proptop

My Feedback: (8)

Couple friends and I were talking the other day, discussing the various isolating motor mounts. Anybody have any opinions? Are they really nessessary on airplanes w/ say 40-50cc engines...One part of the "debate" was that if the engine is bolted directly to the airframe (no rubber insulation/ isolation) the airframe itself will better dampen vibration. With rubber mounts, it seems the engine wobbles all over the place, and at times, seems like it vibrates worse...Any observations/ opinions???

Panzlflyer

My Feedback: (15)

No mount on any of em even a G62 with mag.......IMHO

Volfy

My Feedback: (23)

With a rare few exceptions, most of the "soft mounts" on the market do more harm than good. To "dampen" vibration is to absorb the vibration energy, which means all that mechanical energy has to be converted to some other form - most often heat. IF the vibration dampening mounts truly did their job, the mounts would get hotter than Hades in a hurry. Since I've yet to see a set of soft mounts spontaneously combust, where's all the supposedly absorbed energy go?

The best you can hope for is to "isolate" the engine vibration, but even that's not as easy as you might think. Putting pieces of rubber grommet between the engine mount and the firewall is not automatically going to do the job. For one thing, its DOF isn't oriented correctly to be in the same plane with the principal axis of vibration. Even when the DOF is aligned with the axis of vibration, one still needs to "tune" the spring constant and damping factor to the actual vibration pattern and frequency range. Mismatched, they either act like rigid mounting or be so soft as to exacerbate the amplitude.

This also applies to fuel tanks and radio gear mounted inside the fuselage. Most people think wrapping them with plenty of foam is infinitely better than rigid mounting. Not necessarily so. Short of putting an accelerometer on the fuel tank, it's not easy to apply just the right amount of foam padding on the first try. Besides, what works great at full tank and full throttle may be sloshing the ***** out of the fuel at 1/2 full and 1/2 throttle.

With a model airplane in the typical scale we fly, the best engine vibration dampener is indeed the airframe itself - working best when it is moving through a viscous fluid (i.e. air). So hard mount that engine and go fly. Just don't break in the engine on the plane sitting on the ground - at WOT with all electronics on board no less - it's the worst thing you can do to your airplane.

proptop

My Feedback: (8)

Thanks guys...Volfy, you pretty much said what I was trying to say to my buddies. I already have the 3W-50 mounted to a friends (building it for him) WM Patty Wagstaff Extra 300, and I wanted some extra (bad pun intended) opinions. I ran a gal. thru it mounted on a picnic table before I put it on the plane. Some of the other guys out at the field were checking it out, and a few said some things about soft mounting, but frankly, I can't see the point. (I can, but prefer to solid mt.) I hear ya about Points Of Force, and resonance, harmonics, etc...I'm an AMT (used to be called A&P Mech.) but this is my first "big" gasser, and it's for somebody else, so...My rep. is kinda on the line, ya know?! Want as few variables involved as possable...Thanks again.

Steve Sanford

I mounted my echo conversion on rubber mounts, got them from grainger, only about 1/2 thick for 1/4-20 threads, there are two choices of the rubber hardness, I got the hard ones, it makes a differance in vibration on solid mounted test stand, motor does not move, that I can see with my eyes anyway, but I cannot tell any differance on the air frame. I mounted a converted McCollugha, these were the long (approx 1 inch long, and soft), wow moved so much it scared me, put the short stiff ones on it after that, the long soft ones took some rpms off the engine too. I don't remember how much, but I do remember being suprised as how much it was. I don't know if they help, but don't think it will hurt, unless its moving around too much.SS
Oh yea, the idle had to be a bit higher too.

ZM2000

My Feedback: (1)

Why is it so that all the engines in the automobiles and all the compressors in the aircons and refrigerators are religiously soft mounted. Remove them and see if you can live without them!! Suggest follow the results on your aeroplanes.

Volfy

My Feedback: (23)

Remember I said: "With a model airplane in the typical scale we fly". Automobile and larger industrial engines do benefit from isolation mounts. However, when was the last time you see a Chevy V8 mounted on cantilever beams with isolation mounts on the firewall? Study the engine mounts on a modern automobile engine and you'll see not only are their placement carefully chosen, the types of mount are often different from location to location. IF one spends the same amount of engineering on an RC airplane engine, one might actually have a viable isolation mount as well. But would you be willing to pay for it?

Rember also, that unlike the typical larger scale industrial engines, the RC engine's weight is quite large compared to the "ground" it's mounted on (i.e. airframe). The "spring" between airframe and the ground is much lighter than the equivalent one between the engine an the airfram. Unless you have a very soft engine mount, much of that engine vibration will be transmitted to the airframe anyway. And if you do have a soft enough suspension to isolate engine vibration effectively from the airframe, that prop will be flopping around like mad. Automobile engines don't have that problem - watch an auto engine when someone revs the engine, and you'll see how much that engine heaves around. That's not a problen because the auto engine drives output shafts connection to the driven wheels. Put that big V8 in a homebuilt aircraft and slap a big ol' prop on it, and let's see if you can still allow that engine to move around that much.

ptgarcia

My Feedback: (36)

Please, nobody get offended, but one of the things that, for some reason, hits a nerve with me is the misuse of the word "dampen" or "dampening" with reference to shock absorption. The correct words are "damp" and "damping". We want to absorb the shock...we don't want to get them wet

Volfy

My Feedback: (23)

Actually Pauly, the word "dampen" does describe either:

1 : to become damp
2 : to become deadened or depressed

Came straight from Meriam Wester.

Damping and dampening are used almost interchangeable by engineers. On one hand, we speak of "damping factors" and "critically damped". On the other hand, in my line of work, we often deal with "pulsation dampeners".

I suppose etymologically speaking, the concept of damping and dampening with respect to deadening noise or vibration does have its origins in the concept of becoming damp (laden with moisture). Many things, when saturated and loaded down with moisture, tends to suppress noise and vibration. Think of an empty kettle vs. one full of water.

Language is usage. Try telling the common folk that technically they really should refer to their moisturizer as moisteners.

Volfy

My Feedback: (23)

Okay, an even better example than the tea kettle is the tin cups and string neiborhood communicating device. Works great for kids all over the world, except when it starts to rain. All those raindrop hanging on that string dampen vibration transimission real effectively. Probably at least 1 dB per linear yard.

ptgarcia

My Feedback: (36)

Thanks Volfy, I'm gonna bring that back to my old college "Technical Communications" professor so he'll leave future students alone when they say "dampening"!

heliair

Just a thought.

Rubber mounting is an interesting topic. I would have to agree with the hard mounting technique. It seems that most rubber mounting occurs when there is an acceptable level of comfort expected (ie. automobiles, compressors, motorcycles, etc). Just because an engine doesn't move when it is rubber mounted, doesn't mean it is affecting the structure it is mounted to any less. It just means the energy is being placed elsewhere. If the rubber mounts are designed properly, all the energy could be absorbed in the mount itself. However, if the mount isn't designed properly the energy could be multiplied by the excess movement causing more damage to the structure. Lawnmower engines are mostly hard mounted (that I know of). How often have you seen a lawnmower break due to vibration? The handle may be rubber mounted because there is a certain amount of comfort expected, but durability isn't the issue.

proptop

My Feedback: (8)

That's annother thing. When you see a soft mounted model airplane engine cylinder shaking / rocking back and forth. The cyl. "rocks" in the opposite direction of the prop. when it fires, so the mts. are absorbing some power that would otherwise be transmitted to the prop., right?! And I've seen engines mounted so softly that it seems like more vibration is being transmitted to the airframe/ equipment because the eng. is shaking so violently. Much more so than if it were solidly mounted...

ZM2000

My Feedback: (1)

Remember I said: "With a model airplane in the typical scale we fly". Automobile and larger industrial engines do benefit from isolation mounts. However, when was the last time you see a Chevy V8 mounted on cantilever beams with isolation mounts on the firewall? Study the engine mounts on a modern automobile engine and you'll see not only are their placement carefully chosen, the types of mount are often different from location to location. IF one spends the same amount of engineering on an RC airplane engine, one might actually have a viable isolation mount as well. But would you be willing to pay for it?
[/quote]

So what I am saying is right? Soft mount is needed but the question is simply how to do it.
Apart from the heavy engines you mentioned, even the small motors in the tape recorders are soft mounted. I agree that a lot of research is needed in the case of model aeroplane engines unless an apple falls on someone's head.

-ZM

jacs

At my former workplace we build large engines for boats, power plants they had anti-vibration plates on the gensets but notably that this could be applied if you take and mount these spring plates under the mounting bracket on say a .46 engine and you also have a mechanical stop bolted also. This is to prevent the engine to move to much round axle (why isn't my maiden language English). What you want to stop is the vibration in axial direction, if you mount it with four rubbers on the back it's free floating around three axles this is not what you want. So you may what to mount it in the regular four holes that are on a .46 engine and this have to be applied for these big engines. I have seen that some manufactures makes engines mounted like the small ones.

Volfy

My Feedback: (23)

Yes, it doesn't take a rocket scientist to realize that almost ANY mechanical device that generate undesirable vibration can potentially benefit from a well designed isolation mount. The original poster proptop asked about the various isolation mounts, so my original comments were in regards to currently available commercial units. Over the years, I have seen several designs that I thought looked well designed and promising, but somehow they all died off. It seems the most popular ones that survived are the ones that don't work - go figure.

ZM2000

My Feedback: (1)

Volfy

Have you any experience of famous Hyde mount. I have never used it but would like to know if it is worth.

-ZM

peso

Toni Clark in Germany makes an excellent soft mount. I have seen it and heard it and it work fine. However in my opinion it is too complicated and only justified if you need a very quiet plane.
Here you can find some data. [link]http://www.toni-clark.com/index_en.htm[/link]
/PO

JohnnyJ

My Feedback: (1)

I am an Aircraft Maintenance Engineer and have noted rubber"soft mounts" on all internal combustion engines I have seen including cars, trucks, all fullsize aircraft including jet and turboprop equipment, except for model airplanes.
The engines on full size aircraft are typically mounted to the firewall, yet extend forward much like a model aircraft engine.
Vibration is a destructive thing, damaging the airframe along with any installed components.
Why do you think that your servo gears wear out so soon?
What do you think that the vibration is doing to the electronics inside the servo?
The airframe will absorb a certain amount of vibration and what does this vibration do to the airframe?
Is it causing damage....broken hinges, chafing damage, loosened glue joints, etc?
Is the airframe built heavier/beefier to take the vibration?
Are all the rotating assemblies balanced and aligned as best as possible?
A properly designed "soft mount" will go a long way to preventing premature airframe and component wear.
I use the Dubro mounts for most glow engines and B&B or Great Planes engine vibration isolators for gassers,
cinched up tight enough that the engine does not flop around at all when running. Other mounts are available which may do a better job.
The design of most model airplane "soft mounts" is not optimum, but it is better than a kick in the pants!

Merry Christmas
John

Diablo-RCU

My Feedback: (1)

The major problem with soft mounts is that they can allow the engine to shake quite a bit. This would be OK if there weren't pieces bolted to the engine - like mufflers and carbs. All that shaking does bad things to the mufflers. Either they will shake loose or break from excess vibration. It took the pattern guys years to figure out a way to keep from breaking exhaust headers when using soft mounts. I remember the days when you were lucky to get a header to last 40 flights. The muffler manufacturers typically will void your warranty if you use a soft mount. About the only flyer at the TOC who used a soft mount was Chip Hyde - produced by Chip and his father.

Volfy

My Feedback: (23)

No, too rich for my blood. Don't know what the internals are, but since it appears to be designed specific to each engine, it stands a better chance of working well. I don't get into $500 glow engines, so $100 mounts are not my idea of money well spent.

Panzlflyer

My Feedback: (15)

My own experience is that with vibration mounts none of the screws stayed put, tail wires came loose turtle deck cracked and had 2 servo failures ( wires fell off pots)
No soft mount =problem went away.

I sell air compressors and the reason we soft mount is to protect the welds on the tank. They are bolted to concrete. Models are typically on rubber wheels ?

apalsson

My Feedback: (1)

Apache Products in the UK sell excellent softmounts. I have used one of their sets and can verify that they substantially reduce the vibrations transmitted into the airframe with no ill side effects.
With a G38 turning an 18x10 at 6800RPM, the most noticeable noise is the prop tip sound.
Great improvement on my previous installation.

airtimeallen

In my opinion your not trying to really dampen vibrations as much as power pulses. And the reason a soft mounted engine moves more is because the vibrations are no longer dampened by the airframe. People talk about the rubber mounts absorbing energy, not so. Thats not the point of the rubber mounts. They are only there to flex. The acceleration and deacceleration of the engine mass is where the energy goes. Thats why rubber mounted engines vibrate more, because they are supposed to. But this can cause other problems, including broken mufflers, etc. In fact, the rubber mounts that i found for my zdz on Troy Built Models states that they are specifically for engines with headers and rubber connectors to pipes, and not for bolt on mufflers. All of the extra vibration can cause mufflers to experience extreme fatigue stresses and crack. There is also the possibility of the engine rubber mounts being of the wrong spring rate and causing resonant frequencies within the plane, and those can get transfered to the airframe causing even more damage. There is also the fact that the rubber will eventually wear out from friction loss, and may degrade from fuel exposure. I think that a properly designed rubber mounted system can be benificial and that is obvious in all kinds of industry. However, i'm going to stick to the tried and true bolt on system. My firewall will get fiberglass and my components soft mounted and i have a feeling my plane will die by the inputs of my fingers

av8tor1977

My Feedback: (6)

Where do you stand on this one TOM??

AV8TOR