Bent 049 conrod repaired

Thanks George and Bob...indeed. The hobby has burnt into my everydays, and I feel somehow empty if I have not dealt with my engines and planes in some way for a few days or shorter. But repairing a wounded engine gives me a special satisfaction that you Bob may also feel when completing one of your refurbishment projects.

Great story Andras, I’m glad you were able to save it! I too have a once bent engine that was repaired by a kind forum member and still going strong …Let us know how it holds up for you in the future….Your reports on high time engines have been encouraging as well… I really like running them so it’s good to know they can have a long service life if treated well…

Robert

Thank you Robert. I bought this red postage backplate engine in already used condition, and it has had some heavy runtimes since then. I do not expect its hand-straightened rod to buckle in normal running conditions, but the recognition that a starter spring can also drive it into a harmful hydraulic lock just taught me that no matter what, the first flips and cranks should always be by hand. With its excellent compression still retained I expect many hours of dependable performance still waiting of us.

While you have straightened it, those rods aren't ductile enough to bend and re bend. When it bends it microscopically cracks and when you straighten it, you work harden it even more making it even more brittle.

Ken Cook wrote:While you have straightened it, those rods aren't ductile enough to bend and re bend. When it bends it microscopically cracks and when you straighten it, you work harden it even more making it even more brittle.

Ken, you bring up an interesting topic. I remember at McDonnell Douglas, watching on Test Hangar 41, we had million pound hydraulic jacks applying pressure on a C-17 main wing spar. Using a spectrum tape from the structural design engineers, would detail the load profile to be applied in our test rig, read in by our multi-tasking load servo controlling and data acquisition Perkin-Elmer 3220 test minicomputer, that we had in an enclosed trailer next to the test fixture.

It ran for a number of months, basically at an accelerated rate, to find out when it would start to fail (shut down at periodic intervals for inspection) and final failure through cyclic fatigue. When it did break, there was a huge explosive sound in the hangar (ear protection nearby a requirement), our computer would immediately terminate the test. (I could even hear such an event in the adjacent 41A office/flight test control room/simulations lab annex building where my desk was.)

Engineers wanted to know ahead of time to predict signs of failure, to see if redesign was needed, also to develop the criteria required to adequately inspect the spar so that the plane could be safely be safely withdrawn from service if need be, repaired / replaced so the plane could remain airworthy and prevent accidents.

I think the beauty of this type of repair on a venerable Cox 190 or 290 postage stamp engine, is that if the connecting rod did fracture and separate, from what others have shared in similar instances, worst case is the engine is rendered non-repairable. Sometimes, may be the worst hasn't happened and one can replace worn/damaged parts. It's not like destroying an expensive hard to replace, say, OS .30 Wankel.

Would one act of straightening a mild bent connecting rod cause a problem? Can't say. They do afterall from what I've heard, do a certain amount of rehardening (re-tempering) in the manufacturing process after forming when ductile, so the part can withstand higher strain without failing.

András is safety minded, and in case say there was a failure, given his mode of flying, even if the conrod separated, worst that could happen is loss of the engine. However, the nice thing about these engines is availability of replacement parts from Cox International, Ex Model Engines and his various other sources in his area.

Personally, my gut level feeling is that he probably won't have problems here on out, but that is just my personal impression. If it does, he'll have another story to share.

Yes, Ken and George, a bend means a permanent change of shape of the material, that was stressed beyond the yield strength, yet I hope this will not cause  a fatigue break to come soon. This engine with the Kamtechnik or Galbreath head spins well over 20k rpm when unloads in the air (its brother was audiotached at over 28k when I maidened the Quickie100, see the video in that thread), and after straightening the rod I already had a terrific test flight back in the field with the 290 screaming as usual, and the rod survived. Providing the rod is indeed straight, it is mainly exposed to pressing, and not to pulling forces or bending moments that would widen the micro cracks. So those intercrystalline cracks that may have developed when the rod got bent, will hopefully not, or not too soon propagate deeper into the material to lead to a break of the rod..I repaired a 051 rod with a similar rudimentary method (pliers, what else?) and so far that engine thanks for the question, but feels OK.

Thanks András for the logical explanation.

One recent incident:

Yes George the carbon fiber pressure bearing parts of the Titan were certainly less elastic than the metal parts, which, under a stress below the yield strength of the material will elastically elongate and regain their original shape when the stress or load is removed. The carbon fiber material is more brittle and if not designed for proper thickness/strength, will crack rather than bend elastically.. I have observed this also with the carbon fiber tubes that I substitute for hardwood or balsa wing spars in most of my planes. They take more load than balsa but if bent beyond a point will crack or buckle sooner than the bend radius a wooden spar will endure.
I read the Titan displayed micro cracks in its carbon fiber pressure bearing parts after a few dives, that finally may have led to the catastrophic propagation of the cracks leading to the implosion of the vessel on its final lethal dive.

András --

What plug are you using with your Kamtechnik head?

a--

Andrew,

I use medium hot turbo plugs in the Kamtechnik head. No special brand preference, whatever I find in the few hobbyshops that still exist here.

Good work Andras. It is better to try fixing a conrod than paying 19.95 Dollars. I think that conrod didn't bend too much, so no cracks any. Most materials can bend a little amount without any crack.