Bent 049 conrod repaired

Andras, at your age 64, your enthusisum for flying these little devils reminds me of myself back when I was 14-17 flying Walt's hollow logs powered by Space Bug Jr.s and the early Babe Bees. I couldn't wait to get out there. Keeps you thinking young.

Bob

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

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also to develop the criteria required to adequately inspect the spar so that the plane could be safely be safely withdrawn from service if need be, repaired / replaced so the plane could remain airworthy and prevent accidents.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.

Thanks András for the logical explanation.

One recent incident:

András --

What plug are you using with your Kamtechnik head?

a--

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.