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Old 11-06-2005, 11:38 AM
Bob Hoffman Bob Hoffman is offline
Join Date: Jan 2005
Posts: 206
Default Using Viscos Dampener With Metal Prop

What do the experts think?
Purchased a completed RV-6A .Has a 150 Lyc with wood prop and a Viscos dampening ring that mounts to the flywheel.
Am replacing wood prop with metal fixed pitch prop.
Has asnyone ever used the dampening ring with a metal prop ?? Are there benifits or hazards in doing this ? Anyone else tried it ?
Your comments are welcome
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Old 11-08-2005, 08:13 AM
DanH's Avatar
DanH DanH is online now
Join Date: Oct 2005
Location: 08A
Posts: 11,244

Imagine a simple torsional model representing your crankshaft-propeller assembly. The propeller has a large mass moment of inertia while the crank has a small mass moment of inertia, so we will represent them as a large disk connected to a small disk by a simple shaft. To make observation easy, we will paint a straight line along the length of the shaft. Because this is the world of theory, we will cause the the assembly to float in the air before us.

Now grasp the two disks and rotate them in opposite directions so as to twist the shaft in the manner of a torsional spring. Release them suddenly, and the system will settle into opposing oscillation; when one disk is rotating clockwise, the other will rotate counterclockwise. They rotate until they twist the shaft (a torsional spring) and store the available kenetic energy, stop, start back in the opposite direction, wind up the spring again, stop, reverse again, and so on. The angular displacement of each disk has a direct relationship to the mass moment of inertia of it's opposition. In other words, if the disks are equal in mass, they will rotate an equal number of degrees. If they are unequal, in each cycle the larger will rotate fewer degrees than the smaller.

Now to the meat of the issue. Observe the line painted on the shaft. You will see it form a spiral shape with each twist of the shaft. Note however that one point on the line does not move in relation to the observer. This point is called the "node"; it is the one point in the assembly that does not have any angular velocity. Note the location of the node in relation to the inertia of the rotating masses. If the masses are equal, the node is located dead center in the middle of the shaft. In this case, since our masses are unequal, the node is nearer to the large mass.

In a crankshaft-propeller system, the propeller has a mass moment of inertia 5 to 25 times the inertia of the crankshaft. Thus the node is very close to the propeller (the far larger mass), "very close" meaning right behind the prop flange.

Remember, the node location has no angular displacement, thus a damper mounted on or near the node would be nearly useless. Replacing the wood prop with a metal prop moves the node even closer to the prop. So where did you say your viscous damper was mounted?

The above describes a simple system with a single mode of vibration and thus a single node. A truly accurate model of a crank-prop system would have many disks and shafts, and the actual number of vibratory modes would be masses minus one. However, the simple model is fine here, since you are placing the damper so close to the system's dominant inertia.

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Old 11-08-2005, 10:12 PM
gmcjetpilot's Avatar
gmcjetpilot gmcjetpilot is offline
Join Date: Jan 2005
Posts: 4,841
Default NO take it off

QUOTE: completed RV-6A, 150 Lyc with wood prop and a Viscos dampening ring that mounts
to the flywheel. Am replacing wood prop with metal fixed pitch prop.

Has anyone ever used the dampening ring with a metal prop?
(Don't know but it sounds like a bad idea)

Are there benefits or hazards in doing this ?
(Little or no benefit and potential for hazard is high. If the prop was made to operate with
out one than using one is an experiment. Call the maker of the prop (Sensenich) and I am
sure they will tell you not to do this.)

Anyone else tried it?
(I don't know. These devices where popular in the 80's with wood props as a work around to
some of the draw backs of wood props.)

Your comments are welcome

With a metal prop Sensenich, you will not use the "Viscos Disk" you now have with your wood prop.

The main purpose of the disk with a wood prop, are two fold:
One - add weight to the nose for CG;
Two - give the engine more rotational inertia (flywheel) so it would idle better.

The low rotational inertia of light wood props required you to idle at a higher RPM. This was a pain to taxi with a high idle, The disk helped with the flywheel effect. As far as smoothness, wood props are already smooth because of the natural dampening of wood, God's Composite, if you will. I don't think the disk did much there and was more hype in my opinion. They where popular in the 80's, especially since wood props where used more. Now most opt for C/S props or the metal fixed Sensenich prop, made just for the RV. Wood it great but has some drawbacks, as does metal. Everything is a trade-off.

A metal prop will provide the rotational inertia and weight. Adding any extra mass on the end of the crank is counter productive and could be a hazard. I don't know for sure but call Sensenich and ask, they will know for sure. My opinion is it's a waste at best. Also, far as tensional dampening, if it was necessary and useful Sensenich or Lycoming would have done it long ago.

Wood props are as smooth as you will find. They have many advantages but there are drawbacks. Going metal is a good move. I assume Sensenich. Wood props require more maintenance and are rain restricted to mention a few.

Metal prop will be a little faster (due to thinner airfoil) and maintenance free. Metal, however does not dampen like wood and will be more "buzzy", but if well balanced (dynamically), blades track and are pitched evenly, metal props are plenty smooth.

Wood props have there own natural dampening quality that makes the pilot "feel" it as smoother than metal. Metal has harmonic frequency that can be excited by the engine's own crank-shaft natural frequency?s and power pulse at some RPM's. When you get flying you may find just a 50-100 rpm differnce will make it much smoother.

Sensenich has tested this and determined the prop / engine is OK at all RPM's. It is also made for RV's and there flight speeds.

Bottom line you do NOT need the little gizmo with a metal prop and it will do nothing but add more weight. Also some of these "disks" have cracked, especially if you do aerobatics.

EXPERIMENTING by using "The Disk" and could shift natural frequencies to be counter to safe operations, unless you do definitive flight tests.

If you do any Mod you should call the manufacture and check, including a dampening disk.

Take all the wood prop stuff off and sell it on eBay. Put on the New Sensenich prop, all new matching hardware and new spinner as needed. Enjoy the speed, safety and low maintenance.


(I don't work for Sensenich and am a diehard c/s Hartzell guy, however clearly Sensenich has a great product for the RV's and made their product just for us. I think it is a deal and the performance is outstanding for a fixed prop. I still like c/s better, but that comes at a premium of cost. Also not to offend any wood prop flyers. Wood props are as light, cheap and nice looking as you will find. They give good performance and are safe if operated and maintained as intended. However they are made of made natural materials, subject to normal variations and defects. The big draw back is the bolts need to be re-torqued on a routine schedule.)

Last edited by gmcjetpilot : 11-08-2005 at 10:54 PM.
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Old 11-09-2005, 12:27 AM
ship's Avatar
ship ship is offline
Join Date: Apr 2005
Location: PA
Posts: 100
Default huh?

The short answers are "yes" "no/no" "yes"

Bottom line: Sell the disk along with the prop. Have the metal prop dynamically balanced. Have fun.

Long answer:

Viscous discs are actually used as "balancers" rather than "dampeners". (Landoll "dampeners" were/are quite common on woodies. Is the "dampener" in question a Landoll?)

It's common for most folks to confuse dampeners and balancers. This is a subject that gets hugely complex in a hurry, especially on direct-drive piston propeller aircraft. (see other threads on this quagmire issue)

In simple terms, dampeners (shock absorbers) are designed to dissipate "spring" energy; balancers are designed to redistribute mass. There are many types of dampeners, balancers and combinations. Viscous disks can perform both functions, but the balance function is far more important.

Wood props have balance problems but don't need torsional dampening (at least at the size used on RV's) b/c they have very low polar moments, i.e. they are too light to require torsional dampening. And, as George correctly states, wood naturally provides whatever harmonic dampening it needs via the wood itself.

Metal props are just the reverse: they are stable in balance but require large torsional dampening and they are prone to harmonic vibration.

Composite props are more like wood than metal vis a vis dampening.

Viscous disks have no benefit on metal props, but it has been tried and tested.

DanH provides a thorough and accurate description of torsional node, but it does not apply to why viscous disks are found on wood props. They provide minimal torsional dampening in this application.

The disks are "continuous dynamic balancers" very similar to those used on trucks, i.e. disks with fluid-filled rims bolted between the wheels and hubs. These perform a dynamic balancing function by displacing fluid to the "light" spots to laterally re-center the c.g. of the wheel (or prop disk).

Wood props are notorious for de-balancing due to changing moisture content in each of the blades. Since this is always different for each blade, a wood prop will virtually never be in perfect balance even from morning to evening. This is why you should leave a wood prop in the horizontal position when stopped....the lower blade will be heavier than the upper blade due to gravity pulling moisture down.

Hence the fluid disks, which follow the laws of inertia in rotating bodies. The eccentricity induced by a heavy blade will force the fluid to migrate to the opposite side, thus "automatically" moving the cg back toward the center. Clearly there is a limit to the ability of the disk to offset any imbalance, which is why these disks are sized to accommodate a nominal range of predicted imbalance.

Once balanced, metal props tend to stay that way. No need for viscous disks.

For those who are wondering: torsional dampening for metal props on non-counterweighted Lyco/Conti engines is accomplished via carefully designing the nose of the crankshaft to twist juussst the right amount for a given range of polar masses....hence the need to stay with the "tested" props for each engine configuration.

Ironically, the introduction of metal props required crankshafts to be both strengthened and "softened" at the same time. Quite a challenge!!!
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Old 11-09-2005, 10:06 AM
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DanH DanH is online now
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Location: 08A
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I never paid much attention to Landoll's products, so I did a quick search and turned up a recent ad on Barnstormers. Interesting to find that Mark refers to his product as a harmonic damper (not a balancer). Further, it appears that he requires the damper be installed with a prop extension, which may be to shift the damper's physical location in relation to the F1 node.

From the ad:

"The harmonic dampener consists of a steel ring floating in silicone fluid inside a machined aluminum case. This unit dampens 30 to 40% of harmonic and other engine vibrations. Must use prop extension in conjunction with dampener. Weight approx 12 lbs. Cost $375.00 The steel ring CG adjuster is a one inch thick steel ring. Weight approx. 12 lbs. Cost $90.00 ."

Without knowing some details of internal construction (mostly radial clearances), it is difficult to determine if the Landoll viscous device has any dynamic balancer properties. Maybe so, maybe not. As to effectiveness as a torsional damper, a lot depends on physical location in the system. The actual damping values of the device are a function of ring mass, ring diameter, fluid viscosity, and again, internal clearances. Note that increasing radial clearance to improve performance as a balancer would decrease performance as a damper.

I find Mark's description of the "steel ring CG adjuster" to be accurate. If it is a uniform one inch thick and weighs 12 lbs, then it would only be about 7.5" diameter. That is too small (compared to propeller diameters) to add any significant rotating inertia, and he apparently does not make that claim. If it is thin in the middle with a thick rim, diameter might be around 10 inches. It would have more inertia, but still not a huge amount.

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Old 11-09-2005, 10:57 AM
warren hurd warren hurd is offline
Join Date: Jan 2005
Posts: 38
Question Conical Mount?

Hi Bob,

I had thought that the harmonic dampener was recommended for the conical mount with a wood prop. Exactly what I have. I am not flying yet and have not bought one either. So could I get a good price on yours? Any other thoughts on dampeners with conical mounts?

Thank You
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Old 09-30-2022, 06:59 PM
cgarts1 cgarts1 is offline
Join Date: Mar 2020
Location: Lyons CO
Posts: 76

I flew my RV14 175 hours with a FP Sensinich aluminum prop and the Mark Landoll balancer. Ran smooth as silk without any problems.
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Chuck Garton
RV-14A flying 140600
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Old 10-05-2022, 09:47 AM
cheeselion cheeselion is offline
Join Date: Aug 2019
Location: Vancouver, BC
Posts: 31

Since this thread was brought back to life...

What is the brain trust's thoughts on dampeners with modern composite ground-adjustable props?

I have a 3-blade Sensenich GA on my RV-4, and a dampener waiting to go on. The thinking behind this was that I was told by the guys who dynamically balanced the new prop that the blades being ever-so-slightly different in pitch (because of the ground adjustable nature) made it really hard to get a consistent balance.

So I thought, well rather than having the assembly balanced and then messing that up during an annual or other work on my prop/hub, I'd just put on a dampener and let it figure out the balancing for me.

Was that a simplistic and wrong assessment?
Tim E.
RV-4 2080 (bought)
BC, Canada
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