oily conditions and in the end the ACU invoked a rule where a wire strap was required so that when silencers fell off they did not become projectiles.

The thought of any IC engine at WOT moving around or breaking loose is very concerning + if the mounts are behind a cowl how do you visually inspect before each flight where as holding the prop prior to starting instantly confirms all is bolted tight.

The alternatives made for the specific purpose of engine mounting look okay and as said above I would seek the advice of the manufacturer as to their recommendation.

Off topic, but my observations is that significant noise comes from the stalled prop at WOT and the drumming effect of the airframe when noise testing.

[David Ovenden]

I have a Saito 45 in my Hawker Tomtit model. It is mounted on a standard glassfibre engine mount. There is minimal vibration and the engine is pretty quiet. One of the quietest models at the club. It has been working fine in the model for 20 years! So I doubt that a anti-vibration system on a Saito 40 would be worth the time/effort/money involved.

On the other hand I have a Saito 115 in a Great Planes PT-19 82" span aircraft. That set-up suffered from loads of vibration and was noisy. Mostly I think due to the design of the model and weird way the firewall / mount was fitted on wooden stand-offs! I fitted a Dubro quiet mount to the model (as suggested by Bob) and that did make a huge difference. Smoother, quieter and really transformed the model. So they can work - if you need it.

However, I would try the .40 in your Super 60 as is. Fitted with the right prop it should be fine.

 

Edited By David Ovenden on 30/03/2020 07:39:23

[David Ovenden]

Posted by Jon - Laser Engines on 29/03/2020 23:24:06:

be very careful with that sort of thing as they can cause damage. If they arent man enough the engine will thrash about all over the place which is really bad for it.

Its not always easy to see due to the frequency of the vibration but due to the way the camera captures the moving images this video highlights the problem you might face.

 

all that waving around puts massive strain on the exhaust and in the case of a saito with a heavy cast muffler you could easily break something in the harmonics are right. The rubber will also degrade over time so that is another thing for the maintenance list.

 

Given the size of the engine and the task its intended to perform i would just use a standard glass/nylon mount and not over complicate things.

Looks just like the flex mount system that came with my World Models giant Zero kit years back. I fitted my Saito 180 to the model using that mounting system. Just one attempt at running the engine was so scarry (as per the video) that I removed it and fitted a proper/normal engine mount instead.

Edited By David Ovenden on 30/03/2020 07:47:44

[Nigel R]

Posted by Chris Walby on 30/03/2020 06:52:50:

Off topic, but my observations is that significant noise comes from the stalled prop at WOT and the drumming effect of the airframe when noise testing.

Not that far off topic hopefully

As far as drumming effects go, there is a lot to be said for the older style foam wing structure of veneer or balsa sheeting over beaded foam. An all sheet balsa fuselage and fully sheeted built up wing is not too far behind.

Anything loose (cowl not quite tight, wheels that don't fit the axle, slightly loose control runs) can make a phenomenal noise when it flaps about. On something with acres of open structure, the covering can make a lot of noise too, quite literally like a drum.

[Colin Carpenter]

For what it's worth , those bobbin type rubbers were in use in IC multi racer boats in the 80's ! A Speedo cable type shaft took care of wobble at the shaft end. They made a big difference to the noise ! Very few multi boats these days unfortunately ! Colin

[Engine Doctor]

Don't bother with those mounts they will allow your engine to wobble all over the place. We used to use them years ago on backplate mounts on fun fly models like the Panic but they were made from a hard rubber so didn't wobble as much. If you take time to balance your prop properly , ex pun , it will greatly improve any  vibes/drumming . Saitos are pretty quiet and smooth running engine. I have used rubber Rawlplugs to hold Mount to the firewall but no noticeable difference. Dubro aluminium engine  mounts are very good with rubber isolation but expensive. A decent  covering like solar Tex if available or oratex if you can afford it will also help.

Edited By Engine Doctor on 30/03/2020 10:20:21

[Nigel R]

ED one of the worst offenders I had, many moons ago, was covered in Solartex. At certain throttle settings (not just full throttle) the drumming from the fabric was easily the loudest component of the total noise.

[Engine Doctor]

I know meanwhat you but you say certain revs so vibes are from something out of balance and rev critical. I have over the years had engines that are just rough running out of balance engines ,usually the cheaper brands. No amount of prop balancing would help. Change engine and hey presto , a quiet model ! Even some props in years gone by were so bad that balancing was impossible.

I think it really comes down to Individual model engine combinations / actions to get quiet drum free model.

Edited By Engine Doctor on 30/03/2020 10:31:36

[Martin Harris - Moderator]

Full size propeller driven aircraft often have "avoid continuous operation" rpm ranges due to harmonic vibrations so it can't be an easy fix to eliminate them, although changes in propellers can shift them to different rpm bands - one often overlooked reason why a prop change can have a dramatic effect on noise testing.

Edited By Martin Harris on 30/03/2020 13:35:17

[Mike T]

I used the bobbin (Lord) type on a GP 1/3 Pitts. Ended up using 6 - 4 for the mount, plus two to damp the radial oscillations!

The old Apache Aviation mounts were good and relatively inexpensive, but you have to bolt them down quite hard to keep the harmonics high up. I've got them in my 1/4 scale H9 Cub and DB Pup, where they seem quite effective.

Re the DuBro mounts mentioned above, apart from being eye-wateringly expensive, I'm pretty sure I read somewhere that they should only be used for upright or inverted mounting, NOT sidewinder (I can't think why?)

[Phil G]

Wow! What a brilliant and rapid response to my query. Thanks to all of you for providing your insights and advice.

Just to clarify with a little more detail..... The (Ben Buckle) Super 60 has been built as per plan with Hardwood beams, but outboard of any likely engine. The engine is to be mounted on an alloy plate, which in turn is screwed to the hardwood bearers. (Taking advice from another thread, I will be bolting to the beams rather than self-tapping screws.)

My thoughts were either to use small m3 rubber backed washers top and bottom of the engine lugs to mount to the alloy plate, OR put small 'o' rings under the alloy plate when bolted to the beams.

Both options would limit the possible movement to a very small amount, and would be pretty easy to renew if /when needed. But whether that would significantly reduce noise I can't say.

I note what Jon from Laser Engines says about adverse effects so I am now somewhat cautious. And a couple of guys suggested it really isn't worth the effort, given the small engine size, and a reasonably quiet engine anyway.

So, I think I'll go with the easy option and do nothing at first and see whether noise is a problem.

Thanks again to all. Any further insights given the information above will be gratefully received.

[Jon H]

You could mount it on a sheet of paxolin. That used to be standard fare many moons ago.

Mike, i wont warranty engines run on those Dubro mounts. They can work well..but usually dont and they just leave the engine falling about the place and this causes damage. Setting those mounts up and keeping them maintained is a fair amount of work and as you say, they cost a fortune.

I use radio active glass/nylon mounts part no RAA1520 on all my big stuff (70+ 4 stroke) and whatever decent looking glass/nylon mount fits my smaller engines...Not that i have many of those

[Nigel R]

Is paxolin still available? SLEC sell small lumps of flat glass sheet for the same purpose.

Phil, I must admit I'd just use self tappers with shakeproof washer to secure the plate to the beams, not sure what the logic is behind using bolts and nuts. Soak the screw holes with cyano when you've cut the thread in the wood.

[eflightray]

Go electric, problem(s) solved

But then you are already partly electric, electric starter

.

[Piers Bowlan]

If I remember correctly, Brian Winch, Wizard of Oz, Engine Guru and RCM&E columnist, was not a fan of these soft mounts at all.

[Bob Cotsford]

You can certainly buy Tufnol which I believe is tougher than Paxolin. I shied away from using self tappers for mounting IC engines to wood as fuel will migrate along the screw and soften the wood even with CA hardening, given enough time. What I have done is to put a slice of bicycle inner tube rubber between the engine plate and the wooden beam and tap washers under penny washers under the bolt heads. I even went as far as drilling the engine plate holes oversize and lining them with fuel tube to completely isolate the engine from the airframe and it did make a slight difference to the noise level. Enough to warrant the effort involved? Not so much.  The engines were held tight enough to not flop about but there was enough give to stop the nasty high frequency vibration being passed to the airframe.

Edited By Bob Cotsford on 31/03/2020 10:30:50