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

Brantel

Well Known Member
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:
 
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.
 
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/community/showthread.php?t=36763&highlight=oil+canning

+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!
 
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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.
 
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.

No way would I fly with that bulkhead removed.... just removed it right before Osh to change ELT batteries and perform annual condition inspection items behind there.
 
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.

Nothing unusual found....and the sound appears to come from behind the cockpit.
 
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.

Already in progress. I checked out the flaps and links and nothing out of the ordinary found.
 
Take care when adding stiffeners - secure the ends to some structure else the skin may crack at the end of the 'floating' stiffener.
 
Smoking Gun Found!

0710Gun.jpg


So after my last test flight with stiffeners....BAM! Did it again.... RATS!

So I landed almost defeated and as I was taxiing back to the hangar I thought to my self: "Self, what else has changed?" And I answered back to myself "Nothing but the recent smoke system install..." WAIT A MINUTE, HOLD THE PHONE!

You see I have flown many flights since installing the smoke system and did not have any issues like this BUT I never flew with the tank almost empty! You see right before Osh, I knew I would be heavy so I pumped all but about a half gallon of the oil out of the tank!

When I ordered this smoke system, I took the option to have the non vented cap along with the spring loaded check valve/vent.

Is this starting to make sense yet?

The SMOKING GUN is the oil tank! It was oil canning due to the changes in pressure around it. The non vented cap and check valve won't let air out and it takes a few psi of pressure to get the spring in the check valve to open so this was causing the tank to oil can.

The force required to make it oil can is significant and when it does it is loud! I now know why I thought it was an exhaust pop in the beginning!

To simulate what is going on, I used my vacuum in reverse to blow air into the tank. BANG! A very sharp, metallic sounding BANG!, press it back in and BANG! Even with the vac running, it is very loud!

Links to video of issue:

https://goo.gl/photos/arhQXRyvDi3zd5f79

https://goo.gl/photos/eQ8ueiNSmn3nq8A87

I am off now to test fly with the cap loose just to be sure but I am 99.9999% sure this is it! I am sure that the reason it did not show up sooner was because it always was always at least 75% full!
 
I was reading through the posts and thinking of my mustang I had that would randomly make a bang sound which I finally figured out that it was the fuel tank collapsing because the previous owner removed the charcoal vent system and plugged it. I was going to post my experience thinking that the possibility of that happening to your fuel tank was very unlikely but wow! pretty close.
 
I was reading through the posts and thinking of my mustang I had that would randomly make a bang sound which I finally figured out that it was the fuel tank collapsing because the previous owner removed the charcoal vent system and plugged it. I was going to post my experience thinking that the possibility of that happening to your fuel tank was very unlikely but wow! pretty close.


It can and has happened to RV fuel tanks. Some have collapsed from blocked vents. I had already checked mine for that issue. Never dawned on me that it could be the smoke tank!

For the final test flight of the day I stuffed a rag in the tank fill hole and went flying. All the climbs, decents and speed changes I could muster and there was no bangs! Smoking Gun Confirmed! :D

Maybe I can sleep now. I have not slept well since leaving Osh thinking about this issue! :eek:
 
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Reminds me of the time I was in the right seat of a 737 in a descent, and I heard a sharp "crack!" I started looking around furiously for a cracked window, and the Captain said: "What are you looking for?". I said I heard a loud crack noise and was looking for a cracked window pane.

He said, "........... Tha was your water bottle collapsing!"


:eek::eek::eek: :rolleyes:
 
The other common source is the bag of chips you threw in for that mid flight snack exploding at 10,000 MSL.

Erich
 
Glad you found the true issue. It is too bad you started on the most expensive side, both in effort and $$, in your troubleshooting attempts. The good thing though is that after doing all those tests, upgrades, changes, etc. to the engine means you now have a great running engine and high level of confidence in its performance.

This reminds me of the adage that whenever troubleshooting a problem start with the simplest/easiest/cheapest solution and work toward the difficult/hardest/expensive solution. Many times it is those simplest/easiest/cheapest solutions that solve the problem.
 
Glad you found the true issue. It is too bad you started on the most expensive side, both in effort and $$, in your troubleshooting attempts. The good thing though is that after doing all those tests, upgrades, changes, etc. to the engine means you now have a great running engine and high level of confidence in its performance.

This reminds me of the adage that whenever troubleshooting a problem start with the simplest/easiest/cheapest solution and work toward the difficult/hardest/expensive solution. Many times it is those simplest/easiest/cheapest solutions that solve the problem.


I agree. The effort was free but the $$ was not insignificant. However I was planning to add a second EI at some point anyway, this just accelerated that. As for the engine, I did find some goofs by the original builder that make all of that work worth it to me. I would have never found that stuff until it was too late if I had not torn into it. I also learned a ton about how the valve train works on a Lycoming. Hands on experience of all of that is priceless. The old cork gaskets on the rocker covers, oil drain and intake tube hoses were starting to leak as well so it was good to replace all that anyway. The intake and exhaust gaskets appeared to be OK but it definitely did not hurt to change them out as well.

As for what I would do different if I were starting over again, I am not sure. It was very hard to describe what was happening to others. It took some time for the patterns to become obvious. It was something you had to experience. The sound it was making in the air was very loud and very scary and for me the only thing I have every heard that was even close to that level of sound was an exhaust after fire bang. It was getting to the point where I was gonna have to find the issue or stop flying the plane because the noise was so loud and scary sounding that I knew it had to be something unusual going on and the fear was that it was some sort of pending disaster that I could not find.

In my defense...nobody ever recommended to check the smoke tank :p

The funny thing is that in my day job I am known for someone that always defaults to checking the easy, cheap stuff first as that turns out to be the problem 98% of the time. :eek:
 
In my defense...nobody ever recommended to check the smoke tank :p

Weak my friend - to our defense, you did not say you had a smoke tank with flat sides and no vent . . . . :eek: The possible description of "oil canning" was a red flashing arrow.

All in good fun, you are good company, the best part is you kept an open mind and went seeking better data.
 
Weak my friend - to our defense, you did not say you had a smoke tank with flat sides and no vent . . . . :eek: The possible description of "oil canning" was a red flashing arrow.

All in good fun, you are good company, the best part is you kept an open mind and went seeking better data.

It has a vent, just a one way vent....one way being it lets air in and does not let oil/air out. Even with the checkvalve/vent it it puts a slight vacuum on the tank due to the tiny spring holding the ball closed.

I talked with Marvin via email and he said I am #2 to report this issue (I am always the lucky one). He and I agreed that an overboard vent would be best and to ditch the supplied check-valve vent.
 
At least you found and fixed a couple engine issues that could have developed in to real problems - especially the P-Mag stud hanging by 2 threads that were about to let go.
 
At least you found and fixed a couple engine issues that could have developed in to real problems - especially the P-Mag stud hanging by 2 threads that were about to let go.

I agree, I am definitely happy to have found a few issues that could have been big trouble down the road!
 
Nice work, Brian. The longer a problem takes to diagnose, the more satisfying the solution...or should I say the expensive is the solution.
 
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