Debate about Mark Langford's 3rd crank failure

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Will Aldridge

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Those who are reasonably familiar with the Corvair movement will recognize Mark Langford's name. He is/was the posterchild of the Corvair movement. Near the end of Nov 2011 he suffered his 3rd crank failure and damaged his plane in a forced landing(he walked away unharmed).

This incident brought up a serious question about the reliability of OEM Corvair crankshafts. I will not take any side in this debate but thought I would start this thread with links to both Mark Langford's and William Wynns comments on the subject for the convenience of those who are interested.

1. Marks initial report on the crank failure. Crank Break #3

2. William Wynns Rebuttal: Mark Langford’s Crank Issue « flycorvair

3. Marks Response: WW
 
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autoreply

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Thanks for sharing Will, interesting read and a good reminder to myself why I'm using a proven engine.

I can especially encourage the initial report:
It's a good text on decision-making with a low engine breakdown and it shows that even the worst-case scenario is almost always survivable if you do it right. The only thing he didn't do was to have an engine failure plan before he turned the ignition key. That saves you a few precious seconds and would have probably left his plane undamaged in this occasion..
 

Vector

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Yeah, internet feuds is the norm today. I have communicated with WW on a few occasions and I did not get a good vibe from the man. That is not why I moved to a VW engine; I will still fly a Covair.
 

Autodidact

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Aside from any personal charactarizations the two parties involved may or may not have made about one another, I thought they both made good points on strictly technical grounds: better steel and larger fillets to reduce stress risers is a good idea; as well, not adding more inertial mass to the back end of the crank while at the same time reducing the bearing area that is normal to the crank axis also sounds like a good idea.

The data for this is always going to be empirical, each person has to use their noggin and make the best decision they can. It does look like this has nothing to do with the WW fifth bearing.
 

Jan Carlsson

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Lots of questions we don't know the answer to.
With WW's 5:th bearing, it replace the thrust bearing, at least on Mark's engine, what if there is 2 thrust bearings, one at each end of the crank, will that not be bad when the crank and block is up to running temp and have different heat gradient. looks like that setup Mark had was only on his engine, what about the rest with WW's 5:th bearing? do they have the original thrust bearing?
Did Mark grind all his cranks at same company? In the picture of his boken crank it looks like there is a crack going between the balance drilled hole, and where the break origin at the fillet. was this before the brake or as a result of the break?

Mark did not use the original damper. was the replacement made for his setup?

How many other crank failure have there been, is it just the big engine or also on 2,7 liter version?
 
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Autodidact

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what if there is 2, one at each end of the crank, will that not be bad when the crank and block is up to running temp and have different heat gradient.
From what I understand, Mark's was the only flying WW fifth bearing and that there was no longer any thrust type bearing on the rear as in a stock Corvair.
 

rheuschele

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Has anyone done a percentage calculation of the difference between crank failures of Corvairs to Lycomings?
Just curious.
Ron
 

Vigilant1

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Has anyone done a percentage calculation of the difference between crank failures of Corvairs to Lycomings?
Just curious.
Ron
I'm sure there's not enough data on the Corvairs to yield any significant info. There's no certification/paper trail for how the Experimental aircraft engines were prepared and the parts used, there's no database for the hours flown with various engines, etc. That's why the experiences of a few guys who are blazing the trail with equipment and flight patterns similar to ones we plan to have are watched so closely by the rest of us. No one would give up on Lycomings because one guy broke a crank--we know that's a rarity.
 

RJW

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I agree that both WW and ML make good points, though there are errors in reasoning and judgment here and there (one example is ML’s claim that grinding a crank decreases the fillet radius when actually the opposite can be true).

I doubt that adding a fifth bearing eliminates bending—it just reduces it.

Misalignment of a damper or starter ring could contribute to bending fatigue.

Bending is almost certainly increased by using larger pistons due to increased mass and/or increased combustion pressure.

This kind of break is interesting because it might have been caused by something I have been wondering about for years. The cranks in car motors are machined in a very course manner (older cars anyway). The machining leaves huge gouges across the cranks all the way to the rod journals. You can see these gouges pretty clearly in the pictures. It appears that these gouges in this particular case extend just to where the break on this crank originated, just at the top of the fillet. My guess is that unless these gouges are polished out, even the use of large radii will not eliminate stress risers. It is also interesting that aircraft cranks are machined with much more care in this regard.

Anyway, my guess as to the actual cause of this break (and the other 2 that ML experienced) is simply the desire to run the motor beyond its design limits. As is usual it is not the fault of the machine. The failure was caused by a human decision—the seemingly universal (and understandable) desire to increase the power per dollar ratio.

Rob
 

BBerson

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not adding more inertial mass to the back end of the crank while at the same time reducing the bearing area that is normal to the crank axis also sounds like a good idea.

The data for this is always going to be empirical, each person has to use their noggin and make the best decision they can. It does look like this has nothing to do with the WW fifth bearing.
Quote from the book The Airplane Engine (1922):
"The free end of the crankshaft is subjected to much more severe conditions than the propeller end. The free end is, as it were, wound up when the maximum torque is applied to it and released when the torque diminishes. At certain speeds this alternate winding up and release may coincide with a natural period of vibration of the crankshaft, and in that case the crankshaft will vibrate excessively and the reciprocating masses attached to it will also vibrate and impart their vibration to the whole structure. Such TORSIONAL VIBRATION could be reduced by the use of a flywheel on the free end."

Without a complete vibration analysis and test program, your second comment that luck plays a factor, good or bad, is probably valid.
 

autoreply

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I wonder if the analysis part, done digitally, is beyond the realm of possibility?
No, it's not. Billski has made some very insightful comments on HBA about this very subject. The big problem is getting your data, I don't think there's such thing as a complete set of data about the Corvair available on the internet or for a fee. You could scan a crank in a 3D scanner, determine the material and go from there though, but that's not exactly a small job.
 

Dan Thomas

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Quote from the book The Airplane Engine (1922):
"The free end of the crankshaft is subjected to much more severe conditions than the propeller end. The free end is, as it were, wound up when the maximum torque is applied to it and released when the torque diminishes. At certain speeds this alternate winding up and release may coincide with a natural period of vibration of the crankshaft, and in that case the crankshaft will vibrate excessively and the reciprocating masses attached to it will also vibrate and impart their vibration to the whole structure. Such TORSIONAL VIBRATION could be reduced by the use of a flywheel on the free end."
And that there vibration is the reason for the torsional damper on the front end of most auto engines. It stops the "ringing" of the crank. It has a steel hub, a heavy steel or cast outer ring (usually with pulley grooves in it) and a rubber ring between the two.

Dan
 

Vigilant1

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Anyway, my guess as to the actual cause of this break (and the other 2 that ML experienced) is simply the desire to run the motor beyond its design limits.
I suppose it depends on what we mean by "design limits." In these direct-drive aircraft applications the engines aren't being asked to produce more torque or HP than they were designed to produce in automobiles. So, we're staying within design limits in that sense. But it's other stuff--bending due to prop loads, torsional vibration that is different from car use, maybe even the prolonged operation at a steady RPM that may be outside the design limits and causing trouble.

The VW-based aircraft engines are now reliable, but it took many decades of trial and error by lots of competing experts to get things ironed out. Crank issues figured prominently in those dramas.

Speaking of that: Revmaster had an engine, the R3000, that I think was well along in development but was put on the shelf due to a drying up of developmental funds. 100 HP and about 200 lbs. I wonder if the current questions about the Corvair will be cause for Joe Horvath to get that project back on track.

Mark W.
 

Max Torque

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Thanks for sharing Will, interesting read and a good reminder to myself why I'm using a proven engine.
Curious. Which proven engine are you using?

Continentals & Lycomings have been breaking cranks for decades. Haven't heard anything about Rotax.

I've helped recover more than one certified aircraft with a broken crank. A most memorable one back in the 80s up in Alaska had a recent factory overhaul with new crank. Pilot managed to land it on the muskeg without damaging anything. I slung the replacement engine out to the plane, but they had to wait until freeze up to fly it out.

Anyone remember Interstate's successful lawsuit against Lycoming? Apparently, one of their claims was that Lycoming's crankshafts were poorly designed and without safety margins...

Mark's breaking three cranks is definitely alarming. The big question, for me anyway, is: is this problem peculiar to this particular engine/set up or is it systemic to all flying Corvairs? If it's systemic, there should probably be a lot more Corvairs with broken cranks out there than has been reported.
 

autoreply

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Curious. Which proven engine are you using?
Rotax. But then it went quickly downhill with my own principle to use proven components by whacking in a long drive shaft. You can't win them all...
Mark's breaking three cranks is definitely alarming. The big question, for me anyway, is: is this problem peculiar to this particular engine/set up or is it systemic to all flying Corvairs? If it's systemic, there should probably be a lot more Corvairs with broken cranks out there than has been reported.
The only approach to an answer I've seen were the hotly debated statistics on Van's Airforce that compared many of the auto-engines with failure rates of dedicated aircraft engines.
 

Dan Thomas

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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
 

Max Torque

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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

Dan,

I agree prop strikes to be one of the main culprits for crankshaft failures. Corrosion another.

According to the FAA, there has actually been more than one Lycoming crankshaft that actually failed. Corrosion pitting cause at least one failure.

They list twelve failures as the basis for proposal and the resulting AD2005-19-11. Think they were all Interstate produced crankshafts. Faulty heat treating or the improper addition of vanadium or improper hammer forging or whatever. Engines will fail for whatever reasons, so be prepared.

The good news is the FAA concluded in one of its reports that Lycoming's crankshafts have sufficient design margin between operating stresses and material endurance limit to result in an effectively infinite service life for defect-free material.

Getting back to the Corvair - at this point in time, I'm leaning towards Mark's crankshaft failures as being unfortunate coincidence for whatever combination of factors/reasons.

I'm building up another Corvair for a new project. It will have a magnafluxed, polished, nitrated crank with a fifth bearing. In addition, it'll also retain the original thrust bearing...
 
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