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pauldan181

Well Known Member
My 8A forward belly skin has developed a crack around the center floor rib most aft rivit. This is about a foot behind the exhaust ramp hinge.

crackj.jpg



Has anybody had a crack at this location? Any idea what might cause this and any ideas on a permanent fix? For now I was just going to stop drill it, bond a small patch over the area and keep an eye on it.

The inside of this area is completely inaccessable so a real structual repair would require major surgery.

Thanks

Paul Danclovic
Jamestown NC
RV-8A N181SB
 
Yes, yes, yes!

I had very similar cracks, and we determined that it was related to having straight exhaust pipes with no turn-down. I stop-drilled the cracks, put in a few Cherry rivets between rivets that were "working" (yes, this will get much worse if you do nothing...), and added turn-down tips on my exhaust pipes (from Vetterman). No further crack development, and less drumming on the floor under your feet.

Paul
 
I discovered one such crack at about 100 hours flight time. I stopped drilled it. No further cracking observed at 330+ hours. I still have the straight exhaust pipes.

Chris
 
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vibration damper

I've read of this occurring with several other folks as well. The same thing happened with the BD-4 I built. I ended up bonding 1/2" stryofoam to the inside of the floor skins. Not only did this prevent any further cracking but the sound levels in the cabin decreased.

I'm getting closer to installing the forward bottom skins on my -8 myself and will probably do the same thing I did with my BD-4.
 
it's interesting to see that others have had problems in this area. I've seen damage in the exhaust ramp area from exhaust pulses but not this far back. It makes sense though, the back of floor ribs flex a bit and probably oil can in flight.

I went ahead and stop drilled the crack, installed a small patch with two cherrys and some click-bond to help stiffen it up. Maybe it's a little overkill but seeing that crack was kind of like seeing that first shopping cart dent in a new car. It's gonna happen but still......

I'm going to call Larry V. this week and see what he says, also I'm going to get a dynamic prop balance just to be sure some weird vibration isn't working it's way down there.

Thanks for the info guys!

Paul Danclovic
Jamestown NC
RV-8A N181SB
 
Same

pauldan181 said:
Has anybody had a crack at this location? Any idea what might cause this and any ideas on a permanent fix? For now I was just going to stop drill it, bond a small patch over the area and keep an eye on it.

The inside of this area is completely inaccessable so a real structual repair would require major surgery.
Click to expand...

I recently discovered the same cracks, at the ends of both ribs. Stop-drilled for now, will apply a patch if necessary. I do have exhaust down-turns but they're very gentle, and the exhaust tends to pound the floor during climb out. The price of speed...
 
We had a recent thread about "root cause". The root cause here is an unsupported rib end. Lack of support merely makes it susceptible to all kinds of stresses, including exhaust-induced vibration. What we need is some support.

For QB kits and finished airplanes, the support would either have to be external (skin doubler, external rib, etc) or something injected internally between the belly and floor panels.
 
DanH said:
We had a recent thread about "root cause". The root cause here is an unsupported rib end. Lack of support merely makes it susceptible to all kinds of stresses, including exhaust-induced vibration.
Click to expand...

Agreed, but just for argument's sake, couldn't we also say that exhaust pipes that are too short are the root cause? Without both conditons there would be no cracks.

There were plenty of places that looked like needed beefed up as I was building and had to resist the urge to add doublers etc. I do recall the plans calling for tank sealant on these ribs much like the flight control stiffeners.

Surely there are some 8's out there with hundreds of hours, short pipes, and no cracks. Maybe the root cause is something not yet thought of.

Paul Danclovic
Jamestown NC
RV-8A N181SB
 
Turbulence

pauldan181 said:
Surely there are some 8's out there with hundreds of hours, short pipes, and no cracks. Maybe the root cause is something not yet thought of.

Paul Danclovic
Jamestown NC
RV-8A N181SB
Click to expand...

Not sure about the first part of this question, but I think that in addition to the exhaust there is considerable turbulence at and downstream of the cooling exit. There has been at least one video posted of tuft tests that show this. You can also see it in the pattern of oil streaks on the belly - usually not straight back. This turbulence might contribute to flexing the belly skins?
 
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