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"Banging noise"

gblwy

gblwy

Well Known Member
For some time now I have had a banging noise while warming up the engine.

The noise appears after maybe five minutes running at 2500 rpm. At lower and higher rpm the noise goes away, and I have never heard it in the air, or while taxying post-flight.

At first I thought it was the exhaust banging against the lower cowl. I checked the exhaust springs. All were intact, but on a couple the safety wire I had threaded through the spring had broken. I repaired this and the noise persisted. I also enlarged the exhaust outlet hole in the lower cowl because there was possibly some evidence of contact. No joy, so far.

At the annual (which I am doing) I checked the four engine mount bolts, because I suspected that might be a possible cause. All the bolts are secure. So now I am running out of ideas.

As an aside, I also found a small (half-inch) crack in the lower starboard firewall, adjacent to the lower cowl mounting point.

Cheers...Keith
 
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We would sure like to see a photo of that crack!

gblwy said:
For some time now I have had a banging noise while warming up the engine.

The noise appears after maybe five minutes running at 2500 rpm. At lower and higher rpm the noise goes away, and I have never heard it in the air, or while taxying post-flight.

At first I thought it was the exhaust banging against the lower cowl. I checked the exhaust springs. All were intact, but on a couple the safety wire I had threaded through the spring had broken. I repaired this and the noise persisted. I also enlarged the exhaust outlet hole in the lower cowl because there was possibly some evidence of contact. No joy, so far.

At the annual (which I am doing) I checked the four engine mount bolts, because I suspected that might be a possible cause. All the bolts are secure. So now I am running out of ideas.

As an aside, I also found a small (half-inch) crack in the lower starboard firewall, adjacent to the lower cowl mounting point.

Cheers...Keith
Click to expand...
 
Hi Keith,

I had a similar noise when I first built the airplane. Nearly drove me nuts trying to figure it out. Only heard it at certain RPM's. Turned out to be the lower cowl rubbing against the skin of the airplane at one point where I hadn't allowed enough gap. 30 seconds with a piece of sandpaper and it was fixed!

Hope your fix is as easy.

BTW, thanks for the lesson in weight conversions!;)

John
 
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Upper and lower canopies?

Huh? Can you elucidate or even explain, John?

Thanks...Keith
 
Well, first off I said "canopy" when I really meant "cowl". Duh. The noise was being generated when the fiberglass of the lower canopy rubbed against the metal of the skin. This was on the right side, where the cowl extends rearward toward where the short hinge pin is.

Text from my log that day:CJ helped me look for the infamous "rattle" using Bill's stethescope contraption. Tracked the noise to lower right passenger side foot well. Further investigation revealed a small portion of the lower cowl rubbing against the metal fuselage skin at certain RPM's and making a snapping noise. A little sandpapering and problem solved!
 
Ahh

So canopy = cowl! Based on this, I'll investigate. I have to say that sitting on the pilot's side, the noise seems to be coming from the other side...

Cheers...Keith
 
Cracks

Hi,

I was asked for a photo of the firewall crack. The crack is less than half-an-inch long, and I stop-drilled it with a #36 bit.

2uh1wmf.jpg


I also subsequently found a larger one in the port fuselage skin right where the firewall shelf is. I'm still wondering whether the banging noise I have heard is in some way related to these cracks. This one is maybe an inch-and-a-half long and stop-drilled with a #30 bit.

PS Never paint a plane black :)

2941hqf.jpg


Cheers...Keith
 
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Start a new thread?

Hi Keith,

I have a suggestion. Start a new thread with a title more descriptive of the cracks you have discovered. As the fleet ages we will want to keep track of fatique issues, and track whether there are any spots prone to cracking that should receive special attention during inspections. If you were to start that thread with your photos and a more "searchable" title, it would benefit the RV-12 community.

Maybe a title something like "Fuselage fatique cracks" or some such.

What do you think?

John
 
Cracks

I think you should send those pictures to Vans. They should be interested in seeing them.
 
I think we all need to be adding this to our checklists as something to watch out for. Has anyone else ever seen this on their aircraft?
 
BigJohn said:
Hi Keith,

I have a suggestion. Start a new thread with a title more descriptive of the cracks you have discovered. As the fleet ages we will want to keep track of fatique issues, and track whether there are any spots prone to cracking that should receive special attention during inspections. If you were to start that thread with your photos and a more "searchable" title, it would benefit the RV-12 community.

Maybe a title something like "Fuselage fatique cracks" or some such.

What do you think?

John
Click to expand...

....Or maybe expand it to cover all in service problems. Then if it catches on we could suggest DR to make it a sticky!
 
With those hours (and presumed low cycles), fatigue is not the first word to come to mind. If I was back at Boeing, we'd probably cut those out and have the met lab do a detailed analysis of the fracture faces to understand what's going on. Not suggesting you do that, but my curiosity is in high gear on these low time cracks.

In addition to the stop-drill, you might want to consider a repair doubler.

Bob Bogash
N737G
 
I am at the stage that perhaps I need to be looking at putting doublers in those areas for insurance against those cracks.
 
No hard landings, or nosewheel landings...

...and the plot thickens as I have two cracks in the nosewheel spat brackets and a tiny crack in the fibreglass of one of the main gear spats.

Cheers...Keith
 
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gblwy said:
Hi,

I was asked for a photo of the firewall crack. The crack is less than half-an-inch long, and I stop-drilled it with a #36 bit.

I also subsequently found a larger one in the port fuselage skin right where the firewall shelf is. I'm still wondering whether the banging noise I have heard is in some way related to these cracks. This one is maybe an inch-and-a-half long and stop-drilled with a #30 bit.

PS Never paint a plane black :)


Cheers...Keith
Click to expand...

Seems odd that the larger crack decided to bypass the nearby rivet holes. That's usually where you'd expect them head to or start from. From the photo it looks like the crack is in the thick paintwork rather than the underlying skin, but I assume this is just an optical illusion? (Apologies in advance for the dumb question).
 
Keith,

Could your prop be out of balance or have the blades set out of pitch with each other causing unusual vibration thru the air frame. Thats strange having cracks show up in so many different areas. You may want to find another 12 to compare the amount of vibration or find someone that can dynamically balance the prop assembly.

The first time I flew my 12 I thought everything was going to fall off of it due to the blade pitches being slightly different. Took a lot of tweeking to get a smooth run.

Gary
 
Prop vibration

I did have some prop vibration, but since then I have a new prop, and the vibration is much less. I still intend to get it balanced.

The fuselage side crack is definitely a metal crack, not just a paint crack. I took some paint off the outside and examined with a good magnifying glass. I can also see the crack from the inside. Same with the firewall crack.

There is some contact between the lower cowl and fuselage, between the two sets of hinges, but I cannot see how this could lead to cracking. I'll sand down these edges as a precaution, though.

Cheers...Keith
 
Is is possible

for the orbit of the prop shaft, or some other part of the engine as it moves about, to hit the cowling?

Is that crack in the firewall at the end of the cowling connection?

Just some wild questions, so forgive if they are way off base.
 
gblwy said:
I did have some prop vibration, but since then I have a new prop, and the vibration is much less. I still intend to get it balanced.
Click to expand...

Prop balancing was very effective in reducing vibration in my case.

gblwy said:
There is some contact between the lower cowl and fuselage, between the two sets of hinges, but I cannot see how this could lead to cracking. I'll sand down these edges as a precaution, though.
Click to expand...

But - this contact could cause the noise you are hearing. It did on mine.

John
 
Guess

Latest guess (still to be investigated) is the firewall "oil-canning" as it warms up. If I repeatedly press the engine side of the firewall in the open space between the battery and the bottom of the firewall I get a loud bang-bang-bang sound, exactly like the noise I hear when warming up the engine.

What could cause the firewall to resonate like this, and why has it started doing it?

Cheers...Keith
 
Oil canning of relatively thin sheet metal structures is normal and shouldn't cause cracking - but, they could cause the banging noise.

Boeing airplanes are full of oil cans - on the exterior skins and internal structures - like the aft pressure bulkhead. 737s and 747s are full of them. Check out some B-52 pictures if you want to see some really large areas of oil canning.

Our customers didn't like the oil canning - especially the appearance - and for a while, we chased that ghost. But without success. In a pressurized airplane, you can take pictures showing the oil cans coming out when you pressurize the airplane. Before we gave up on this quest, we would try to shim out the oil cans on an airplane, only to watch them reappear or move around, when the airplane was fueled or dropped off jacks.

Bob Bogash
N737G
 
albums


One of the VAF members asked for a picture of the area on my airplane where the noise was emanating from. Apparently at certain RPM's the the fiberglass resonated and the vibration against the metal skin caused the noise.

Sorry, I still can't seem to master the technique of getting an image in a post. I don't understand why it has to be so hard.
 
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Good info NASA. The 12 is a very thinly skinned bird and I can't imagine trying to treat all the oil cans it has. Mine is full of them. I keep any eye out for areas of fatigue and abnormalities, but don't sit around and worry about all the noises. The most annoying noise mine has is the right side brake puck rattling with no hyd., pressure on them.
 
Talk about oil canning. When I pull mine from the hanger into the hot GA sun the leading edge (top) wing skins sound like corn popping. Of course I have very dark blue paint in that area. Seems to be the nature of the beast tho.
Dick Seiders
 
Well, I took some material off all around the cowling where it could interfere with the fuselage. In particular I found the bottom end of the top hinge on the port side was touching the forward fuselage, right near the crack.

However, I do still get a banging noise when warming up, maybe less noticeable but still there. I got someone to sit with me and monitor the firewall in the passenger footwell. There's definitely vibration there, and it does get warm. As it warms up I guess it expands a little and starts to "oilcan". It pushes out towards the cabin. If you push it back, it jumps rearward again. I have no idea whether the small firewall crack is the cause of the oilcanning, the result of it, or a completely separate phenomenon.

Cheers...Keith
 
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