Crank Break #3

Crank Break #3. Third time's a charm!

created December, 2011, updated March 2016

First off, let me just say that I've always expected that I'd roll my plane up in a ball some day, perhaps reducing it to a pile of sticks, but that I'd walk away from the mess with a few bruises or maybe a broken bone or two. Given that scenario, I don't kid myself about flying, or that it'll never happen to me. When I go flying, I leave my car in the hangar with the key in the ignition, just in case somebody wants to drive it when I'm gone. I try to keep my affairs in order, and have made an effort the last few years to teach the family where things are and how they work, just in case I don't come back. Flying is that important to me.

I recently had a pretty hard landing, and managed to collapse my landing gear. If I stopped right there, I'd be way ahead...y'all would just think I finally met a gusty condition that I couldn't handle, and that would be it.

But I've done 2824 landings in my plane so far, and I'm no novice at landing a KR, if that means anything. I'm bad about having to tell the whole story though, especially if I think there may be a lesson in it for others. Unfortunately, there are ramifications that may cause doubt among Corvair builders and flyers, and for that, I offer my apology in advance. If you're the kind that doesn't want to hear something that may force you to make hard decisions, then go check your Facebook page instead of reading further.

The whole story is that I was departing from my father's farm strip the day after Thanksgiving, which is a generously proportioned grass strip...about 3600' long and 80' wide, with the departure end a gradual slope downhill, and no obstacles on the far end other than two large lakes on either side (not bad, as far as obstacles go), and then woods out past that. My brother had driven me to the far end of the strip, and when I took off he raced behind me in the car, just for kicks. I lifted off a third of the way down the strip, and started climbing out, then turned south to go toward my grandfather's place for "cousins day". The engine suddenly started missing and lost power. The problem developed 27 seconds after I went full throttle for takeoff, and swapping out ignition and fuel systems with the "magic" DPDT switch made no difference. 21 seconds after going full throttle, the prop was stopped. Given that I only had 160' of altitude when it started, I was looking for how and where I was going to hit the ground.

The choices were limited...lakes, woods, or two hay fields. As soon as the engine problem started, I'd hauled back on the stick to convert my 117 mph of airspeed into some more altitude (that got me to 280' and 90 mph "best glide" speed), while turning toward the two hay fields. No way would I make it back to the runway with that kind of altitude, and falling short would put me in one of the lakes. I was faced with two hay fields in a row in front of me, the closest one shorter than I'd have thought I could land in (and I was already half way across it and still too high anyway), separated by a substantial tree line from another hayfield of about the same size, but I had to clear the tree line to get to the second one. Both were heavily terraced, however, so I'd have to follow the terraces in either case, and it was probably going to be a wild landing. The only "good" news it that my father owns everything within gliding radius of where the engine quit, so at least recovery of the airplane was going to be relatively easy! Ordinarily I'd have an APRS/Google flight path I could show you, but I never got high enough for a HAM repeater to pick the plane up during this "flight", so the image above with my best recollection of the path overlayed is the best I can do to show you what kind of bind I was in. The times and altitudes mentioned above come from the EIS, which records about thirty different data points every second.

It quickly became obvious that I couldn't make it over the treeline that separated the two fields, so I had to put it down in the smaller field, and unfortunately I'd already overflown most of it. I couldn't make it over the trees, and I sure didn't want to go plowing through that line of trees (and a fence and deep drainage ditch) with any speed. That would certainly tear the plane apart. I'd thought about this scenario many times, and I was already prepared to sacrifice the plane if I needed to. I "slipped" it in to bleed off as much speed as possible, and touched down hard, right at a stall, but dropping a little quicker than I'd have liked. But lowering the nose and picking up more speed to get better control wasn't an attractive option either, given the terraced field, so I slipped off the speed until the plane's energy was reduced as much as possible while still under control. When I touched down I damaged the right side landing gear leg and the plane turned sideways and slid to a stop within 30 yards or so. Looking at the photo above, you can see that I was in a bit of a bind. If nothing else, I think I did a pretty fine job of pulling that landing out of a hat! Perhaps with the benefit of 20/20 hindsight I still had another 50 yards to dissipate some more energy, but I'm OK with how it turned out. 16 seconds after the engine quit, I was on the ground. That's a 64 second flight, with an ending that could have been a lot worse!

Meanwhile, my brother had seen me turn south and then abruptly north, and quickly drop below the horizon. He tells me he set a new record for crossing the pond dam on the big lake. My father saw him whiz by the house and decided he ought to see what what he was up to, so he joined the chase. He hadn't even heard me fly over, since the engine was toast already. The fuselage is undamaged, the right wing was compromised on the underside to the extent that it'd need some rework before I fly it again, and ailerons and flaps are fine. It's probably fitting that I boogered up the only part of the plane that has nice paint on it. Spars and WAFS are OK. It would take a couple of hundred hours to get it back in flying shape, but it's certainly repairable. As for me, I didn't get a scratch, a bruise, or even a crick in the neck in the process...just hurt my pride is all. It didn't even set my ELT off. As far as "worst case" engine failures go, the situation couldn't have been much more critical for such an untimely event. Maybe I shouldn't say stuff like that could have been at 60' and out of runway when the engine quit.

What happened to the engine? I was worried the bolt that broke during the I-65 incident had done something similar, or I'd taken off leaned out, was lower on fuel than I thought, or something else that would make me question whether or not I've gotten too stupid to continue flying an airplane. But upon engine disassembly, the "new' bolt was still tightly installed into the end of the crank, and was still solidly safety-wired in place, and putting all of my 145 pounds on an 18" breaker bar, and then my pneumatic impact wrench, couldn't loosen that bolt (thanks to Loctite 620). Eventually a three-foot pipe on the end of the breaker bar did the trick, but it fought me well.

The flywheel and harmonic balancer were flopping around with about a quarter inch of play, with the distributor drive gear (and therefore the aft end of the crank) all in one piece. Given my limited vantage point through the distributor hole, I thought maybe the aft end of the crank had broken between the rear-most main bearing and the fuel pump eccentric. But once I had the top case cover removed it looked like maybe the the crack may have originated with the factory-drilled balancing hole between #1 and #2 rod journals, despite that normally being a low stress area (hence the place of choice for balancing holes). But once I had the crank out of the case and saw the failed joint, it was clear that the crank experienced a bending fatigue failure, and it originated at the #2 rod journal and progressed through the #1 rod journal. as would be expected by a crank that was over powered by connecting rod forces.

Here are the two crank halves, face to face, with the #2 on top and #1 on bottom. The "beach marks" point to the origin of the crack, near the top of the picture. In this photo you can see a thin dark circumferential ring adjacent to the break, but when viewed from the other side, it's clear that there is some polished surface between there and the break, so I don't think it was the edge of a bearing creating a stress riser or anything like that. It should be mentioned that WW's crank replaces the aft thrust bearing with a normal bearing, since the front bearing does the thrust duties, and that would be rod journal #1, not journal #2 anyway, but I don't see that as a possible factor.

It appears as though the crack started just above the red dot.

Although there was no physical damage to the crank from the I-65 incident (other than the keyway being wallowed out), I might envision how the forces might have set up a stress rising crack that took a while to develop into a break, but I sincerely doubt that such a crack would have been between #1 and #2 rod journals, and would have lasted 120 hours before it actually broke. I did cut a new tight tolerance keyway 180 degrees out from the original keyway on my Bridgeport, but that connection is still pristine, and far removed from the fracture. I also installed a NEW Clark's balancer, which was a press fit, and was Loctited as well.

Few people would deny that a Corvair crank isn't going to have the reserve factor of safety that a typical certified aircraft engine would have. Although I still have some level of confidence in the Corvair crank, only when it's backed up with a fifth bearing to handle prop forces, and even then, the jury may still be out. I've considered simply replacing the crank with a high quality reground and nitrided crank, and fly that same 3100cc engine again. I'd have to come up with another front bearing setup though, and I have ideas for that as well. With 460 hours on it, I must say the internals of this engine are near-perfect. The crank journals still look freshly polished, and last time I checked the compression, it was still 80/80.

One could argue that the rear starter's ring gear was a contributing factor, but I would counter that there are no alternating bending moments on the crank from the rotating ring gear like the propeller puts on the nose of the crank, and the vibration level has been just as low as when I first put William Wynne's front bearing on my engine. And crankshaft expert Ken Smith agreed with me that it's very unlikely that a problem on the very aft end would result in a break two crank journals away. And like my two other crank breaks, there's no sign of a torsional component, only bending fatique as one would expect from a connecting rod in an internal combustion engine. It should be mentioned that I've had no trouble out of that balancer/gear connection until that bolt broke last year, and Joe Horton has almost seven hundred hours on his similar installation with no crank problem.

I expect some folks will think of all kinds of reasons why my engines have been different from other Corvairs, and indeed, the first two had 1.25" more length on the prop hub than WW's front starter prop hub, and this WW fifth bearing engine is a match for that length. It can be argued that I have 15% more displacement (and therefore power) than a typical Corvair engine, turn the engine RPMs up too high, and make sharp turns, and I do. But by far, most of the flight time on this engine is at 3050 rpm going in a straight line. If nothing else, maybe we now have a reason to keep displacement and RPM low, and to make a point of flying more gently.

I have to admit can argue that all types of engines break crankshafts, including the certified aircraft cranks, but three broken cranks in 1130 hours of flying says something. I'll be the first to admit that the first two breaks should have been expected, given the lack of nitriding in the first, or a fifth bearing in the second. But the level of Corvair knowledge at the time did not extend to nitriding the crankshaft, or that fifth bearings were going to be necessary for the engine to do duty as a flight engine.

The Volkswagen conversions had similar problems, and they learned their lessons, nitrided and incorporated a front bearing, and that ended the problem. But this break out in the middle of the crank for no apparent reason makes me wonder. I've been in communication with Ken Smith, Continental's aircraft crankshaft failure expert, and at this point his primary comment has been "Very interesting...a major bending fatigue failure". I expect a more detailed report soon. At the end of the day though, it's still another broken crank!

I'm probably famous for sounding the fire bell, but I'd like to see a high quality forged crankshaft developed for the Corvair engine using the latest metallurgical technology. That sounds expensive to many, but it's cheap insurance when you consider the cost of your airplane, or your life. Given the takeoff timing of my crank break (and that's when it happens...high load), I'm fortunate to have had a place to put the plane down. There are plenty of airports in this country that are far more hostile environments.

The plane is repairable, but it's no secret that I've been planning on designing and building "the next generation" for several years now. I've just been lacking the kick in the butt to do it. I've also had a long list of things I needed to do with N56ML, the least of which is paint. I'd been torn between just building a new plane and not even bothering to finish painting the old one. I can build another plane so much quicker, much better thought out (now that I know what I want and where I'm headed), lighter, more fun to fly, and with better attention to detail. And that's the direction I've been leaning for the last couple of years, but haven't had time to do anything...either finish up the first one or start a new one.

In a perfect world I'd have continued to fly my plane while building a new one, although that never seemed to happen before...but now I have a reason. Rather than spending time reworking N56ML, I plan to start construction on the next generation plane, incorporating a long list of changes that I have in my head, including Light Sport compliance so I won't need a medical to fly it. I can do this one right, given the knowledge I've picked up over the years, and the experience of having flown N56ML 1130 hours with over 2800 landings. I'm ready to order the spruce and plywood from Wicks to start the next project. The neat thing about having the old plane in one piece is that I have virtually everything else "on-hand" to build the next one. I know exactly where I'm going, what I'll need, how to do it, and it will be a much better plane for what I need. Despite my current workload, this will pretty high on my priority list. I'm cleaning out the basement shop to make room for the fuselage assembly table, and I expect progress to be made quickly, given enough time off from work, so I'm a bit excited about the future.

Update - April 2012 - Fillet Radii analysis

So how about those critical crankshaft journal radii? After a few months passed, I picked up that broken crank and started looking it over really well. While Corvair crankshaft journal radii are always meager looking once you've seen a Continental or Lycoming fillet, the fillet radii on my broken crankshaft sure looked small.

Up until recently, my problem with crankshafts is that I've trusted others to provide a crankshaft that is up to the task of aircraft use, but failed to quantify them myself. I've always checked them over to ensure no obvious flaws or shipping damage, but have not taken the next step, verifying the integrity of the entire crankshaft by quantifying the radii with a hard measurement. And in this particular case, I never even SAW the fillet radii, because the crank was installed in the case before it arrived at my house, as assurance to WW that his fifth bearing was properly assembled by one of the "Corvair Consortium", rather then me. The thought was that if I assembled it and it croaked, I might be blamed, whereas if it were assembled by Petz with his machinist expertise, proper installation would be assured. The crank regrind, with front bearing surface installed, was provided by William, and he's recently told me that he does not recall exactly where it came from.

Keep in mind that the GM manufacturing drawings for the Corvair crankshaft call out a radius of .090" to .110" for the rod journals, and a minimum of .062" radii for main journals.

This radius is on rod crankpin number one, meaning it's located between the rear-most (balancer end) main bearing, and number two, the journal that broke. Given that this is a .062" radius gauge, this is clearly not in compliance with the minimum .090" radius called for in the drawing. It doesn't even appear to be .062". Someone will no doubt ask about the dark line that runs circumferentially around the journal..."is that where the bearing cut into the journal?" No, that's way outside of where the bearing is allowed to travel, given the .017" total lateral play (on this particular crank) that the rod had between the crank cheeks. Neither the bearing nor the rod can get there. It has to be a remnant of the crank grinding or polishing process.

Here's a closeup view, which shows that it may very well be a .062" radius, but has a discontinuity between the radius and the bearing surface, which constitutes a stress riser. I would argue that the notch at the bottom might be lucky to be a .015" radius.

This fillet is on number two, the crankpin that broke (180 degrees around the pin), except it's not .060" either, is closer to 3/64" (.047"), and has a similar small-radius discontinuity. It's not exactly a notch, but the radius is probably less than .010". There's a huge difference in resistance to fatigue fracture propogation in the .090" minimum radius and this thing. I think this is the usual Corvair crank failure smoking gun, an insufficient fillet radius. The fillet is where all the failures I've seen have initiated.

This is the adjacent main bearing journal, showing something like an overall .062" radius, but with multiple discontinuities.

This is not the secret to crankshaft longevity, and I am not impressed with this grinding/polishing work. I realize that grinding a crankshaft .010" undersize is a bit of a challenge, given the geometry of the fillets, but I've seen much better regrinds. After this third crank break there was discussion that cast doubt on my rear starter installation and the manner in which I flew the plane. But I'd argue that if you have a .010" radius notch in one of your rod journals, it's going to fail prematurely! The good news is we know how to fix this, but the bad news is it's requires attention to detail, isn't easy to fix, and can be expensive!

This is a stock crank I had laying around the shop. It doesn't ring well, and I assume it's cracked, but I never throw anything away. Note 3/32" gauge, and how it's not far from that...maybe .083" or so. But more importantly, it's a nice smooth transition with no abrupt discontinuities.

Here's another un-ground stock crank, again it's closer to the GM drawing, at maybe .083". I should have wiped the journal down a bit before taking this picture.

The bottom line is that getting someone to regrind a Corvair crank is a crap shoot at best, and absolutely requires that you check the work when they're done. If you have a crank sitting on your bench and not yet installed in your engine, you need to invest in a set of radius gauges and a magnifying glass and make sure it has reasonable radii with no notches or polishing burns in the fillets. And trusting someone else with sourcing your crankshaft and assembling your bottom end is done at your own peril.

People won't listen if I tell them they ought to remove, inspect, and magnaflux their cranks at 500 hour intervals, but that's going to be my policy once I build another plane and start flying behind a Corvair again. And this is only AFTER I check for proper radii when building the engine.

When I first published this episode, I challenged somebody with more time on their hands than I have to please take the Corvair crankshaft to the next level and solve this problem. Dan Weseman had already been considering a new Corvair crankshaft, so it didn't take much for him to get an initial buy of six crankshafts in work. This will be a new 4340 crank with crank radii so large it will require clearancing the bearings and connecting rod big ends to clear them. Bearing area isn't critical on our slow turning Corvair engines, but fillet radius is.

When I called Dan Weseman to plead my case for the largest fillet radii possible, he pointed out that he and William Wynne had measured something like 30 used but unground Corvair cranks and found many of them didn't meet the minimum radius dimensions given on the factory GM drawing. That makes regrinding a stocker even more of a crap shoot. I'm proud to say that one of those crankshafts has my name on it.

Update - March 2016- SPA 4340 crankshaft

In contrast to the images above, this is a photo of my new SPA (Sport Performance Aviation) 4340 crankshaft journal. This looks considerably better than any of the options previously available, with a 1/8" fillet radius with no sharp discontinuities. This crankshaft is in a league of its own, and is real insurance against crankshaft breakage. Just as the 4340 crank and front bearing solved the crankshaft issue in VWs, I am convinced this will do the same for a Corvair. I have no doubt that this crankshaft will fare far better than the one depicted above, since the base material is already something like 50% stronger, and the radii will be much larger (and hopefully, more consistent with longevity). After each improvement we've made to the crank (nitriding first, then the fifth bearing) I've always said "maybe this will fix the Corvair crank problem", but this time I'm almost ready to say "this WILL fix the Corvair crank problem.

Also see more on the first and second crank breaks, see Return to more on the first crank break (with forty hours on it) and the second crank break (which had 181 flight hours on it). Number three had 456 hours on it. Better, but not good enough to bet your life on, or your airplane. Trust me on this...

So what now? I bought Jim Faughn's VW powered KR2, mainly because I thought it would be the quickest way to get back in the air, and I've always wanted some stick time in a KR2 so I could compare the improvement between the KR2 and the KR2S. The story of that trek is detailed at I'm still a strong believer in the Corvair engine (especially with the new SPA 4340 crankshaft). And having rebuilt this VW engine twice now, the Corvair is far simpler to rebuild (especially the 2700cc version) compared to a 2180cc VW, which involves a LOT of modification and careful attention to detail, not to mention the hoaky tapered prop hub arrangement. And the Corvair runs MUCH cooler than the VW, at least 50 degrees, thanks to much more cooling fin area and larger air passages. I'll wrote another article on this topic later.

I will build another KR2S, and it will have the SPA 4340 crankshaft in it!

Also, I've bought a Continental O-300 powered 1946 Globe Swift to keep the itch satisfied and to continue the sunset photo tradition while I'm building "the next generation" plane. Swifts are somewhat "KR-like" in the control feel, and best of all, the roll-down windows leave nothing between me and the sunset! It has most of the "nice" modifications that are made to Swifts, like sticks in place of the yokes, electric trim, Cessna 150 seats, the "new" cleaner instrument panel, gross weight upgrade, auto-fuel STC, and many more. And although the retractable gear is a little worrisome, there's a big bright red flashing light right in front of me that warns when the throttle is pulled back past 1800 rpm with the gear up. I only have to change my radio call sign slightly to N59GL, so that makes it easy too. This plane looks expensive, but it wasn't. If I ever sell it, I'm pretty sure I'll get it all back.

I've been flying the Swift for several weeks now, and can tell you landings are completely different, given the draggy retractable gear, large tires, and real shock absorption. The KR landings really taught me to be a better pilot than I'd thought. I'm now signed off and flying it. In the last month I've read just about everything ever published on flying the Swift and maintenance of this classic plane. The downside is that it costs almost twice as much per hour to fly (not counting maintenance), and isn't quite as fast as the N56ML, but it will keep me going while I build my next one. I already miss how quickly N56ML would climb, and how easy it was to roll in and out of the hangar, and even how easy it was to get the cowling off and do other routine maintenance. The "next generation" will be a big improvement though. I Still love the smoothness of the Corvair, how quickly it starts, and the great fuel economy you can get from it.

Why did I wait a month to publish this? I wanted to have a better understanding of the cause, and because if I'd posted this a month ago, the tone would have been considerably worse! We were late for cousins day, so I just walked away and left it in the field for a week, and the only post mortem I did on the engine was making sure the front of the crank wasn't broken by wiggling the prop. And I certainly didn't want the Corvair community or the folks at work to get all wrapped around the axle over what happened and why, especially before the engine had even been torn down. Now I know what happened, and have bought a nice replacement plane that I'm already quite happy with, and am preparing the basement for construction of the next plane, so no condolences required or expected.

If you have time, check out my aerial sunset pictures.

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