# Debate about Mark Langford's 3rd crank failure

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#### Ramrod25

##### Member
One of the issues that MAY have some relevance here is the type of flying that Mark is apparently skilled enough to do.

By that I mean aerobatic maneuvers that impose significant G loads on the internal components of the engine.

Both Dan Weseman and Mark are apparently skilled pilots and have airplanes capable of performing high

G maneuvers. Just a thought.

After reading both WW's comments and Mr. Langford's, it occurred to me that there probably haven't been many
crankshaft failures on slow flying aircraft with nitrided cranks and 5th bearings installed.

Bringing any engine designed for automobiles along the path of increased reliability for use in aircraft is necessarily

a long, involved learning experience, punctuated by engine and engine component failures. These failures point

us toward engineering solutions which necessarily put us back at the beginning point on some learning curve.

Someday, perhaps with a new crankshaft composed of modern steel and machining methods, 5th bearings and
the other refinements all ready proven, we will get past crank problems and quite possibly move on until we
identify the next weakest component in the engine to fail. Guess what - that puts us right back at the beginning again;
engineering solutions, testing and improving.

To me, the sad part of this is when conflicts arise, as I guess they inevitably must, among the people who have given
so much of their time, energy, effort and $$to bring us an engine worthy of our trust and$$ to hang on our own airplanes.

May I suggest that who said what, when or where is not all that important. Those of us looking to build, or who are
building Corvair engines for their aircraft are quite willing to acknowledge that the combined expertise of WW, ML and DW
is the main reason that there even exists a Corvair movement today.

What we need to acknowledge is that there appears to be a continuing issue that needs to be solved. Perhaps there
will always be continuing issues to solve which is why we AD's and SB's on certified engines. That said, don't we need
to push forward and simply solve the problems? We all want the safest engine we can build to put in our airplanes.

Regards
Rodney

#### BBerson

##### Well-Known Member
HBA Supporter
Torsional resistance is mostly provided by crankshaft stiffness. Nitriding improves the surface hardness but does not change the deflection stiffness and therefor does not reduce torsional vibration. Aircraft engine crankshafts are almost always larger for stiffness (and hollow for lightness) compared with solid automotive cranks.

Again, from the book: THE AIRPLANE ENGINE 1922

"Crankshafts are dimensioned for stiffness as well as for strength. Stiffness for given dimensions depends only on the modulus of elasticity and not at all upon tensile strength. As the modulus of elasticity is practically constant for all steels, there is no advantage, so far as stiffness is concerned, in using a steel of very high tensile strength for shafts."

#### orion

##### Well-Known Member
About four months ago I started looking into alternate engines for one of our customers. The Corvair seemed to be a pretty good candidate given the simplicity, power and weight. Since I've owned two Corvairs (many years ago) I was familiar with the engines and felt reasonably assured that this is a potential candidate. But looking closer I found that I did not fully agree with some of the assessments regarding the design and especially the reasoning behind the fifth bearing that is being installed in these conversions. The theory is that this fifth bearing will provide the prop end of the crankshaft with sufficient support to handle all flight loads, thus protecting the crank from what might be damaging loads and deflections. But in practice this is only partially accurate.

The fifth bearing does provide end support but it primarily addresses thrust loading and loads perpendicular to the crankshaft axis - it however is incapable of handling any other imposed load. When installed in a car, whether clutched or with an automatic, the crankshaft sees only pure torque. All components bolted to, or taking power from the crankshaft are rigid and piloted so the crankshaft end sees only axial moments. Yes, there is a small side load encountered during starting (due to gear mesh of starter with ring gear) but this is short period and pretty minimal and thus not significant.

In aircraft application the transmission is of course removed and a prop is generally bolted on the end of the crankshaft. The main load of the prop is, as with the car application, torsional. However we have two other loads to deal with now. The first is a force vector perpendicular to the axis of rotation and it represents the weight of the prop. The prop is displaced from the end bearing so its weight times that distance is a bending moment imposed on the crank's end. The prop's weight is then multiplied by a combination of turning Gs and pitch (or yaw) accelerations. These multiply the prop load, significantly increasing the overhang moment.

But the main and very significant moment that's created here is the reaction caused by the rotation of the prop and the pitch or yaw of the plane. Even relatively mild turbulence for instance can generate some pretty impressive rotation rates, resulting in sizable "gyroscopic" loads. So while the bigger end bearing can handle the axial vector forces, it is generally incapable of doing anything with the moments. Even a double row bearing has insufficient rigidity and distance between the contact points to counter what can be a sizable bending load imposed on the end of the crank, thus allowing measurable crank deflections. Given the typical geometry, it is no surprise that these conversions are seeing failures.

The fix for this would be to isolate the end of the crank completely from the loads of the prop. This would take a somewhat elongated housing with two bearings, the pair separated by some distance. How much? Hard to tell without any analysis but as a first cut rough guess, I'd suggest at least four inches. This would then create a reaction couple, which would get transmitted by the bearings to their housing and then in turn into the block, and then the engine mount. This way all vector and moment loads would be handled by the bearings, thus isolating the crank. This type of mounting would also provide the opportunity to create a flexible coupling between the crank and the output prop shaft, thus also addressing the torsional feedback issues, if any.

But as things sit right now, for our project I decided to pass on the Corvairs.

#### Autodidact

##### Well-Known Member
Orion, would the crankshaft, when coupled to the prop drive by a flexible coupling, also need a flywheel?

#### orion

##### Well-Known Member
The flexible coupling is flexible only from a torsional standpoint and only as compared to a rigid steel on steel connection. As such, probably not but don't quote me on that - I haven't gone that far in my research on this. I know other engines that use any type of dampened coupling don't use a flywheel or if they do, it is substantially machined down, so the Corvair application might need a bit more investigation.

#### Max Torque

##### Well-Known Member
I think I'll go with a kevlar belt re-drive to turn the prop on my Corvair. For my application (tandem STOL bush plane), I need to turn a large diameter prop at a lower rpm anyway. (Using Jan's prop program www.jcpropellers.com for the prop.) That should eliminate/reduce chances of a crank problem, yes?

#### Jan Carlsson

##### Well-Known Member
M.T.

The small spocket must be unloaded from the belt tension and drive force, so it don't put a side/radial load on the crank, and a flex coupling, like centaflex, to take the T.V. the belt will not flex much if any. consider it can be a need for 10-30 degree of tortional movment at some rpm. Flywheel - you need a starter ring anyway, but designing a PSRU isn't for the experimenter, it need the knowlege and experiance of a pro, or, a lotto miljonär's amount of pure luck.
there are people that know this on there five fingers.

#### Dan Thomas

##### Well-Known Member
Someday, perhaps with a new crankshaft composed of modern steel and machining methods, 5th bearings and
the other refinements all ready proven, we will get past crank problems and quite possibly move on until we
identify the next weakest component in the engine to fail. Guess what - that puts us right back at the beginning again;
engineering solutions, testing and improving.
Developing a new, strong crank might cost so much (especially for so few crankshafts) that it would probably cost the end user as much as buying a nice new Lycoming.

I would wonder, after the new crank is in, how long the bearings and case will last. Those parts were designed for handling only torque, not dealing with thrust and precession.

As far, again, as broken Lyc or Continental cranks go, an insurance company will now usually demand a complete engine teardown and NDI of all components if an accident involving a propstrike occurs. They'll often pay for it. They have found that it's cheaper to do that than to continue to insure an airplane that has had a propstrike without any inspection. Even if the prop flange dials out OK, cracks have often started far back along the crank and it eventually fails and drops the airplane and occupants into a forced landing that sometimes gets really expensive. Small Continentals are known to crack between the #1 and #2 journals, which in that engine are at the back, far from the prop. My inflight break was right there, too, and of the O-200s we had propstrikes with (flight training accidents) we found cracks starting in that same spot. No amount of checking the prop flange runout will find that.

To make a crackproof crank might be possible but it would be massively heavy. Using a wooden propeller is a better way to avoid it. It absorbs the shock and splinters when it hit something and saves the crank, and imparts much less precession loading.

The FAA has an AD against Lycomings that have suffered any propsrike, including encounters with water or tall grass. Such decelerations cause the bolt that retains the accessory drive gear on the crank to come loose, and can partially shear the drive dowel. That's because of the inertia of the mags and other stuff back there, but the same AD also warns of crank cracking and the AD applies even if the prop wasn't turning and ran into a hangar door, for instance. Things bend a bit and start cracks. And that's on a really strong aircraft crank, not a much weaker auto crank. A well-designed PSRU sounds like a much better bet to me.

Dan

#### Doug2233

##### Well-Known Member
There are a few reasons cranks can break (at least so far as I am aware) beyond just a weak or damaged crank itself. Cases can become warped, can flex, can suffer non linear distortion as they heat. Increasing the cylinder forces with increased power will also result in case flexing.

I suffered a crank failure in a vw engine, thankfully in a car not an aeroplane. This was just a 40 hp engine too! I put that down to not having the case main bearings line bored prior to reassembly.

Lycoming and continental use enormous steel through studs to stabalise the crankcase halves, even then fretting is sometimes discovered on the mating faces due to movement. Does the Corvair have through studs?

Personnally I doubt just a forged crank in a more appropriate steel will do the job. The case needs to be reengineered as well. Nothing less than a full FEA model and analysis of the bottom end of the engine.

I believe the Corvair is the genesis of a great aero engine an fits a niche, but it is just that.

##### Moderator
Small Continentals are known to crack between the #1 and #2 journals, which in that engine are at the back, far from the prop.
Any idea why they would crack there first? My "seat of the pants" feeling says they would be loaded most of the forward journals?
A well-designed PSRU sounds like a much better bet to me.
The clutch (as used on many of the Rotax engines) apparently works great too.

#### psween

##### Well-Known Member
Might be an interesting follow-on post from Mark Langford Crank Break #3
Way down at the bottom is a follow up detailing inspection of the crank radii on his most recent break. It shows that none of the radii were up to the original spec, and showed regrind errors and grooves that would seem to create multiple stress risers. His recommendation (which I agree with) is to do a detailed inspection of any crank prior to assembly, especially those reground, and consider waiting for an aftermarket crank. Apparently the first batch of 6 aftermarket cranks are in works now, from Dan Weseman, so they might become reality. I don't have a dog in this show, but am leaning strongly toward a Corvair to power my KR derivative. Considering that the alternatives for me are VW (not quite the power), Rotax 912 ($$), or o-200/o-235 (weight and$$$), I'm very interested to see what comes of these new cranks. Even if they add$3000 to a build, they would still result in a cheaper package than all but the VW, and might rival the reliability of a certified motor. Only getting them into airplanes in significant numbers and flying a lot of hours will prove them out, but I've got time.

Patrick

#### Marc Bourget

##### Well-Known Member
I have an engineer friend who was one of the memebers of the core design group for an engine. Another friend who had switched to a larger engine used the first engine's Harmonic Balancer on the second - and the Balancer "grenaded" in Rush Hour Traffic, making the evening news as he was stalled in the middle lane of a 6 lane freeway in LA on Friday. Well, when we started discussing the situation I now discovered from the first friend that there were things like "Sixth Order Vibrations" and the like. He made my head hurt.

But other discussions focused on the Journal Radius. If you're serious about longevity, a diligent Crankshaft manufacturer will "roll" the radii with Hardened Rollers, under great force, similar to shot peening, to eliminate the stress risers. I think this will be hard to find from suppliers of after-market and nitrided crankshafts, but, having suffered a catastrophic driveline failure (transaxle) for this "inside radius" problem, I wouldn't run a Corvair crankshaft without the technique. Take away the "opportunity" (and maybe run a wooden propeller) and the stock crankshaft may be just fine.

Onward and upward

Marc Bourget

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#### john affleck

##### Member
The people getting interested in the new aftermarket crankshaft for the Corvair need to read this; as it correctly states the facts. Mark Langford stated that 4340 would be twice as strong as 5340 in a crankshaft; this is ridiculous. He has a talent for getting the wrong answer; I've communicated extensively with him by e-mail; I'm the only person qualified to speak on this subject who has communicated with him; but he decided not to listen. at that time he was in love with his guru WW; who is a MECHANIC; and not an engineer. He's a good mechanic and a disaster as a engineer; but that's what you would expect. I'll explain exactly why his crankshafts broke on another post here which you can look up; I'm not going to write the whole thing twice. DO NOT BUY THE AFTERMARKET CRANKSHAFT; this is just more amateur screwing around. Okay; this was intended to follow on the man's post to the effect that only the modulus of elasticity is relevant; but I don't understand how forums work; so this is in the wrong place.

#### Vigilant1

##### Well-Known Member
I'm the only person qualified to speak on this subject who has communicated with him
WW; who is a MECHANIC; and not an engineer. He's a good mechanic and a disaster as a engineer; but that's what you would expect.
Umm, well where to begin? Welcome to the forum. Please don't be a shrinking violet, let us know your opinion on things. Mark Langford and William Wynne are names we know. They've produced much, written much, tried many things, and have been very clear about their qualifications. That helps everyone know how much weight to put on the things they say.

#### stol

##### Well-Known Member
The people getting interested in the new aftermarket crankshaft for the Corvair need to read this; as it correctly states the facts. Mark Langford stated that 4340 would be twice as strong as 5340 in a crankshaft; this is ridiculous. He has a talent for getting the wrong answer; I've communicated extensively with him by e-mail; I'm the only person qualified to speak on this subject who has communicated with him; but he decided not to listen. at that time he was in love with his guru WW; who is a MECHANIC; and not an engineer. He's a good mechanic and a disaster as a engineer; but that's what you would expect. I'll explain exactly why his crankshafts broke on another post here which you can look up; I'm not going to write the whole thing twice. DO NOT BUY THE AFTERMARKET CRANKSHAFT; this is just more amateur screwing around. Okay; this was intended to follow on the man's post to the effect that only the modulus of elasticity is relevant; but I don't understand how forums work; so this is in the wrong place.
Let me get this straight..................

according to the sig line this is your first post on HBA and yet you say

"disaster as a engineer; but that's what you would expect. I'll explain exactly why his crankshafts broke on another post here which you can look up; I'm not going to write the whole thing twice"..

care to share the link to that post ?

#### Pops

##### Well-Known Member
Log Member
Add me up on a Lyc crank break. Lyc-290 with about 350 hrs TT. Found out latter that it was in a batch of bad cranks. Pops

Broken Lyc or Continental cranks are most often attributable to propstrikes in the engine's history. When that prop strikes anything it sends a shock down the crank and twists it enough to start a crack. I had an engine fail this way, and have seen cracked cracks come out of engines torn down for propstrike inspections. Other than that, broken cranks are extremely rare, even when Lycoming had some problems with cranks they contracted a foundry to build for them. Those were all recalled even though only one actually failed.

Dan

#### Autodidact

##### Well-Known Member
john, could you at least clarify something? Do you - by "the second U shaped structure" - mean the second one forward from the oil pump drive end of the crankshaft or from the propeller end of the crankshaft?
The provision of the front bearing moved the critical bending, (NOT TORSIONAL BENDING), back to the second U shaped structure; not too surprising.

#### TinBender

##### Well-Known Member
if the front cylinder is at top dead center, or bottom dead center, doesn't matter, then the prop. MUST BE STRAIGHT UP AND DOWN. And that's it. that's the cure. Now there are no more broken crankshafts.
From what I read here, Corvair crank failure (5th paragraph), ML's been indexing his prop 90o out since 2005, perhaps at John's suggestion. Ahh, nothing in life can be simple, can it? I agree with John's general arguments, but find his tone disrespectful.

As a student of mechanical engineering I now understand why the university is making the current crop of engineers write so many formal technical reports. It is just as important to be able to share your ideas clearly as it is to have them. A few carriage returns would do wonders to the readability of the above. "Clear and Concise," is my mantra.

Regards,
Jamie

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