Sticky valve? Nope! Ignition? Nope! Oil canning? Maybe...
 

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 :confused:

Sounds like time for a camera mounted underneath pointing towards tail. I would suspect the skin on the bottom of the tail, behind the baggage bulkhead first...... many reports of that area popping

Some reading for you.
http://www.vansairforce.com/communit...ht=oil+canning

Brian,

Since you mentioned that you removed the baggage bulkhead and the flap covers, it gave me a thought. It's a long shot, but who knows.

Could it be that a screw (or screws) of your baggage bulkhead or a flap cover aren't quite tight and not quite lined up? My theory is that they might seem tight because the nut plate is grabbing and may be galling just a little. So, you screw goes in almost all the way, but not quite. In addition, the hole in the bulkhead may be off to the side so it is catching a thread. Then, when the airplane gets a certain air load on it, there is enough stress, in the right direction, and the bulkhead jumps the thread and produces your bang.

When putting all the panels in my 10, if I don't be careful and line everything up, I occasionally get a pretty loud bang when screwing the panels in and they jump off of a thread.

+1 for the list. Mine would pucker with just the fuse while building. It was sensitive to up and down forces on the tail with the forward fuse held steady. I added some ribs. (flame suit on) I suppose internal dynamic air pressure could also be a driver. Pucker in, pucker out.

Brian, good to hear you are getting to the cause, or at least closer.

i would be inclined to find an RV buddy who could fly around my plane and inspect it in flight for deformation.

Agree with the "look outside and have someone help" advice. One other thought: Have you tried removing/loosening all the screws on the baggage bulkhead and maybe the smaller side panels you moved prior to this starting, and reinstalling them?

Smoking Gun?
 

Is this the smoking gun? Here are three different cell phone videos that I took. Using various techniques of pushing on the skin. This is much louder in person.

I tried it with my Lightspeed's on and it sounds very similar to what I have been experiencing.

Click image below to go to video's:



Update: Turns out this was not the problem....real root cause further down!

Did you fly with the baggage wall out or just removed and reinstalled? it is structural and required. I have seen on here a pic taken where a close inflight shot during aerobatics shows obvious oil canning in the tail cone.

Bang sound
 

The first thing I would check is the wings. Look at the wing root fairings and any smoking rivets underneath. It sounds like a wing loading problem in my opinion, I would look hard at attachments for galling or any kind of shifting. From what I understand, the bang appears around 150 kts ( High pressure underwing ) then reappears when slowed to around 70kts nose low ( wing pressure unloading ).....just my 2 cents.

Hey Brian,
Sorry to hear about all the problems you are having since OSH. Looks like it would be easy to eliminate the oil canning with a temporary lightweight stiffener double side taped to each side for a short flight. I almost tend to lean toward a flap actuator or flap linkage problem.