The saga continues to find the source of the bang my airplane has developed since departing Osh a few weeks ago. Prior to, no unusual noises ever heard. Been flying it since 2010.
You can read about how I got to this point here
So in one of my threads Dan Horton mentioned that it is difficult to be sure what one might have heard. He is right! I can tell you that when you are not used to any unusual noises coming from your airplane and you have a nervous wife on board and something less than severe clear weather, bangs in flight get your attention fast!
So today after checking for sticking valves and finding none, doing the wobble test, compression test, borescoping the cylinders, finding a suspicious distributor block on the remaining mag and replacing it with a new Pmag, installing new intake and exhaust gaskets all the way around, new intake hoses, changing oil and cutting open and inspecting filter, test running for a while on the ground... etc. etc. I can tell you that the engine runs like a sewing machine and sounds great!
Test flight this morning. Engine fired right up, taxi out and runnup and all is normal. Takeoff and power level seems to be normal. All temps and pressures in the green. Expedite climb to safe altitude for a return to the field if needed. Climb on up to about 4000msl.
As soon as the speed started to build around 150kts IAS, bang! Same sound I heard leaving Osh, same sound I heard on the test before departing KDNV, and the same sound I heard departing KDNV for KMOR. Now that I am almost certain that it is not engine related, my mind starts to get off its brain block and realize that all of these have something in common. They all appear to be airspeed related. Once the speed builds I get a bang.
So I flew around after the bang for a while testing major power changes, accel decel climb dive etc. trying to get the engine to do something other than pur like a kitten. Nothing!
I then slow down to about 70kts IAS hoping to get the bang to reset. It never did. So I decided to land and on short final as the speed dropped to 68-70kts, bang! Just like the sound I heard when landing at KDNV, just like the sound I heard when landing after my test flight at KDNV and just like the sound I heard when landing at KMOR the week of Osh. These bangs also all have something in common. They happen when slowing down on short final with the nose down.
So I said to mayself "Self, this is too much of a coincidence! You can't see the forest for the trees! Get off your brain block and realize that this must be something else besides the engine!"
That being said, I decided to go up for another lap around the patch to confirm my suspicion. I took off and all was as normal. Flew around over the field at 3500msl. Got the airplane up to 150kts IAS and waited. Nothing happened. I shook the controls around a little and waited. Bang! The same sound happened again.
Come in to land and on short final...you guessed it! Bang! Somewhere around 68-70kts with the nose down.
I that pretty much confirmed for me that there must be some sort of oil canning going on that appears to be fairly repeatable.
So if this is oil canning that makes a bang that is uncomfortable to ignore, where is the most likely place to start finding it? I have pressed around on the skin of the entire tail cone and can't find it that way.
Also, why now? I have never had this issue before. I do very mild aerobatics to include aileron rolls and some ugly loops. Never had a bang when doing those.
I can tell you that I did have the baggage compartment bulkhead out before Osh to change ELT batteries and I also removed the side covers over the flap linkages to inspect the ball joints as well. Have no idea what issue that could cause but I wanted to put it out there as something I did before this issue showed up.
Also, when this happens there is nothing felt in the stick and the engine does not appear to miss or anything.
I can tell you this, I must find and fix this issue or my wife will never ride in my plane again