Motor Mounts: Hard or soft?

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Monty

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I've noticed that most large multi-cylinder engines have hard mounts : Allison/Rolls/Daimler Benz

The Wittman tail wind V8 was also hard mounted.

simple, light, easy to construct.

It greatly simplifies the cowling design if the engine is hard mounted.

Smaller engines have compliant bushings and the engine floats in the cowl. The cowl has to be self supporting.

So my thoughts are anything with more than 8 cylinders you can get away with a hard mount.

Anything I'm missing?
 

Battson

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What do you mean by "hard mounts" - do you mean to imply that there is no vibration damping / separation between the engine and airframe on aircraft like the P-51 or similar?

I am fairly certain you would still want to isolate the engine from the rest of the aircraft, somehow. I guess as the engine gets heavier, the way you do that becomes more complicated.

Ps. I have a tonne of family where you live, dozens of them, small world ain't it!
 

Monty

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Yes, simply bolted with some steel shims to align the assembly.

Keep in mind the larger the number of cylinders, the smoother the engine and the lower the vibration.

Just reporting what I've seen. It certainly makes building the cowl easier. You just bolt it to the engine.

Yes, a very small world.

Monty
 

BBerson

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VW is bolted solid to firewall on the Volksplane.
Riding mowers with one cylinder are bolted solid.
 

cheapracer

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Riding mowers with one cylinder are bolted solid.
And things fall off them often from vibration. Longer term use, if you can call 50 hours long term, bolts break out from cracks developing around them, again from the vibration.

Not a worthy example.
 

Monty

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And things fall off them often from vibration. Longer term use, if you can call 50 hours long term, bolts break out from cracks developing around them, again from the vibration.

Not a worthy example.
yep, that is what worries me. We use compliant bushings to mount our enigne/trans in our little 10 hp single cylinder race cars. A lot of teams hard mount them, and we used to also. Everything on the car would rattle: body panels, seat you name it. Bolts would back out, fatigue cracks would appear. It sounded awful too. So we started using bushings and mounting the whole trans/engine assembly so it could move. The difference is night and day. The car is quiet and civilized with no buzzing in your spine or rattling body panels-MUCH nicer.

The fighters with the V12s weren't exactly expected to have a very long design life. Maybe the designers knew it would cause problems, but expected the airframe to be a smoking hole in the ground before the issues showed up.

I don't know. I wish somebody with some experience working on some of the war birds would comment.
 

BBerson

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And things fall off them often from vibration. Longer term use, if you can call 50 hours long term, bolts break out from cracks developing around them, again from the vibration.

Not a worthy example.
All three of my mowers vibrate minimally. Less than any airplane I have owned.
Nothing falls off. lasts 10 years or more.
Of course, these mowers have Briggs anti-vibration recipicating weights.
Sometimes a looser mount will shake worse. Especially with an opposed twin with a rocking couple.
I watched the o-100 at Oshkosh, it had a visible movement of about 1" at idle because the rubber mounts let it move.
The mower engine movement is not detectable by eye, and very slight when you put hand on it.
Vibration occurs at different rpm, frame stiffness, damping, etc. It is too complex to make general statements.
 

Dana

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Riding mowers also have frames that are much heavier and more rigid relative to the engine's weight than an airplane.

Larger multi cylinder engines run more smoothly and have less vibration than smaller engines with less cylinders, as Monty pointed out, to the point that a hard mounted V8 may well vibrate the airframe less than a soft mounted one or two cylinder engine. Then again, some engine's mounts aren't very well thought out. Proper vibration isolation design takes into account the frequency (rpm), the weight of the engine, and the mount stiffness and geometry are all critical.

Dana

"I don't practice what I preach because I'm not the kind of person I'm preaching to."
 
E

ekimneirbo

I've noticed that most large multi-cylinder engines have hard mounts : Allison/Rolls/Daimler Benz

The Wittman tail wind V8 was also hard mounted.

simple, light, easy to construct.

It greatly simplifies the cowling design if the engine is hard mounted.

Smaller engines have compliant bushings and the engine floats in the cowl. The cowl has to be self supporting.

So my thoughts are anything with more than 8 cylinders you can get away with a hard mount.

Anything I'm missing?
IMHO, a mount which relieves any vibration from being transferred to the mount or fuselage is the preferrable choice. All builds have unknown or unforseen variables

such as the amount of vibration produced by each individual engine. Vibration WILL break things eventually, its just a matter of when. Mowers are made of heavy material while your airplane will consist

of much thinner and lighter construction materials. Have you considered adapting some of the mounts used by the Corvette or late GTOs? They are a gel filled cushion and you can find them on Ebay

for a reasonable price. There is also an aluminum mount which might be adaptable. Summit has the gel/rubber mount on Ebay for about $17 each.
 

BBerson

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Most of the rubber mounts on airplanes are relatively hard rubber and compressed even tighter. I don't see any difference from a solid mount.
A really soft rubber might help. I have some really soft rubber mounts from an old Onan generator. But the generator had about as much weight in mount structure, heavy flywheel etc.,as the engine weighed.
For airplanes, weight is a big factor.
Most airplanes engines don't have counterweights on each throw likes cars, for example.
 

Dan Thomas

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Most of the rubber mounts on airplanes are relatively hard rubber and compressed even tighter. I don't see any difference from a solid mount.
A really soft rubber might help. I have some really soft rubber mounts from an old Onan generator. But the generator had about as much weight in mount structure, heavy flywheel etc.,as the engine weighed.
For airplanes, weight is a big factor.
Most airplanes engines don't have counterweights on each throw likes cars, for example.
The old conical mounts used on small Continentals and some Lycomings tend to be more rigid but do absorb small movements. The bvery common Dynafocal setup on the modern Lyc, and the much larger bed-mounted Continentals move a LOT. Very soft, very smooth. I can grab the prop on a 172 and shift the whole front end of the engine a half inch just with manual input.

Dan
 

BBerson

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Yeh, and all that movement just lets the engine shake around at different rates. The prop is really the big shaker, and soft mounts can make it worse.
Get the prop dynamically balanced first.
 

Dan Thomas

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Yeh, and all that movement just lets the engine shake around at different rates. The prop is really the big shaker, and soft mounts can make it worse.
Get the prop dynamically balanced first.
The prop isn't the big problem. It's the firing pulses of that engine kicking against a heavy propeller and the torque reaction rotates the crankcase itself. We see the result of that movement in the wearing of baffle seals against cowlings, for example. A hard-mounted engine won't move much but it will impart a lot of damaging vibration into the firewall structure.

When Lycoming came out with the Dynafocal setup we couldn't believe the difference in smoothness.

I used to dynamically balance propellers with an expensive Vibrex. It made little difference to what the average pilot could feel unless his prop was way off, but the airframe, radios and instruments knew it. Target was 0.2 inches per second, which is very little movement, and that's measured radially. The engine's firing pulses are still kicking the whole engine around torsionally and there's not much anyone can do about it other than minimize the transmission of that to the airframe.

Dan
 
E

ekimneirbo

The prop isn't the big problem. It's the firing pulses of that engine kicking against a heavy propeller and the torque reaction rotates the crankcase itself. We see the result of that movement in the wearing of baffle seals against cowlings, for example. A hard-mounted engine won't move much but it will impart a lot of damaging vibration into the firewall structure.

When Lycoming came out with the Dynafocal setup we couldn't believe the difference in smoothness.

I used to dynamically balance propellers with an expensive Vibrex. It made little difference to what the average pilot could feel unless his prop was way off, but the airframe, radios and instruments knew it. Target was 0.2 inches per second, which is very little movement, and that's measured radially. The engine's firing pulses are still kicking the whole engine around torsionally and there's not much anyone can do about it other than minimize the transmission of that to the airframe.

Dan
I agree with Dan, you should do everything possible to balance the moving components and achieve the best conditions possible . Regardless of the chosen mount, you should not expect the mount to cancel out a poor

situation. While Dan and I often view things from different perspectives, getting things balanced right is the proper way to start.I am a proponent of V8 engines for the most part.Smooth operation of these engines should be relatively

easy to accomplish. Moving to six cylinders, you don't have the natural balance of firing pulses like a V8. Manufacturers of auto engines often employ different engineering to prevent vibration and harmonics. Some use a balance shaft

to cancel or hide the vibration. I think the vibration in the engine still exists, its just offset by the antivibration of the balance shaft. Another solution is to offset the crank journals to achieve a balanced firing order. While this works

well in an automotive application, in an aero application I would be concerned about it weakening the crankshaft. In a four cylinder application you deal with larger power pulses on each power stroke, but they are balanced at 90

degrees. You have to expect more vibration from them. Four or six cylinder aero engines have very large power pulses due to the size of the moving components and lack of mass to absorb vibration. They can be made to run

reasonably smoothly but are helped greatly by softer mounts. So at this point you have whatever you have......or in other words , your choice of engine "is what it is". No matter what you chose, a softer or gel filled mount will help

prevent the transfer of whatever vibration you have from being transferred to the airframe......and thats a good thing.
 
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