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Guts of the Landoll Balancer

Zero4Zulu

Well Known Member
Bought this balancer recently and have disassembled it for inspection/rebuild. I thought some would like to see what’s inside…

It has two areas of galling between the steel ring and aluminum channel. The ring has not been rotating in the housing, so probably only acting as a flywheel rather than a Damper. I think the ring is supposed to rotate, maybe not because those large “O” have a lot of pressure on them.

I’m thinking of anodizing the aluminum parts to help prevent the galling.

Any input on this balancer?


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The overhaul/service manual should list the limits for wear and galling, make sure to measure and compare to those numbers.

Anodizing can weaken a part so it has to be designed around that loss in strength. The manufacturer can tell you if there’s enough engineering limits to work with the anodizing.
 
Steve, thanks for posting this internal view. It is a little surprising that it galled when bathed in high viscosity silicone, but it certainly happened.

Here is a link to a company that has made this type TV damping device for many many years (formerly Houdialle "who dye"). The tech service may get you some contacts with the company and they might help with the details for your restoration.

Do measure and document the clearances for your discussion, though. I would imaging they are pretty small as any offset would result in imbalance. The installed assembly may have been out of balance as it seems the galling is a specific clocking. In theory, if not rotating in an orbit (i.e. rotating concentrically) there would be no radial loads at all.
 
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Thanks Bill. I found that both the steel ring and the aluminum housing are out of round by .003"-.004". Which tightens up the clearance between the two. It looks like there is about .003/side of the steel ring, clearance.

I dialed in the parts on my CNC mill for measuring. The Aluminum housing was not machined perfectly parallel so I have fixed that. Looks like they probably distorted the part while clamping for machining. Then when released the roundness is screwed up. All fixable.
 
Thanks for posting the pictures. I have been intrigued by the Landoll Balancer. That is great you can get it repaired and working again. I wish I had those skills!
 
Thank you Steve for the pictures and Bill for the link. I have one on RV-7A along with a Catto three bladed prop. I think it helps with the lighter prop at low power settings and makes for a smooth operation. Now I have a good idea about how it works.
 
The balancer is an unnecessary part. If the Propeller is properly balanced and tracked, it should be very smooth in operation. If you need weight on the nose for w&b get a heavy crush plate cut to the desired weight
 
How many ounces of liquid did the unit hold?

Maybe only about 4-5 ounces of fluid I think, maybe less. The most space is between the id of the steel ring and the id of the slot it sits in. That gap is about .200” all around and 1.025” deep.
 
It's a harmonic balancer

The balancer is an unnecessary part. If the Propeller is properly balanced and tracked, it should be very smooth in operation. If you need weight on the nose for w&b get a heavy crush plate cut to the desired weight

It balances out the harmonics of the engine . You would look long and hard to find someone who isn't happy with theirs .
As to servicing .....I had a limited production run of 10 ( 11 actually) made last June and the company that bought the rights from Mark ( fine gentleman RIP ) will service the unit for a VERY reasonable price ...if memory serves $175 ....
I have a new one in the box and my used one will be going there soon for service as I really believe it is best left to those with the expertise and the price and turn time seems more that reasonable to me
 
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weird

It is weird I don't see any balancing marks on the Balancer. I would think one would see balancing marks because nothing ever seems to be perfectly balanced right off a mill.

It seems even a lightweight prop would seem to have a bigger moment of inertia given the 72" diameter, than this 12" dia balancer. But that is just me and TLAR engineering...

Glad others have found it worthwhile.
 
Harmonic balancer

I have installed probably 50 -60 Harmonic balancers in my hot rodding days and not one had any balancing marks that I remembered, they are keyed to crankshaft ....as stated earlier I am convinced by my discussions with the late designer and the current manufacturer that it wasn't intended as a " Prop " balancer .
 
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It balances out the harmonics of the engine . You would look long and hard to find someone who isn't happy with theirs .
As to servicing .....I had a limited production run of 10 ( 11 actually) made last June and the company that bought the rights from Mark ( fine gentleman RIP ) will service the unit for a VERY reasonable price ...if memory serves $175 ....
I have a new one in the box and my used one will be going there soon for service as I really believe it is best left to those with the expertise and the price and turn time seems more that reasonable to me

Stew, most folks who bought a balancer did it on recommendation from those who already owned one. So there is bias in their opinion. I will add, most of my experience is with Wood props and am positive the balancer is not needed with a properly tracked and balanced Wood prop. I am a proponent of reducing weight and see the balancer as a bandaid when one should fix the underlying problem
 
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Harmonic balancer

You are more than entitled to your opinion….but again isn’t a prop balancer ,but intended to help balance the harmonics of the engine ( I know that the very reputable prop manufacturer that I purchased my prop from recognizes the impact of engine harmonics and I certainly do as well ) ……is it absolutely necessary ?? …obviously not…lots of engines running without them ….however I believe that most home builders rely on more than anecdotal evidence when making decisions and I spent a good deal of time investigating before I went to the effort of getting new ones made . I think it is a bit presumptuous to assume that most or perhaps even any use this as a band aid as you suggest ….my experience ( hate to say it but 48 years worth ) is that once again most home builders from their very “ hands on “ experience leave very few stones unturned and I can tell you unequivocally that the designer of this in no way suggested that this was a replacement for good prop balance….again because that is NOT it intended purpose
Like you I ( most are ) very conscious of weight ….but if weight is needed forward …as is often the case with a wood / composite prop this actually has a function for the weight as opposed to steel crush plate which is basically “ dead weight “ ….I see it as a “ win -win” so to speak ….but again you are entitled to your opinion ,there is no right or wrong here
 
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Seems like a lot of the problem is the name. It is really a torsional vibration damper. "Balancer" is a bit of a misnomer.

I'm sure someone here has the instrumentation that could show a before v. after amount of torsional vibration. DanH I bet can tell us how to do it!
 
@Stew,

The name appears to be a bit of a misnomer, at least to me. If it were called a Landoll Harmonic Dampener would there be less discussion?
 

The link I provided earlier has all the information necessary for one to understand show this works and what the principles are. There are papers and plots of TV stresses and displacements that show before and after.

Cost is a huge factor. The elastomer rings simply split the resonant frequency to keep TV stresses within limits. It does no true damping, but the visconic dampers will damp over a wide range. Engines are not designed with nobility of being the best that can be made, it is the best that meets the production cost.
 
Not really applicable to RV's but this unit will help the non -counterwighted 4 banger Lycs tune out the TV issues when running long (80+ inch) metal props.

IIRC, Its part of the STC to run the Hartzel "Top Prop" kit on the 4 banger. If the engine has dynamic counterweights, the extra dampener is not needed.
 
Why would It not be applicable to RV's

Please educate me as to how the length of the propeller would dictate the benefits of using a harmonic balancer or not ? I get the dynamic crankshaft balance,but I can't see the prop diameter as a deciding factor ? Thanks in advance
 
I am only a customer

@Stew,

The name appears to be a bit of a misnomer, at least to me. If it were called a Landoll Harmonic Dampener would there be less discussion?

I agree...but I didn't name it .....my sole interest was to have one on my plane ....so much so that I arranged the limited production run as I think I do understand it's function and only commented because an easy service is available, hoping to save the OP a bunch of work ... ( I have NO connection or vested interest in the company...just a customer ) Also there were statements made as to its purpose and function that from my fairly extensive reasearch that I believe were incorrect .
 
Guessing here. It not about RVs as much as allowable prop length; or more specifically, the stiffness/elasticity of the rotating mass.

The longer the blade (RMOI), the more (lack of) stiffness/rigidity will influence adverse TVs. I probably just got BillL excited.

Willing to learn, here.
 
Guessing here. It not about RVs as much as allowable prop length; or more specifically, the stiffness/elasticity of the rotating mass.

The longer the blade (RMOI), the more (lack of) stiffness/rigidity will influence adverse TVs. I probably just got BillL excited.

Willing to learn, here.

Yep. The long, metal prop is a tuning fork and when combined with the firing frequency of the 4 cylinder will set up a resonance that can break the crank. The dynamic counterweights (in those certain models) takes care of that resonance. The type of dampener discussed here is the alternate method of taming that resonance issue.

This all ties into the other thread about why PSRU design is "hard"... Every propeller length, construction, engine firing frequency, CR, ignition timing, induction and exhaust tuning all combine to create a TV signature. Sometimes that signature is benign, sometimes wildly destructive.
 
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Yep. The long, metal prop is a tuning fork and when combined with the firing frequency of the 4 cylinder will set up a resonance that can break the crank.

Break the prop.

The dynamic counterweights (in those certain models) takes care of that resonance.

Note pendulum absorbers are tuned to a specific order rather than a frequency.

The type of dampener discussed here is the alternate method of taming that resonance issue.

In this specific application, it's more likely simply acting as a flywheel mass, as its location near the node of the first torsional vibratory mode makes it largely ineffectual as a vibratory damping device.

I've attached an illustration from DenHartog's Mechanical Vibrations. It's a classical representation of mode shape for the two lowest natural frequencies. From left to right, it represents a large driven inertia, a flywheel inertia, four crank throw inertias, and the accessory inertias. The values are relative amplitudes of angular displacement (shaft twist), the maximum ("1") being found at the free end of the system.

I've also attached a sketch in which I've lumped the crank and accessory inertias together. It illustrates one aspect of what the mode shape diagram is representing...the direction of vibratory rotation when the system is being excited with a forcing frequency equal to one of these natural frequencies.

See the little circles where the plot lines cross the X axis? Those are the vibratory nodes for the two modes of vibration. A node is a point along the chain of connecting stiffness members where there is no angular displacement.

Now read carefully...if we have to equal inertias connected by a shaft, the node will be located precisely halfway between them, and the two inertias will oscillate with opposite and equal amplitude. If we connect two unequal inertias, the node will move toward the larger inertia, because the large inertia oscillates with proportionally less amplitude. If the large inertia is much greater than the smaller, the node will be very close to the larger, as the larger acts almost like an immovable object.

That's what you see in the DenHartog figure, smaller inertias oscillating with far more angular displacement than the large inertia. The first mode has a node close to the large inertia, as the vibratory amplitude of the large inertia is only about one fifth as much (0.2) as the free end of the system. In the second mode, there are two nodes, one close to the large inertia and one somewhere along the crank. The node near the large inertia (the prop) is moved even closer to it, as the angular displacement of of the large inertia is tiny (0.06).

Here's the point...there is no angular displacement at a node, thus a viscous torsional damper located at a node is useless. I have made no attempt to determine the precise location of the node for the first mode (or any other), but you can be sure it's quite close to the prop flange in the case of a Lycoming four banger connected to a propeller.

Compare with the typical auto engine application...the damper is at the free end of the system, the location with the greatest angular displacement. Same is true for the location of pendulum absorbers. Put them near the prop and they would be far less effective.
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Great Stuff DanH. If I understand... then, it would be more likely for the Landoll to have some effect if the prop is a light (wood / composite / etc.) type than if it is a heavy metal one.

If someone wanted to actually measure reduction in torsional vibration (somehow), then it would be important to test with both a light and a heavy prop.

Similar influence might come from a lightweight flywheel.
 
If I understand... then, it would be more likely for the Landoll to have some effect if the prop is a light (wood / composite / etc.) type than if it is a heavy metal one.

As there is no connecting stiffness between the Landoll's inertia and the prop hub inertia, the effective result is an increase in prop inertia...making it more like the metal prop.
 
Explain please

Hi Dan ….not sure I understand how there is no connecting stiffness between the props inertia and the LANDOLL’ s inertia . I must obviously be on the completely wrong track ….as would not the prop hub extension be a “ connecting stiffness “ as it certainly connects the prop to the crankshaft which of course connects to the flywheel …which is the mounting surface for the LANDOLL . Completely confused on that point …..also given an installation with a prop hub extension in the above diagram there would in fact be displacement at the “ flywheel “ location ( second from left if I understand) and while admittedly small in comparison to the “ free end “ it would nonetheless exist and would that not in fact render some validity to the function of the LANDOLL as it is mounted to the flywheel ?? Hope I am understanding that one correctly …...thanks in advance. Stew
 
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….as would not the prop hub extension be a “ connecting stiffness “ as it certainly connects the prop to the crankshaft which of course connects to the flywheel …which is the mounting surface for the LANDOLL .

Obviously yes, assuming a shaft extension exists in a particular installation.

However, the question becomes "How stiff is it?". To see the reality, it's necessary to quantify, because we're not talking Reno F1 here, i.e. no 12" aluminum extensions. The shaft extension Vans sells for a 360 is only 2.25" long, but roughly 6" diameter in the shaft section.

The equation for torsional stiffness of a simple shaft is...

(G x J / L) / 12 = ft-lbs/radian, where:

G = shear modulus of the shaft material
J = pi x (outside radius^4 - inside radius^4) / 2
L = length of tube

Note "J" is based on radius to the 4th power. Short and fat is a big deal.

How big? Let's make a comparison. The nose section of a 360's crank is 2.375"D with an ID of 1.9". Let's call it 6" long, just because I'm too lazy to walk down to the shop and measure. Shear modulus ballparks around 12 million, so torsional stiffness is about 307,205 ft lb per radian.

Now the shaft extension. Shear modulus for aluminum is way less, ballpark being 3.5 million. However, if the shaft is 2.25" long and 6" in diameter with a 0.75" center bore, stiffness is about 16.5 million ft lbs/rad. The short, fat extension is roughly 53 times stiffer than the propshaft nose, despite being made of softer material.

For all practical purposes, the extension is just part of the prop hub inertia.

The exact location of that node would be a function of relative intertias, prop vs crank, not stiffness, so don't be confused. Bottom line, in the big picture, anywhere near the large inertia is a poor place to put a viscous damper, because there is little vibratory angular displacement in that location.
Without it, the damper can't be very effective.
 
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So far we've been thinking about the prop as a constant consistent rotational inertia (a big flywheel if you will).

I suspect (I don't know) that the prop experiences varied resistance (for lack of a better term) as rotates in front of an asymetric cowl that is quite near the rear of the prop. Does that induce a torsional vibration in the prop?
 
The blades on a propeller bend like a willow in the wind at every firing inpulse from the engine. They retreat at the firing event and then rebound. Similarily, the crank winds up like a spring at every firing event and also rebounds. If the "store and release" cycle of these two "springs" do not add energy, then the system remains within the structural limits of each component within the operating range, then life is good. If you change the properties of one component (longer prop, for example), then you can drive a self feeding (energy additive) resonance condition, which can quickly exceed the structural limits of the crank or prop. This is why you have a requirement to add an external dampener on an STC if you want to add a particular prop to a particular engine, or you have placard on the instrument panel to warn you of specific RPM/MP settings. The external dampeners add inertia, yes, but also drive angular displacement of the crank/prop hub into a dampener ring through a viscous fluid. This picks off energy in the form of friction and heat, which the dampener is designed to deal with. This rouge, self exciting energy is powerful stuff and baffled Jim Bede for years with the redrive system of the BD-5, Pratt and Whitney engineers developing the R-2800, Rolls Royce on the Merlin, and is also why you -12 drivers have such a high idle speed on your Rotax.
 
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Still confused

As there is no connecting stiffness between the Landoll's inertia and the prop hub inertia, the effective result is an increase in prop inertia...making it more like the metal prop.

So …help me here please DAN …. No argument that LANDOLL would be of more effect on the accessory end of the crank …but obviously not an option …..however a couple of posts up the fellow suggested that ( assume a prop shaft extension) the LANDOLL would have more effect ( not just flywheel effect ) with a wooden prop than a metal …..that would seem to be supported ( sorry could get “ quote “ to work ) with your statement .” The exact location of that node would be a function of the relative inertias prop vs crank not stiffness ( lots in that extension apparently) so don’t be confused ( I only wish !)
So the wooden prop has less intertia than a metal one of same diameter would not the LANDOLL have more positive effect ( even if negligible) that if a metal prop was used ? Or have I got that backwards ? Thanks again
 
Exactly

The blades on a propeller bend like a willow in the wind at every firing inpulse from the engine. They retreat at the firing event and then rebound. Similarily, the crank winds up like a spring at every firing event and also rebounds. If the gyrations of these two "springs" remains within the structural limits of each component withing the opperating range, then life is good. If you change the properties of one component (longer prop, for example), then you can drive a self feeding resonance condition, which can quickly exceed the structural limits of the crank or prop. This is why you have a requirement to add an external dampener on an STC if you want to add a particular prop to a particular engine, or you have placard on the instrument panel to warn you of specific RPM/MP settings. The external dampeners add inertia, yes, but also drive angular displacement of the crank/prop hub into a dampener ring through a viscous fluid. This picks off energy in the form of friction and heat, which the dampener is designed to deal with. This rouge, self exciting energy is powerful stuff and baffled Jim Bede for years with the redrive system of the BD-5, Pratt and Whitney engineers developing the R-2800, Rolls Royce on the Merlin, and is also why you -12 drivers have such a high idle speed on your Rotax.

……. This supports what I was told by people that I believe were in a position to know when I was looking at purchase of new balancer….prop is NOT just a disc spinning around ,but a very dynamic unit …….my head is swimming….think I need another coffee. Cheers
 
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So far we've been thinking about the prop as a constant consistent rotational inertia (a big flywheel if you will).

An accurate model of an engine-propeller system does incorporate blade root stiffness as well as the torsional stiffness of shafts and shaft equivalents. The FEA models now used in industry are above my pay grade, but my understanding is that they simultaneously determine stiffness and inertia for every molecule in the system.

Typical explanations treat the propeller as a simple flywheel in order to make torsional vibration understandable...and even then, success is sadly limited. In any case, adding blade bending into the picture does not change the concept of vibratory node. Placing a viscous damper close to a node renders it ineffective. To be most effective, it should be placed at the point of maximum angular displacement when resonant.

Again, let's look at the mode shape illustration from Mechanical Vibrations. In the first mode, the damper would be most effective at the far right, the accessory end of the system. For the second mode, the damper would be best at the accessory end or at the flywheel (0.82). These are the points of maximum angular displacement when the system is excited at a frequency equal to the natural frequency of the mode.

BTW, the number of possible modes is equal to the number of inertias minus one, so there is also a 3rd mode, a 4th mode, etc. In order to place the damper in an effective position, you need to know which mode you wish to damp, and its mode shape.

I suspect (I don't know) that the prop experiences varied resistance (for lack of a better term) as rotates in front of an asymetric cowl that is quite near the rear of the prop. Does that induce a torsional vibration in the prop?

Generally, the prop is a victim, not an antagonist. Put in a more technical terms, it has one or more natural frequencies, and only vibrates in a resonant manner when that natural frequency is excited by a forcing frequency, or an integer multiple of that frequency. Put a third way, the engine excites prop vibration, but the prop rarely excites engine torsional vibration.

There is one common exception sometimes seen with twin engine aircraft whose propeller tips pass close to the fuselage sides. Each blade tip passes through disturbed air, and thus the exciting frequency is equal to (number of blades x RPM) / 60. Like all exciting (forcing) frequencies, it's not a problem unless something in the system has a matching natural frequency. (EDIT...some pusher applications too.)

I'm not aware of any such disturbance due to a prop rotating in front of a cowl of any practical shape.
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Not really applicable to RV's but this unit will help the non -counterwighted 4 banger Lycs tune out the TV issues when running long (80+ inch) metal props.

IIRC, Its part of the STC to run the Hartzel "Top Prop" kit on the 4 banger. If the engine has dynamic counterweights, the extra dampener is not needed.

Mike, got a link to the STC information?

The blades on a propeller bend like a willow in the wind at every firing inpulse from the engine.

"Like a willow" is perhaps an excessive visualization. Strictly opinion, but I base it on having spent time around spinning props with a powerful variable rate strobe light. A strobe allows the operator to visually freeze the blade so it appears stationary. Dialing the strobe just off the freeze rate allows watching the blade motion as it "rotates" at some very, very low rate, like 10 seconds per "rotation". In general, blades bend like a willow only when something is wrong and bending load is well beyond normal, like when the system is resonant, or a blade has gone into flutter. I've seen both.

Still, blade stress can obviously reach high levels without visually detectable motion. Blades do bend, but generally not like pool noodles, or willows ;)
 
Dan, Back in the days when I was more interested in tube and fabric bush type aircraft, there was significant discussion about the static thrust of "big props". That led down the rabbit hole of the Hartzell "Top Prop" retrofit offering for various 4 banger Lycomings. I really wanted the 80 inch Hartzell for my project, dreaming of a 20 foot takeoff roll.... However, digging revealed the 80 incher was only offered on those models of 360 that had dynamic counterweights. The non - counterweighted crank in my engine required the addition of the Hartzell damper - a heavy and expensive option I was not looking forward to exercise. I started looking for "good deals" on the Hartzell damper and almost pulled the trigger on a few, but didn't.

Lots of typing to say that I've lost track of the STC info over the years. And frankly, in the decade or so since I was "into" bush aircraft, its possible I could be remembering wrong.

As to the "willow in the wind" - yep, there's some hyperbole there, but it was meant to illustrate that the blades do bend, rebound and vibrate like tuning forks. The prop is more than a solid flywheel. My oversimplification was an attempt to keep the concept relatable to a wide audience.
 
Hartzell C-1576. Appears to be little more than a mass ring in the limited photos I find on the net. Anyone have an exploded diagram?
 
I am a fan of old SAE and NACA papers, in particular the pre-WWII years, the golden age of piston aircraft engine development. They tend to be written in plain language, in an era when it was all new to the authors as well as the readers. Some of them are serious classics. One of those is the Lurenbaum paper.

Karl Lurenbaum was a researcher at the Deutschen Versuchsanstalt fur Luftfahrt, Germany's equivalent to our NACA. The DVL was controlled by Hitler's Reich Air Ministry after 1934, yet in June of 1936 Lurenbaum presented his "Vibration of Crankshaft-Propeller Systems" at the semi-annual SAE meeting in White Sulfur Springs WV.

Think about that. And does anyone know of a Lurenbaum bio?

Returning to topic, the paper was published in SAE Transactions, December 1936, Volume 39, #6, and can be be found in many university libraries, or possibly online. Highly recommended.

Edit: I've scanned the paper into pdf format and posted it here for download:

https://www.danhorton.net/Articles/Vibration of Crankshaft-Propeller Systems.pdf

If you're really curious about the interaction of crankshafts and propellers, make a pot of coffee, find a quiet spot, and go for it.
 
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This is a really good discussion, but I can't help poke a little light-heartedness into it, while hopefully some additional insight.
The FEA models now used in industry are above my pay grade, but my understanding is that they simultaneously determine stiffness and inertia for every molecule in the system. .

In principle, if the FEA was of such fine resolution that every molecule was modeled as a discrete element, then this would be true. The element stiffness matrix for each molecule would include all the vibratory degrees of freedom of that molecule.
Of course that would require a mighty big computer! So, well, no, typical FEA models don't resolve the characteristics of every molecule. FEA still requires "lumped parameter" modeling of discrete portions of the system. Dan's point is that the resolution (the number of elements) can be way more than the 4 or 5 element models that can be easily analyzed in closed form (by hand).

In any case, adding blade bending into the picture does not change the concept of vibratory node. Placing a viscous damper close to a node renders it ineffective. To be most effective, it should be placed at the point of maximum angular displacement when resonant..

While it is certainly true that a damper at the free end of the system would be most effective (and one wonders if Lycoming ever explored the idea - it would not be difficult to design in a damper on the accessory case end), including the blade dynamics DOES change the node location some, since the prop hub undergoes torsional displacement as a result of blade bend. We do know that for some propeller-engine combinations, there are some RPM ranges where the prop dynamics do couple with the crankshaft dynamics. Whether the node moves to a point out in front of the prop, or to a point between the prop and the first crank throw likely depends on the particular mode. Because of this, for at least one mode, the damper at the crank flange would be more effective than would be found if the prop were just a flywheel mass.

It is also tempting to consider that the prop extension that is used in concert with the Landoll damper moves the node farther away from the crank flange, so that the damper may be more effective, but the fact that the extension itself is so torsionally stiff means that the node is probably still right at the crank flange.


There is one common exception sometimes seen with twin engine aircraft whose propeller tips pass close to the fuselage sides. Each blade tip passes through disturbed air, and thus the exciting frequency is equal to (number of blades x RPM) / 60. Like all exciting (forcing) frequencies, it's not a problem unless something in the system has a matching natural frequency. (EDIT...some pusher applications too.)

.

The best example of this is a pusher prop arrangement where the propeller has to pass through the wake of a lifting surface (or multiple lifting surfaces). The turbulence in the wing wake can excite a broad range of frequencies. You can HEAR the prop vibration quite plainly in this case. Listen to a Gates Piaggio Avanti, or if you happen to see one of the few left, a Beech Starship. Even a LongEZ exhibits the classic sound of a prop getting beat to death by wing wakes. I'm just not quite old enough to be blessed with ever hearing a B-36 fly over, but some of you may have. I can just imagine the sound! No doubt propeller service life is shortened by this, and perhaps even engine front bearing life.

The type of damper Dan is talking about that most of us know of from wrenching on American V-8s in our youth is called a Houdaille damper. A remarkable characteristic is that it effectively damps all the modes of the system. You can see from Dan's mode-shape picture that the free end of the crank experiences large torsional displacement from those two modes, and in fact from all the modes.

In the classical model, this really is a "free" end. In reality, it is less so because of the inertia of the gears and accessories being driven by the end of the crank. While that inertia is certainly small compared to the prop, it is not much smaller than the other inertias in the system. Evidence the fact that the Service Bulletin for inspection after a prop strike involves replacement of the bolt retaining the crankshaft gear, and inspection/replacement of the drive pin. It is only because of the inertia of the accessories that the connection to the crankshaft sees significant torque from a "sudden stoppage". Because of this, there is a vibration mode of the system where the accessory case end of the crank is close to a node and a damper would not damp that one mode very well. But it is surely the highest-order mode in the system, and likely the least important.
 
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Harmonic......something....balancer.....dampner... .

I have read and appreciate all the technical discussion regarding a balancer. I put mine on before flying as the theory at the time seemed reasonable. And the car racing industry has used them for years.

I found it fascinating to watch airplanes (mostly RVs I was watching) that, when the engine/prop combination stopped producing power, the prop "SLAMMED" to a stop: it was spinning and SUDDENLY/ABRUPTLY it was stopped. My educated mind couldn't help but imagine where all that power suddenly went. It was NOT to keep the prop moving: not enough weight. It made sense that the energy was transferred to the internal working parts of the engine: likely mostly the crank, piston rods, pistons, etc. Ouch. Warm wet slap in the crank.....;)

Our Lycoming engines are relatively "big bore" in that they are "small" engines with a comparatively large bore/displacement. The power strokes sends a lot of energy somewhere, mostly to the prop when the engine is running. The crank when it stops.

When I cut power to my engine (wooden prop with the balancer), it 'slowly' ticks to a stop rather than the abrupt stop described above. Not scientific but merely an observation. Looks and 'feels' better to me. And the engine runs smoooooooooothly when running....IMHO....
 
I meant to also add that if the prop truly was at a node, the prop blades would not get excited. The fact that there are certain prop/engine combinations with certain no-continuous-operation conditions suggest that at those frequencies, the prop and the crank do couple. In that case, a damper on the hub tuned to that same frequency would also get excited, and dissipate energy that would serve to protect the prop.

I also note that there are certain prop/engine combinations for which Hartzell actually supplies a damper assembly. I think that makes the case that at under the conditions that might be damaging to the prop, a damper on the hub can be a benefit.

While it is certainly true that a damper at the free end of the system would be most effective (and one wonders if Lycoming ever explored the idea - it would not be difficult to design in a damper on the accessory case end), including the blade dynamics DOES change the node location some, since the prop hub undergoes torsional displacement as a result of blade bend. We do know that for some propeller-engine combinations, there are some RPM ranges where the prop dynamics do couple with the crankshaft dynamics. Whether the node moves to a point out in front of the prop, or to a point between the prop and the first crank throw likely depends on the particular mode. Because of this, for at least one mode, the damper at the crank flange would be more effective than would be found if the prop were just a flywheel mass.
 
Steve, great comments.

I meant to also add that if the prop truly was at a node, the prop blades would not get excited.

Absolutely, for the same reason that placing a damper at a node is ineffective.

I also note that there are certain prop/engine combinations for which Hartzell actually supplies a damper assembly. I think that makes the case that at under the conditions that might be damaging to the prop, a damper on the hub can be a benefit.

Traded a note with Les Dowd today, and will follow up with another engineer. The Hartzell C-1576 is sort of a hybrid device, mostly a heavy mass ring. However, there is a silicone grommet at each point where the mass attaches to the carrier, with spring loaded friction damping. I'll explore further and report.
 
C-1576 drawing below. As static friction is typically higher than dynamic friction, the mechanical damping function (silicone grommets in parallel with friction damping) would only come into play when oscillation exceeds some angular acceleration threshold. Most of the time it's a simple 8 lb flywheel, locked to the carrier by friction.

I'm told it was originally developed to deal with a particular application which had an unacceptable blade tip vibration driven by a 12th order. Later it was tried with a problematical 80" blade applied to a 360 4-cyl without pendulum absorbers on the crank (probably the app Mike spoke of), resulting in acceptable blade stress. It was found to be ineffective in other applications, and totally unnecessary with crank pendulums, which are far more effective.
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question about the Landoll dampner

I've had one of these for 800 hrs on my RV 6 with an MT fixed pitch prop... made for a very smooth combination and added some needed weight to the front of the airplane. I've always assummed that it not only only damps harmonics but also balances the prop. Correct?

I've recently replaced the MT with a Whirlwind 330 controllable pitch prop, which is great, but surprisingly after replacing my old Lycoming starter with a light weight starter and re-weighing the aircraft, I discovered that I could still use more forward weight, so am thinking about using the Landoll again. It has the aerobatic plate, so it would shift the o-ring position on the prop flane forward 1/8." What do you folks think... good or bad idea?

Thanks, jp
 
harmonic dampner, not prop balancer

Aha, I just went back and read all the posts... and discovered on p.2 the answer to my 1st question re: prop balancer vs. dampner. Still interested in opinions from you experts about use with my Whrilwind 330 and parallel valve IO-360. Incidentally, the WW is already dynamically balanced. Thanks, jp
 
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