pendulum dampers

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Aviacs

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Humor me here.
All my non-aircraft engine experience dates from British sports cars 50 years ago, and American full size pick-up trucks since. I did work occasionally as a motorcycle engine mechanic/rebuilder in HS and early years briefly. VW engines are new to me and i have not been inside one yet though there is one on the nose of my Sonerai 2 project. I am a "capable enough" old school manual machinist & have made airplane parts.

People develop forged crankshafts for all the engines used in performance apps including experimental AC.

Why has no one put some blades (at the forging stage) on some of those cranks and offered pendulum damping?
How hard would it be (could it be done) to machine an existing crank & add pendulum damping?
Would it be useful on a flat four such as a VW to enable reduction drive or heavier propellers?

A little more personal history: my first airplane/ learned to fly in, was a Cessna 175. Such a beautiful, tough, smooth, engine. I did not realize what clattery, banging, vibrating lumps so many others especially "reliable" Lycoming 4's were until later. The pendulum counterweights, quill shaft, and the extremely simple straight cut gear reduction of the GO300 fascinated me & I've been corrupted ever since.

I am aware that in systems like the C175, all the power train components including the prop were designed (tuned) specifically for given system parameters.
Let the brickbats fly. :)

smt
 

TFF

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Blades= counterweights?


Your 175 engine had a full counterweight engine. That and being flat 6 is why it’s so smooth

Lycoming, depends on stage of the game. The original series engines were outgrowths of small engines. Good enough to work is good enough. The later 200 hp engine versions and 6 cylinder Lycomings have counter weights. Some are even tuned by movable weights.

It’s not can, it’s when do we have to? If you got the capacity to make a crank and you can make it fit, do it. It does add weight, so you you eat that performance advantage. You are making a crank which is a pretty big deal, is your time being spent well? Welding one on? Not regular engines. A standard Lycoming crank looks like you pulled it out of a TriumphTR3. About as basic as it gets. The Lycoming with counterbalance cranks have different cases than the regular.

Some of the racing VW cranks have some weights, but they still are space limited. What ends up on noses of planes is very variable.
 

Aviacs

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Blades= counterweights?

NO.
= "place to hang pendulum counterweights"

The GO300 had a pair of pendulum weights.
(Have to dig out manual to verify location, total #)
I remember setting them in motion, in fascination, with cylinders removed.

BTW - I have no intention of building a crank myself. My metal shop equipment, except the planer and shaper, are standard size turret mills, smaller lathes, and typical to large surface grinders. Basically i don't have a large lathe. My mention of machining was to virtue signal that i am not naive or uniformed about machining processes. :)

My Q is how applcable pendulum dampers would be large V8 cranks, or for flat 4 cranks.

A pendulum damper should not add significant weight over a typical integral (solid) counterweight & could possibly be lighter in the whole system.

smt
 

Pops

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I have had 3 straight tail C-172's with the Cont-0-300 engine. Great dependable smooth engines. Believe putting the C-85 pistons in one for a CR boost would be a nice engine for a Tailwind. Would have to burn all avgas. Get maybe 160 hp. Local friend had a C-175 , 1962 or 63, (the last year), and had a constant speed prop. Very nice airplane.
 

Aviacs

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Pendulum dampeners are used "if" there is a problem with TV. If you dont need them, you dont need them

So based on your experience, you are saying there are no TV issues with auto V8's matched to PSRU's in AC application? That it is "something else"?

Flat 4/ VW's especially with the nose drive cannot use "heavy" (including ground adjustable) props due to TV. Mount a PSRU and apparently it (TV) gets even more interesting. If you have experience there, i'd be most interested in the numbers.

Thanks!
smt
 

Toobuilder

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Maybe, maybe not. Its the specific properties of the various elements in the system that determine "if" there is an issue. How springy is the crank? What is the firing order? What configuration is the crank? What kind of PSRU configuraton (belt, chain, gears)? What the MMOI of the prop? How many blades? What constuction?
 

TFF

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The easy way is there is no easy way. Auto V8 with a reduction and prop. Those things were never ever thought to be together by the original designers. To do it right it needs to be a clean sheet. I’m not saying you can’t get away with a V8 and the other stuff, but you are fixing stuff that should not have existed in the first place. To say it’s TV for a certain engine is the reason is not quite right. The TV exists because the part is under designed for the task asked of it.

Pendulum dampeners like a tuning fork? Used on helicopters. You have to have room for them. I doubt you are going to stick one under a cowl and make it look good.
 

Aviacs

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Maybe, maybe not. Its the specific properties of the various elements in the system that determine "if" there is an issue. How springy is the crank? What is the firing order? What configuration is the crank? What kind of PSRU configuraton (belt, chain, gears)? What the MMOI of the prop? How many blades? What constuction?

Exactly!
It's clear you get it.

And, if you go to my OP, this was summed up:
I am aware that in systems like the C175, all the power train components including the prop were designed (tuned) specifically for given system parameters.

Here's perhaps a simpler version of my Q:

1.)People mate V8 auto engines to geared PSRU's for aircraft and the history is mostly eventual failure. Or at least the odds are not long against failure (based on history)

2.)VW's are popular and reliable engines in direct drive, but they can't be used with "heavy" props, and when, as sometimes done, mated to PSRU's the potential for failure goes up mostly in relation to the PSRU as opposed to the increased HP.

3.)"Many" people have sunk gobs of money and effort into trying to make such systems work usually with add-on appliances

4.) "Many" piston aviation engine manufacturers have included very successful geared PSRU's in some of their models. (Most military prop planes) Usually the engine included dynamic counterweights

Which to some minds that should be thinking work thoughts but would rather be staring out the window at the sky :) begs the question: Has anyone made efforts at dynamic counter weight balancing of those 2 types of engines in terms of developing a "whole system" for aviation application?

smt
 

wsimpso1

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First off, you should read this:


It will give you the basics of how torsional vibration occurs and is transmitted, and how it is managed.

After that, it gets more complicated. The torsional pendulums attached to classic airplane engine crankshafts have their resonant orders selected to suit the engine, prop, etc. A lot of instrumentation and test time was spent to figure out what orders were troublesome, and then design pendulums to absorb those. Sometimes they make all kinds of sense, like absorbing at firing order or at twice firing order. Other times they were at some odd orders. Taylor wrote about this stuff in his books. Have fun with that.

The gadget from Rotec Munich that used to rest in my cabinet at Ford cost $70k in 2007. That stuff allows measurement of torsional vibration, and should be about minimum to actually instrument an engine and figure out what it is doing.

Classically, the pendulums were put on the crankshaft in places where there would otherwise be a counterweights, which are put on to tone down some of the vibration transmitted to the crankcases, so they do not add much weight. But they do tend to need more volume, and that volume inside the cases is not usually available. The place we put them on cars and trucks is on the flywheel (manual trans) and on the isolated side of the torque converter clutch (auto trans). Yeah, the research done indicates that a car or truck gets to acceptable vibe at lower weight when you put the pendulums on the tranny input shaft side. I suspect that this is also where we might prefer to put the pendulums on a modern automotive engine with a PSRU. Also, in cars and trucks, they only suck off firing order. The other orders are allowed to fend for themselves.

Something to take away from all of this is that a soft element is still in the system between engine inertia and the gearbox, and it is tuned to drive 1st resonant order at least 1-1/4 octaves above firing order at cranking speed and at least 1-1/4 octaves below firing order at idle. You still need to tune resonance well below lowest firing order... Then once you have that, your can start absorbing firing order with bifilar torsional pendulums.

Oh, and forged crankshafts are not just stronger than cast iron, steel is about 10% stiffer than cast iron, which helps to drive crankshaft resonant frequency up a small amount. Making the journals and cross section bigger everywhere also drives up crankshaft resonant frequency too.

Billski
 

rv7charlie

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It should also be said that some of the questions are based on erroneous assumptions. VW (or Corvair, or....) props running direct drive can and do fail the crank for reasons other than TV. If you've looked very long at those engines, you've seen '5th main bearing' (or Xth main bearing) conversions to the engines. The reason isn't TV; it's the fact that the stock engine has a *short* (not a diameter issue) main bearing supporting the prop. The spinning prop puts *bending* loads into the crank, and the vulnerable point is typically somewhere around the 1st crank throw. It's a fatigue failure, not a TV failure.

Also, one of the most impressive VW powered a/c I've ever seen was the Valley Engineering 2 place monoplane (later converted to a biplane), and it ran a reduction drive. The reduction drive is what allowed the engine to run a 'heavy' (~8' diameter) propeller successfully.

I wouldn't pretend to know the stats on what percentage of V8 (or any other) reductions are failing, but my opinion is that the failures have more to do with the designer's lack of engineering knowledge than any inherent shortcomings in the idea. We hear a lot about alt engine failures, but there are a lot of successful builders who don't bother with the interwebs. They just ignore all the naysayers, build, and fly.
 

Aviacs

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So other that (most likely?) Rotax, do people do any vibration analysis when they build a PSRU and mate it to an engine?
Or is it mostly cut, try, and see if it fails?

RV7 - I understand the issues you raise.
Part of my curiosity is that people don't seem to take a second thought about forging cranks, making them with larger journals, adding main bearings. In the case (no pun intended) of VW's some like my GP 2276 have had a lot of thought and work in integrating a much larger and longer bearing into the propeller end.

But vibration analysis & remediation seems to be haphazard.

I always appreciate reading Billski's responses.
I read the linked post from time to time. Things stick better with iteration.

As an aside, it would seem that regardless # cylinders, the 1st order impulses will span about 2.5 octaves for the rpms most consider running an engine with PSRU (800 idle to 4500 or so max TO power )? However, the frequency which 1.25 Octaves above/below represents is twice the HZ for an 8. Hmmmm. That assumes even fire, though. I don't know enough to understand if 5 octaves total separation is significant or not. I'll keep studying. :)

(Billski - the torsionally "soft" element in a GO300 is a quill shaft. Presumably in other geared piston engines and maybe turbines? as well?)

smt
 

wsimpso1

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In an 8 cylinder engine (as compared to a 4 banger) the whole spectrum is at higher frequencies while the magnitude of the torque ripple is smaller. The combination results in the 8's "soft" element needing to be stiffer than in a 4. It also results in all of those other modes needing to be higher too, which means higher ratios of stiffness to inertia. The beefiness of the reduction drives in the big radials and V's of WWII was there not just to stand the big torque, but also to put other modes out of range high. And yes, carefully designed and developed quill shafts were "soft" elements to isolate firing pulses from the gears and prop.

Oh, in turbomachinery they deliberately run blade counts on each wheel and stator as prime numbers, and different prime numbers on adjacent blade sets all to keep the lowest blade passing frequencies as high as possible, and different between each set. Combine that with the high rpm of turbine engines, and your lowest pulse frequencies are way up there. Torsional vibe input then becomes really small from the turbines themselves, and other issues rise to the top as vibe inputs. Other things happen periodically, like duct resonance, gear tooth passing and irregularities, bearing irregularities, and combustion stability. Turbines have plenty of inputs too. Firing frequency is not one of them though.
 
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Aviacs

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The turbine wheel blade counts is interesting, I had not thought about that.

Regarding gears, i remember being bothered (30/40 years ago) that reductions not just for AC, were often stated in integral ratios.
A very early lesson for gears (Sorry, not the type/sophistication of the units you dealt with) is to always use a "hunting" tooth count. Something for which the driver at least was a prime starting somewhere at or above 13. It reduced resonance, but also in non-generated gears cut with form cutters on a mill or shaper, the wear was less, and "evened out". Even with non-hardened, generated gears ordered from a power transmission supplier like Martin or Browning

smt
 

wsimpso1

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With gears, you have pitch line frequency and then one per rev on each tooth position error and/or shape error. The higher the quality of the tooth position and tooth shape, the better we do on all of these. Helical gear teeth and herringbone gears both reduce the effects of pitch line frequency.

While the "spreading the damage around" argument for using gear ratios of prime numbers of teeth is still used, it is of little value that way. In places where high efficiencies and low mass are high on the required's list, the gears are typically hard and gear errors do not readily "break in". You still pick two different prime numbers of teeth in each mesh to drive torsional inputs off range high and take out the regular one-per vibe input, changing it to more of a broader band noise.

Billski
 
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wsimpso1

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Then why are they on some Lyc 4cyl engines?

To pick off bothersome orders, just like anywhere else. We had a post a little while back where some unusual orders on Lycoming fours and sixes were listed. Have not been able to re-find that post yet.

Lycoming book (cited above) has a different story:
  • VO540-xxx3 has six 3rd order counterweights
  • O540- and IO540-xxx5 has a 5th order and a 6th order counterweight
  • Fours IOxxx-xxx6 has a 6th and a 8th order counterweight
  • Sixes IOxxx-xxx6 has a 6th order and five 3rd order counterweights.
On six cylinder engines, 3rd order is firing and 6th order is 2x firing, so those all make a ton of sense to me - those are the big orders in an even firing six. 5th order has me guessing - normally that in-between order is pretty quiet.

On four cylinder engines, firing is 2nd, 2x firing is 4th, and these normally active vibrations are not directly tuned. 6th and 8th makes sense if either of two things happen:
  • 6th and 8th order excite some higher frequency modes in stuff like props or accessories
  • 2nd and 4th are absorbed by 6th and 8th order pendulums - it will do this but make some of 2nd and 4th order energy show up as 6th and 8th order vibe.
To select these, Lycoming must have found that these tuned counterweights make a significant difference, or they would not put them on the engines.

While at OSH, I talked with Mark Kettering at AeroMomentum, and he has RAM pickup with a V-6 diesel engine. IIRC, that has a torsional pendulum on the turbine of the torque converter. He says that it is smoother feeling during accels than at steady speed on the highway. That is likely because both suppliers (LuK and ZF) believe in tuning their pendulums to a little higher than firing order for the purpose of making them work well during acceleration - where big torque and thus big firing accelerations are more often encountered. Mark opined that he would rather have a little more vibe when accelerating (when you expect guttiness and vibration as performance feel) in exchange for being smoother on the highway.

Billski
 
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