Debate about Mark Langford's 3rd crank failure

[ekimneirbo]

Apparently, it heightens your senses of humor :lick:

.

I always thought Colorado was high enough even before they decided
to make medicinal weed legal. Now in addition to thin air, the clouds you fly thru may prove hazardous too.
One has to wonder with all the dubies being lit up out there on a daily basis will the snow capped peaks be in
jeopardy.....will global warming rise even faster? No longer do you have to listen to the steady drone of an
engine in order to fly, just the steady buzz between the ears will take you on a trip.....but coming down won't
be as much fun and could be hazardous.......Now they have to deal with the FDA instead of the FAA. :roll:

 

[fredoyster]

I don't think there is really much debate about Mark Langford's crank failure here. There's a lot of debate about Corvairs in general, mostly driven by prejudice and factors not related to Mark's crank failures. The various accounts of the circumstances around these failures are fairly consistent, not much to debate there. I think we know that the pre-5th-bearing failures were mostly due to bending. The 3rd failure was due to something else. I don't think we know for sure what caused it.

There is some information from the Corvair factory engineering team on Dale Manufacturing's website indicating the maximum angular displacement they expect can be tolerated by the crank, and the reduction they found when a tuned damping mass was added to one end. I haven't seen an analysis of whether this is the best frequency for reducing torsional strain on a crank loaded by a propeller of a given inertia. WW has a hunch about this (in other words, that a metal propeller has too much inertia for a Corvair.)I think the "vibration damper" might not matter, but it could just move the point of maximum strain from one end of the crank to the other.

I think we all want to build reliable and safe engines for our aircraft, and we bring whatever tools we have to make that happen. WW brings his observation, skills and experience and that's the best we have so far. We know from his record-keeping what has worked, the rest is conjecture. I think as a community we can build on that and do better. I don't think there's much point in criticizing WW for not delivering the engineering analysis that we would expect from Pratt & Whitney.

Ross mentions a Wheatstone bridge; the sensing element for shaft twist is a strain gage which forms one arm of the bridge. They are cheap; the problem historically has been getting the signal from the rotating shaft to the instrumentation. That's not nearly as expensive as it was a few years ago, there are wireless strain gage systems from several vendors that cost much less than the quoted price of a torsional vibration analysis. You can also measure strain by looking at angular displacement between two points on the shaft using rotational encoders. But in each of these cases you're looking at the strain at one point, or the overall twist from front to back of the crank, a far more simplistic view than what's really going on with six pulse generators and different amounts of torsional compliance for each. I have a hunch that intelligent modeling of the strain throughout the crank using finite element analysis would be far more useful than whatever measurements are mechanically practical. Surely there are engine guys who have the chops to do this, and who could figure out whether there is reason to be concerned once the bending loads have been reduced by the use of a 5th bearing. It's my hunch that Mark's 3rd crank break was due to entirely different issues as WW suggests, but that's conjecture too.

I'm an EE, not an ME, so I can't help much. But if someone wants to tackle figuring out where to put strain gages or rotational sensors, I would be very interested in helping with the signal and telemetry part of the problem.

 

[Himat]

Just so that everyone reading this understand your specific experience with Corvair flight engines, please start your next response by taking 20 seconds to answer my 6 yes or no background questions below. I think in the absence of your real names, perhaps you would like to share your experience:

Have you ever:
(1) Seen a Corvair engine turn a propeller in person?

Not that I am aware of, but maybe on some air show.

(2) Seen a Corvair powered plane fly in person

As for question one.

(3) Flown in a Corvair powered aircraft yourself?

No.

(4) Fully rebuilt a Corvair flight engine yourself?

No.

(5) Completed and flown your own Corvair powered plane?

No.

(6) Attended a Corvair College and met me in person?

No.

And I must admit I would probably dismiss a Corvair engine as a candidate for an airplane for much of the same reasons I would dismiss a converted air cooled Tatra V8 car engine.

Here is the Reality Check: On any discussion of structural analysis, there will always be people like who want to show up and claim they can do a calculation, or someone could run a test to predict behavior. If you are new to engineering, understand this basic tenant: ALL calculations, mathematics, and evaluations have elements in them that make them a MODEL of what they are describing or measuring. In the case of aircraft crankshaft, all of these models, even with the best of equipment, are approximations. They can not model the system perfectly, and they are a simplification that can't begin to evaluate factors like variations in materials, surface finish, harshness of pilot input, or something really complex like an intermittent misfiring cylinder.
.
Here is the Good News: flying the plane is a 100% accurate model for flying the plane. No theory nor calculation is as reliable a predictor as several hundred hours of flight time. Thus anyone who is really interested in buiding a plane, with an engine, can have a good degree of confidence that it will work, if they build a clone of an engine prop combination that has worked over hundred of hours. This is the subject of this story: Understanding Flying Corvairs Pt. #6, 98% DNA not enough. | flycorvair
.

From experience in my field of engineering and professional work I have some comment on the statement that the plane is a 100% accurate model of the plane. That is true for one particular plane. For most practical purpose a clone will be similar, but not identical. The same goes for operation, there are no clone pilots. Then, without further work there is no knowledge on the margins before failure.

When modeling, calculating and measuring in product development one task is to research what influence tolerances in production, measurements, and approximations done when calculating has. This as a certain amount of margins are wanted so all produced items work and last.

Real life testing is good, but without a systematic approach it is easy to miss events that seldom happen. Another problem is that without a good model of the system, it can be rather hit or miss if critical tests are performed. Experience helps, but sometimes possible failure modes are best found by simulation.

 

[ekimneirbo]

Himat Quote : From experience in my field of engineering and professional work I have some comment on the statement that the plane is a 100% accurate model of the plane. That is true for one particular plane. For most practical purpose a clone will be similar, but not identical. The same goes for operation, there are no clone pilots. Then, without further work there is no knowledge on the margins before failure.

When modeling, calculating and measuring in product development one task is to research what influence tolerances in production, measurements, and approximations done when calculating has. This as a certain amount of margins are wanted so all produced items work and last.

Real life testing is good, but without a systematic approach it is easy to miss events that seldom happen. Another problem is that without a good model of the system, it can be rather hit or miss if critical tests are performed. Experience helps, but sometimes possible failure modes are best found by simulation.



Reply: I agree with what you said, but in all fairness.......whoever is doing the testing.....whether it be a big bucks well known company or a grass roots exercise......there will always
be some variations in the elements of successive tests and no one can ever test for every possible scenario. While a series of standard tests may be helpful, and may detect certain faults,
there really isn't any absolute guaranteed never to fail engine that can be made. Many people think thats the case when they buy certified engines and parts, but just this morning I read
about the FAA/NTSB issuing a recall on thousands of ECI cylinders at either 500 or 1,000 hours because the heads can separate from the cylinders. Most everyone is familiar with the Lycoming
crankshaft fiasco.The point to me is that no matter how much testing is done, there still will be a cloud over the Corvair conversion if even one more crankshaft fails. It does not matter that
each conversion ultimately is applied to an airplane/pilot/propellor/weather condition/maintaince/build quality/etc. combination that is different from all others.........no amount of testing will
prove that there isn't some boogyman in the cranshaft closet. Personally, from a non-engineer viewpoint its obvious that any steps taken to avoid resonance is a good thing, and within reason
basic vibration testing would be nice on each airplane. But no matter what is done, every airplane is a different combination and may yield a different result. WW is doing the same thing the
big time engine suppliers do...........waiting for results and then trying to discern which combinations yield the fewest problems and sticking with them.

 

[rv6ejguy]

That is why I have little issue with Ross saying " Before you were on the scene though, Mark was on the bleeding edge of flying Corvair engines and putting on a ton of flight hours quickly and documenting his trials and tribulations in a modest, informative and readable fashion." As another writer pointed out, It states on paragraph #1 of Mark's website that he was going to use a type 4, but switched to using a Corvair after attending a forum I gave at SnF 1999. Mark's site also shows that he came to Corvair College #1 as an observer, attended #2, and got his engine running at #3. The point of this is not to illustrate that I had been doing this for 10 years before Mark met me, the point is that the title here is about Mark's crank, and Ross states that he read Mark's site for information, but perhaps not as closely as he thought he did. And I think that is a valid point to make when we are speaking of looking at a mechanical event like a crank break.

-----------------------------------------------
There are no 2000 hour engines. Ross's own plane is 400 hours in 11 years. He knows that a tiny fraction of homebuilts that are not used in some type of commercial venture like gyros ever get to 2000 hrs. We overhauled the engine in Bernard Pietenpol's "Last original". It flew 880 hrs 1977-2011. It had no 5th bearing, no nitriding, no crank issues, it was never apart. My Wife has flown it, it cruises at 3,000 rpm and generates perhaps 85-90 hp


Ross obviously thinks that Subarus are great, and they meet his standards, and I am happy for him. I am sure there were issues they had also, and not everyone flew perfectly, and gave every user better experience than a factory new Lycoming, otherwise Jann Eggenfelner would still be building them. Likewise, I don't see the VW's he mentions as immune from issues either. There are no perfect engines nor perfect people, there are just people who will wait for the perfect engine, criticize what imperfect people work to build and fly in reality, and those who choose to do something real, recognizing the limitations of time money and lifespan.
.
The idea that I have been doing this for 25 years, worked very hard at teaching it to others, have worked to make a viable affordable option for builders, and I do this teaching in public at free colleges that anyone can attend means nothing to some people. Other people are thankful for it, not just because they didn't have the money for a Rotax or a Jab, but because I was the person willing to teach them things they wanted to learn, in an era where 'hombuilding' has largely become 'homebuying.' If all some guy wants out of aviation is to sit on the net and use a fake name to tell me my efforts are not valid, while the years go by and he has done nothing himself, never shared anything of his own efforts that when flying, never built his own alternative engine, great, I hope he is happy with his path. My journey has not been perfect, but it has been more rewarding to me than critiquing the work of others.-ww.

It seems there is some disagreement between your version and Mark's version about crank cracks and the need for 5th bearings years back. Most of the gently flown 70-80hp, near stock displacement, Corvairs got along fine simply because they were under low stress. Mark's rapid high flight time, hard use and later large displacement engines brought the potential problems to the forefront IMO, then new investigations and solutions resulted. I have great respect for Mark's work in the field and the way he candidly shared the information on his site. One of the true pioneers for flying Corvairs to be sure. I think we could safely say that Mark learned some things from you and you learned some from him too. I have other information that Mark was indeed "in" for a group financed TV study with a couple of others despite your opposition.

I think a 1000 hour TBO on any auto conversion is a nice goal to aim for since rebuilds are so cheap compared to certified engines. I know a lot of VWs guys happy with 500 and doing a valve grind at 250. That's lots of years of flying for most average experimental flyers and cheap on a per hour basis given the initial outlay.

Subarus do the job, they are light for the power, fit the general cowling shape, have a huge amount of flight time and have many PSRU choices available. They are a more modern engine with lots of parts available for them. With the right engine choice, they have virtually no problems which need modification to use in aircraft. They are not necessarily better than some other brand and I've worked on most engine brands. Back in 1980, I designed a lightweight, very clean, single seat, retractable to use a turbocharged Corvair engine. Later, road racing a turbo Corvair, showed that the heads did not have the thermal rejection rates needed for my hp goals. I looked towards aluminum Toyota engines and finally the Subaru in the late '80. Realizing I'd never have to time to build my own design, I bought and built a Van's RV6A kit. The rest is well documented on our website. This aircraft is primarily used for testing and product development. Available time and the Canadian winter climate does not allow me to fly as much as I'd like. Through several Subaru forums, sharing of information has allowed a rapid rise in overall package reliability. We were a supplier to Eggenfellner and I know the story well as I still support the customers he left hanging. The main problems there were never the core engine, rather lack of testing to mods he made, not listening to people with more experience in the field and poor customer service and support.

VWs today also have most of the bugs worked out through the efforts of GP and Aerovee. The packages are incredibly cheap for the hp and the quality of parts you get.

All auto engine conversions have been on some evolutionary path. Some long and tortured, some not so much. I just liked the way that Mark presented his information and experiences with no agenda and no attitude- warts and all, we got to see that evolution through the years in a very readable format.

As I said previously, whether someone likes Corvairs or not, nobody can discount the impact WW has had on the fly Corvair movement and the excellent way he educates people, gives them hands on experience and gets them into the air. The versions being built now I believe have brought understanding and reliability to high levels.

 

[rv6ejguy]

Ross, that is a significant statement about Corvair crankshafts. I would like to hear further information about the use of these crankshafts (racing,offroad,daily driving,etc) prior to magnaflux. Had the cranks
been previously ground, or did they come from specific year vehicles.....or specific horsepower vehicle? What I'm getting at (and WW is welcome to address this) is whether there may have been minor production or
engineering changes thru the production years? Were all crankshafts manufactured at the same vendor and were they all ground by the same vendor, or were there multiple sources for GM. I know that the LS Chevy
engines manufactured today often come from more than one vendor....same with some of the heads. What I'm getting at here is whether some vendor may have made a production run during one year and discovered
that the radii were being ground incorrectly and fixed the problem in subsequent years. Could the supplier of the crankshafts prior to machining been from more than one vendor? I realize that this info may not be
available, but who better to ask than you guys? If anyone else has personal knowledge of actual manufacturing of these engines, put it out there for the rest of us.

This is a real problem and counters any claims that somebody never saw any cracked Corvair cranks. This is why it's important to look a more than one source for your information, especially where the information is from vendors. Do your own searching, talk to others using a given product or services. This has always been my advice in any field.

This is a problem with using a very old engine design that has not been in production for over 45 years- lack of good cores and lack of new parts. Fortunately now, we have ready access to new cranks, rods, pistons etc. but are still left to find heads and cases in good shape.

Also with 45-50 year old engines, you cannot know the history, use or abuse through multiple owners. Lots of inspection, measurement and NDT are the order of the day.

 

[rv6ejguy]

Ross mentions a Wheatstone bridge; the sensing element for shaft twist is a strain gage which forms one arm of the bridge. They are cheap; the problem historically has been getting the signal from the rotating shaft to the instrumentation. That's not nearly as expensive as it was a few years ago, there are wireless strain gage systems from several vendors that cost much less than the quoted price of a torsional vibration analysis. You can also measure strain by looking at angular displacement between two points on the shaft using rotational encoders. But in each of these cases you're looking at the strain at one point, or the overall twist from front to back of the crank, a far more simplistic view than what's really going on with six pulse generators and different amounts of torsional compliance for each. I have a hunch that intelligent modeling of the strain throughout the crank using finite element analysis would be far more useful than whatever measurements are mechanically practical. Surely there are engine guys who have the chops to do this, and who could figure out whether there is reason to be concerned once the bending loads have been reduced by the use of a 5th bearing. It's my hunch that Mark's 3rd crank break was due to entirely different issues as WW suggests, but that's conjecture too.

I'm an EE, not an ME, so I can't help much. But if someone wants to tackle figuring out where to put strain gages or rotational sensors, I would be very interested in helping with the signal and telemetry part of the problem.

Dan Horton did his own TV measurement years ago with I think $1200 worth of equipment. This is well documented on VAF under various TV threads.

These days days wireless transmitter chips make it easy to get the data off the strain gauge into memory much easier and cheaper than using slip rings and other old methods. Another member on one of our Subaru forums also instrumented his setup for something under $2K I believe.

People can make the choice whether to do measurement or calculation but high price should not be the reason you don't do it. I can see many folks not doing TV analysis for their own one off creation but would see it as money well spent when we are talking about hundreds of engines with very similar configurations. Let's say $5K-$10K to bring someone in to do it spread out over 100 customers... pretty good bargain to have the knowledge IMO.

As far as what caused this 3rd crank breakage in this location, I posted the information from the Corvair engine builder friend about the large percentages of cracks he's finding these days when Magnafluxing cranks for a reason...

The second point here is why I brought up TV in my original post- a 3100cc configuration with a different prop and accessory drive layout could easily have a very different TV signature than another more proven layout and without analysis, you simply don't know if it could be a factor or not. Remember the flex plate that had been deflecting a huge amount? This was 100% caused by excitation/resonance and I've seen similar failures on a Honda Fit conversion. Anything different you bolt to a crank, changes resonant characteristics and damping.

 

[autoreply]

Our Moderator in the Netherlands pointed out that the price for a full TV analysis is 25K. These is nothing in the economics of teaching people how to build Corvairs that would remotely allow this for each Corvair combination. I have never stated nor implied that we have done this, and our builders know this. Mark Langford works to Teledyne Brown, a major aerospace/DOD contractor with 2000 engineers, and out of that group, he was selected as "Engineer of the year" a few years ago. No one pulled the wool over his eyes, he came, saw what we were doing, understood it wasn't certified nor TV tested, and chose to fly it anyway. It broke and he kept working with it, and today he is still a Corvair guy and he will eventually build another one with the Weseman crank that he bought, and I will assure you he will do this without spending 25K on a TV analysis before he flies it. Read his website, or better yet, meet him in person. He is nothing like some of the engineers on this group who probably think that TV is a requirement before they would fly a plane. He likes considering things, but while others waited and wrote thousands of posts, he went out and flew 1130 hours. Yes, he broke things, but it was his life and his choice.

Next time you would do well not to waste time adressing someone you obviously know zip about, but adressing the message.

That message simply contained some hard facts and pro/cons for a TV analysis, no advise one way or the other....

The real message however is that a competent engineer can do one himself as well. If you want to go all the way (including putting it on vibration bank), maybe count on 1-3KUS$ for that.

Then we're talking a few weeks of labor. Contrary to what you seem to think, that gives you a full range of "allowables" for various combinations of variables like prop inertia, displacement etc.

Fill in the new variables and know whether TV is an issue or not.

 

[fredoyster]

With all due respect, it's not quite as simple as that, and frankly I would hope for a less dismissive response from our moderator. If "a competent engineer can do one himself" were true, more of the competent engineers in the homebuilt community would have done it. If you are aware of anyone who has, for strains inside the engine that might be related to this thread, I would very much like to hear about it. Unless you are able to follow the automotive/motorcycle road and characterize the strain in each journal to web fillet, for instance, you won't know whether it's an issue or not. The engineering step before making that effort is to decide whether the answer available from the $Ks and weeks of time is worthwhile. A TV analysis external to the engine will at best tell you the frequencies at which there will be a resonance in the drive system between the excitation of the engine and the inertia of the prop, that's it. It's important for redrive and prop design, as Dan Horton documented very clearly, but doesn't tell us much about what's happening inside the engine.

A second important measure is the twist of the crank from end to end, which also does not tell the whole picture but gives other information. See gm H.B. lab test sheet where part of a GM lab report is shown looking at the maximum permissible twist, and where it is improved by adding a tuned mass and soft element coupling at the end of the engine opposite the load. This is not so difficult to measure, we can use an inductive pickup off the flywheel teeth at one end and the AC signal from a direct-coupled alternator, or optical pickup off the "harmonic balancer" at the other.

Propeller resonance is yet another issue that got dragged into this discussion. Prop makers measure vibration in the blades due to excitation pulses from the engine. WW hasn't done this either, but reminds us that it's far less of a problem when using prop materials with high internal damping, like wood or some composites. That's a broad brush approach to avoiding the problem, whether it exists or not, as a high Q resonance in the prop will reflect energy back into the crank as well as developing high internal stresses. I have heard that a certain new European engine effort is having similar problems these days. One approach is to instrument everything and try to understand what's going on, the other is to beat up a given engine/prop and see if it breaks. Both approaches have error and uncertainty.

People looking at Corvair crank failures have taken a different approach, since they have not been able to measure twist inside the engine. When you see a fracture due to bending rather than twist, you tend to not look further for sources of twist.

Finally I don't think it's fair to fault WW for not being able to provide all the information you want. He's selling the information he has, which is generally worth a lot more than you pay for it. Unlike some others, he doesn't pretend that the topics he doesn't cover aren't important or have already been covered in secret sauce.

 

[Himat]

... If you are aware of anyone who has, for strains inside the engine that might be related to this thread, I would very much like to hear about it. Unless you are able to follow the automotive/motorcycle road and characterize the strain in each journal to web fillet, for instance, you won't know whether it's an issue or not. The engineering step before making that effort is to decide whether the answer available from the $Ks and weeks of time is worthwhile. A TV analysis external to the engine will at best tell you the frequencies at which there will be a resonance in the drive system between the excitation of the engine and the inertia of the prop, that's it. It's important for redrive and prop design, as Dan Horton documented very clearly, but doesn't tell us much about what's happening inside the engine...

As I read this, you put forward good arguments for making either an analytic or finite element model of the system to better be able to do the right measurements and test. Even if the calculations are not that accurate, they might be what is needed to place strain gauges in the right positions or measure displacement between the right spots on the part.

 

[BBerson]

I never faulted WW for not providing Torsional Vibration analysis. I simply gave my opinion, my conclusion, that a GM absorber is not a likely solution. I didn't see discussion of TV in general and I didn't see any discussion at the website when I looked in 2013.

As an EAA Technical Counselor I advise on safety issues. I never claim that I know best. I just offer my opinion, my conclusions based on 41 years experience.
Take it or leave it, but I won't stop talking about potential safety issues when I see them.This thread is about a crank that failed at the back end and so discussion about TV is obviously needed, as I see it. I didn't say a TV analysis was required. We just need to acknowledge the risk.

As I said before, almost none of us have the skills to analize TV. I participate here as an experimenter that seeks ways to do this analysis, if possible. Thanks for all the discussion.

ps. The $70,000 number I mentioned was one machine cost (from Billski's link, I think) not cost to hire someone, I don't know what that costs. I would like to do it myself.
pps. I am not currently an EAA Technical Counselor, since the Port Townsend EAA disbanded.

 

[ekimneirbo]

Slight change in course with some questions for the engineers among us about using a harmonic balancer on Corvairs and some other engines. My understanding may be incorrect, so I'm
looking for both opinions and experience here.

1. The basic OEM harmonic balancer is tuned to a certain rpm range by the hardness/softness of the elastomer.

2. The HB on the Corvair is at the opposite end from the propellor so the crankshaft has masses at opposite ends. Do the masses create higher
bending moments within the crank than if the HB was on the same end as the prop (even if it is absorbing harmonics).

3. Does the HB need to be at the opposite end in order to absorb harmonics or would an HB absorb harmonics from both directions if it were
mounted on the flywheel between the prop and crank? (I realize this would take some adapting)

4. Would one of the aftermarket HBs like ATI Rattler or Fluidamper be a better choice since it reputedly can deal with varying RPM ranges, and
is possibly tunable? (There are also lightweight aluminum versions and some smaller than OEM sizes available.

5. Could two small HBs be mounted on opposite ends of the crank and work in harmony...or would they work against one another?

6. If only one dampener was used, where in the assembly do you feel is the most desirable place? (could thrust bearing location affect this decision)

7. Since rubber tends to harden and crack over time, wouldn't the range of vibration damping also change?

OK, thats all I can think of for now.............

 

[autoreply]

Slight change in course with some questions for the engineers among us about using a harmonic balancer on Corvairs and some other engines. My understanding may be incorrect, so I'm
looking for both opinions and experience here.

I can't answer most questions because I don't know enough about the Convair.

Check this one out for the basics though:
http://www.homebuiltairplanes.com/forums/general-auto-conversion-discussion/14215-torsional-vibration-resonance-basic-theory-issues.html

 

[fredoyster]

I never faulted WW for not providing Torsional Vibration analysis. I simply gave my opinion, my conclusion, that a GM absorber is not a likely solution. I didn't see discussion of TV in general and I didn't see any discussion at the website when I looked in 2013.

From my very incomplete analysis, guided by the Corvair engineering data on the Dale site, my hunch is that the GM "harmonic balancer" is indeed part of a solution. A conceptual, qualitative understanding is more important here than numerical analysis. I'll address some of that in replying to ekimneirbo later.

 

[fredoyster]

Slight change in course with some questions for the engineers among us about using a harmonic balancer on Corvairs and some other engines. My understanding may be incorrect, so I'm
looking for both opinions and experience here.

1. The basic OEM harmonic balancer is tuned to a certain rpm range by the hardness/softness of the elastomer.

2. The HB on the Corvair is at the opposite end from the propellor so the crankshaft has masses at opposite ends. Do the masses create higher
bending moments within the crank than if the HB was on the same end as the prop (even if it is absorbing harmonics).

First, there is very little "absorbing" going on anywhere. The power is huge, any "absorption" of power would be burning stuff up in an instant. The crank has masses at opposite ends, coupled through springs (formed by the compliance of the prop shaft.) There is time delay through the springs. You send a pulse to the prop, after a while that pulse reaches the inertia (mass) of the prop and part of it gets reflected back to the crank. If it arrives precisely out of phase with a new pulse coming from the crank, you have now doubled the torque twisting the shaft. The next pulse can double it again. Of course some of the power gets absorbed by the prop and converted to thrust, but anything that gets reflected contributes to this resonance. This is why you hear stories of gearboxes destroying themselves in seconds.

3. Does the HB need to be at the opposite end in order to absorb harmonics or would an HB absorb harmonics from both directions if it were
mounted on the flywheel between the prop and crank? (I realize this would take some adapting)

A "harmonic balancer" doesn't absorb anything. It stores some energy in the inertia of the outer ring and couples it back to the crank through the delay of the spring formed by the elastomer. You want to reduce the twist from one end of the crank to the other, so you want to time the returning pulses so they don't coincide with outgoing pulses.

4. Would one of the aftermarket HBs like ATI Rattler or Fluidamper be a better choice since it reputedly can deal with varying RPM ranges, and
is possibly tunable? (There are also lightweight aluminum versions and some smaller than OEM sizes available.

A tunable device might be preferable if the factory tuning of the GM unit isn't optimal for a prop load, and one that's larger in diameter might be lighter. I have a hunch that the factory tuning is probably optimal, because it's based on the spring constant of the crankshaft which isn't going to change much.

5. Could two small HBs be mounted on opposite ends of the crank and work in harmony...or would they work against one another?

You have one at one end already, it's the prop. If there were no springiness between the crank and the prop (or in the car drivetrain) you'd have no need of a balancer at the other end.

6. If only one dampener was used, where in the assembly do you feel is the most desirable place? (could thrust bearing location affect this decision)

Unless you can put it on the individual crank throws like in some engines, opposite to the prop end is the right place. It's not thrust but twist.

7. Since rubber tends to harden and crack over time, wouldn't the range of vibration damping also change?

Yes. That's why you send your "balancer" back to be rebuilt when it gets stiff.

 

[gschuld]

From my very incomplete analysis, guided by the Corvair engineering data on the Dale site, my hunch is that the GM "harmonic balancer" is indeed part of a solution. A conceptual, qualitative understanding is more important here than numerical analysis.

Fredoyser,


Just in case you were unaware:

From WWs site from 2005... "The harmonic balancer is a Dale Manufacturing rebuilt model we use on every engine we build." A handful of people have built and flown corvair engines without the balancer, but WW specifically recommends not doing away with it, even though it adds 5 lbs to the engine, not an insignificant amount.

George

 

[fredoyster]

I can't answer most questions because I don't know enough about the Convair.

Check this one out for the basics though:
http://www.homebuiltairplanes.com/forums/general-auto-conversion-discussion/14215-torsional-vibration-resonance-basic-theory-issues.html

There's nothing unique about the Corvair except that you're building it from pieces and get to do your own engineering rather than trusting someone at Lyc/Cont/Jab/Rot/UL or whoever to have done it for you. As it has evolved with Weseman cranks, new forged pistons, heavy wall cylinders, stainless valve seats, different cams, new intake manifold and exhaust, there's really very little left of the Corvair and it's more of a homebuilt engine, plans-built or kit. Bernie Pietenpol and some of the EAA guys figured out how to make a useful airplane engine out of Corvair parts in the 1960s, WW has taken it up 40-50% in reliability and power since then by paying attention to what works.

Billski's essay on theory is great. Add that to what you can read on Jack Kane's site, and Dan Horton's posts in VAF, and you'll be on your way to understanding the practical aspect of it. It's spring-mass-damper theory from first or second year physics but it gets messy when input pulses and loads change dynamically (as they do.)

 

[fredoyster]

Fredoyser,
From WWs site from 2005... "The harmonic balancer is a Dale Manufacturing rebuilt model we use on every engine we build." A handful of people have built and flown corvair engines without the balancer, but WW specifically recommends not doing away with it, even though it adds 5 lbs to the engine, not an insignificant amount.

I know. I have WW's books, many parts from his company and have talked with him at length on many occasions about these issues. I have a fresh balancer from Dale and intend to use it. I take WW's experience as a primary reference because it saves my making the same mistakes over again, but would like to understand why things are the way they seem to be.

 

[fredoyster]

As I read this, you put forward good arguments for making either an analytic or finite element model of the system to better be able to do the right measurements and test. Even if the calculations are not that accurate, they might be what is needed to place strain gauges in the right positions or measure displacement between the right spots on the part.

Agreed. About all that's practical is to measure displacement between the two ends of the crank, and I am planning to do that. I think it's beyond my skill level to try to get strain gages or rotational encoders on different parts of the crank inside the case. There is margin for error in GM's recommendation that the total twist not ever exceed half a degree peak-to-peak from one end of the crank to the other. I would be really happy to see someone do a nice FEA workup, it's probably not all that different from any other old technology 6-cylinder motor.

 

[William Wynne]

Mr, Peoppastie,

As Mr Autodidact pointed out, The photo is of a Dan Weseman 5th bearing. It is on an engine going together in my shop, but this is common, I use Weseman bearings on all our production engines. As you can see in the photo, it is a straight bearing, not a thrust bearing. Weseman and Roy's garage 5th bearings are both straight, and retain the stock corvair thrust bearing in the #1 position. As pointed out, My bearing has thrust on it, and the stock corvair thrust bearing is replaced by a regular bearing. When I designed it 10 years ago, there was still some question if thrust was a component of the crank issue, or was it purely bending? Time has long proven that it was just a bending issue, and having the thrust bearing up front is not required. My design proved to be an unrequired expensive design, The prototypes were about $3K each cost, a Weseman bearing retails for $1050.

I built a number of my bearings for test purposes, but we never sold one, We gladly provided the one to Mark in exchange for his flight experience and feedback. Mark Langfords plane at 450 hrs is the only significant flight testing on my design. The only operational plane/engine combination with my bearing is our own 3,000cc Wagabond. Again, the unique nature of the bearing, its thrust location, and the fact it is not a commercial product, an a number of other factors were part of my position that Mark Langfords engine as a subject was different enough that it was hard to argue that anyone could make a blanket statement about all other Corvairs based on looking at Mark's engine.

The 5th bearing system Mr. Autodidact is describing is a Weseman "Gen 1." It is the retrofitable model. As he said, it has a very clever, simple, fine tuneable, alignment system. It is pretty easy to get it within .0005" runout to the rest of the crank, lock it down and it will stay there. It is essentially a 3 jaw independent chuck, and it is akin to truing a bicycle wheel with 3 spokes, and checking it with the dial indicator in the installation kit.

The one pictured is a Weseman "Gen 2." It is for new engines, not retrofits. It does not have or need the 3 jaws, it is welded onto a new cam gear, (the weld is just an oil seal, it isn't subjected to a load) and this assembly is shrunk/pressed on the end of the Crank proper. It is then ground concentric with the other 4 bearings on the crank. It comes ready to put in the engine, no aligning required. We use Gen 2s in all the production engines we build. Gen 2 is the model that comes on all their new billet cranks.

The 5th bearing and the cam gear shrunk and pressed on the end of the crank. It is 1.60" in diameter. The 1" "safety shaft" is just a redundant prop hub mounting that is about 6" long, and it is threaded into the end of the crank. The castellated nut on the prop end can be seen in many photos on our site. In our numbering system it is part #2503. It is made from 7075 aluminum. The ones BH Pietenpol used were steel and left hand threaded. The whole in the hub clears the shaft by .060", so the shaft is purely in tension from 100 ft./lbs torque. The six "hybrid studs" #2502 are actually doing the main work of holding the gold prop hub onto the front of the 5th bearing/crank.

The red highlighted quote from Fly5th bearing.com (The Wesemans engine site) is about why they decided to go into production with their 5th bearings when the norm at the time was 4 bearing engines with nitrided cranks. For the record, I was there when Chris smith's fractured crank in his sonex was found. It was on the ground, chris just thought the engine ran a little rough. This was about 20 hours after Chris had a very hard landing that bent the titanium gear legs and shattered the prop on his 4 bearing 3100. The reference to Mark Langford is his previously discussed 2nd failure.

Mr Fredoyster: I am not any kind of expert on metals nor TV. But get a look at Mark Langford's site and read the part about how a crank expert, who saw it in person, did not think it was TV. Keep in mind Mark works for Teledyne Brown, the same Teledyne that owned Continental at the time. Mark spent a year traveling the globe for TB looking at UAV engines, and he had direct access to the TCM people in Mobile AL.

Ross: The current rejection rate for core cranks is about 1 in 14. Over the years we have used many different NDT tests on cranks, but we have a number of 10 year old photos on our site with the cranks going through the Embry-Riddle repair station. Today the Wesemans have a separate shop that gets paid to do NDT magnaflux on the cranks. They are paid either way and they don't do the crank work, so everything is incentivized correctly. We have had Corvair cranks X-rayed after they failed Magnaflux, and in many cases the crank was actually fine. The factory forged 8409 crank is a rough surface finish, and a good operator will err on the safe side with many false positives. I read the 30-50% fail rate in Southern California about 10 years ago. I am not there but I expect some exaggeration, because I have driven Corvairs on the street for 20 years, and you just don't see 30-50%, or even 5% with a broken crank. I think SC has a very high percentage of cores that were once someone's dune buggy motor, and a conservative guy with a magnaflux machine may get a lot of false positives. Many years ago, a guy from the LA area showed up on Mark Langfords Corvaircraft group and said he had a 33% failure rate at his car shop, and he claimed to be testing them at the rate of 10 a week or something. I offered to send a friend over with a stack of cash money, and I would buy his cracked cranks for $50 each, or 50X their scrap metal value. He never took me up on the offer nor posted about crank testing again. Today, I am very confident in the Crank process that the Weseman's use. I am sure that several of the people who fractured cranks, the ones that were on 4 bearing motors that got 40 hrs only, were pre-existing cranks that were missed in a low skill magna flux, done in the field, prior to us using the Weseman's and Moldex.

Perspective: It is easy to look at 25 years worth of work, and wonder "why didn't he see this as a potential issue? Why didn't they just build new parts? Why didn't...? " That is how it looks today, but hindsight is good, and it is easy to miss the factors around how we worked with the Corvair: For the longest time, all we sold were prop hubs and conversion manuals It is hard to imagine how much resistance very basic ideas like using ARP rod bolts and TRW forged pistons generated. (and still do, read this: “Local Expert” convinces builder to use cast pistons | flycorvair) Much of the resistance to ideas like forged pistons came from cheap people pointing out that Pietenpol flew cast and also from car parts suppliers like 'the Corvair Underground", who's owner was on the web every day for years telling people to buy his cast pistons not forged. It took a long time to get people to buy $39 timing lights; There were people who were vocally against nitriding; you name it, if it cost $25, there was backlash against it, all saying I was "ruining" Corvairs by making them too expensive.

It was very hard to control where cranks were done; If we told people Moldex was good, someone would be on the net saying they found a place that was cheaper. I campaigned to standardized motors, but you can go back and see in the Corvaircraft archives, that this made me someone to hate for a dozen vocal people who time showed that they expressed their individuality by building nothing. We had people with 4 bearing engine and 3500rpm KR engines with 7" prop extensions and 14 pound props get the "OK" from internet 'engineers.' When people heard that a Corvair can run and fly with a broken crank, the most common response to nitriding was "To save the $150, I will wait until my crank breaks and fly back to the airport and buy another for $50 and have it nitrided," When Mark Langford was flying 4 bearings and it worked, 5th bearings were in slow development, but no one was interested, because they all said "Mark doesn't use one, so neither will I."

When examining the whole effort in the rear view mirror, it is very hard to understand all the factors against it eventually being a respected engine. It wasn't made to happen because I am smart, quite the contrary is true. It got to where we are today because I am just dumb enough to not know when to quit. So if you read the story and think "I am smarter than that guy, I would not have made those mistakes" I probably agree, but the corollary is a smart guy would not started this or would have certainly quit when he learned about some of the fools, critics, 'experts' and special people. I'm not that smart, and that is the #1 reason why we have Corvairs as an option. I am not trying to be funny, just trying to point out why there are not more inexpensive engine options. -ww.