F-808 Smokers

DanH · #1 02-07-2021, 09:55 AM

I’ve developed a few smoking rivets at 950 hours or so, belly skin, at F-808, photos attached. QB fuselage, vintage 2005 or so. The rivets appear to be driven acceptably. There are several possible fixes. Right now I'm more interested in the cause... which may change the best choice of fix.

Offhand (no hard evidence), I’d guess the belly skin between F808 and F809 may be pumping due to exhaust pulse. I use a 4-into-1 with a 45 degree downturn. The typical exhaust belly stain runs the full length of the aft skins.

Mine also lacks the portion of the lower cowl exit which normally extends a few inches below the plane of the belly.

The skin forward of F-808 is braced by the two big baggage floor ribs. I have a dollop of silicone injected between the belly skin and the floor forward of the spar, where the floor ribs come to a point and cracking is common at the aft most rivet. Still crack-free there.

So, looking for comparative data. Anyone reporting loose rivets at 808?

DeeCee 57 · #2 02-07-2021, 10:35 AM

mmmh... are those really smokers? Maybe it’s the picture’s lack of pixels, or maybe gotta clean those glasses, but the paint around the rivets looks intact.
Dirt telltales maybe?

Carl Froehlich · #3 02-07-2021, 12:10 PM

Dan,

I did not like the long run on the bottom skin between F808 and F809 ribs. On both the RV-8A and RV-8 I added a 0.032” 3/4”x3/4” angle between the two bulkheads and riveted to the bottom skin. I have a bracket to attach the angle to each bulkhead as well.

The RV-8A has ~1400 hours and all is well.
Carl

DanH · #4 02-07-2021, 01:26 PM

Quote:

Originally Posted by DeeCee 57

mmmh... are those really smokers? Maybe it’s the picture’s lack of pixels, or maybe gotta clean those glasses, but the paint around the rivets looks intact.
Dirt telltales maybe?

Belly photo was taken after cleaning. Belly cleaning part of my annual. The marked rivets were streaming the usual telltales. Note you can't very well see the unmarked ones...unbroken base/clear.

DanH · #5 02-10-2021, 07:49 AM

Looking for anyone who has experienced belly loose rivets at F-808.

Vans support says it's not typical, so best theory right now says it's specific to my configuration. Comparison to another example might be useful.

Scott Hersha · #6 02-10-2021, 06:41 PM

Your configuration - IO-390, Hartzell BA prop? That configuration is probably not that common, although I know that Vans has approved the IO-390 on the -8’s. Might not find that many similar examples with your flight hours/experience. I remember early RV’s with thin skinned elevators and rudders (.016) having issues with cracks and smoking rivets, attributed at that time to prop pulses from higher horsepower prop pulses. The remedy was .020 skins on those surfaces - I think. That may not be your issue, but I liked the idea from Carl, to add an angle belly support between the F808 & F809. Might be an oil canning issue, which the stiffener would resolve.

scsmith · #7 02-10-2021, 07:22 PM

Quote:

Originally Posted by Carl Froehlich

Dan,

I did not like the long run on the bottom skin between F808 and F809 ribs. On both the RV-8A and RV-8 I added a 0.032” 3/4”x3/4” angle between the two bulkheads and riveted to the bottom skin. I have a bracket to attach the angle to each bulkhead as well.

The RV-8A has ~1400 hours and all is well.
Carl

This was going to be my recommendation for an improvement. I think regardless of exhaust, there is a tendency for that panel to oil-can. Even more so on the RV-7, I've seen it.

PaulvS · #8 02-10-2021, 07:33 PM

Quote:

Originally Posted by scsmith

This was going to be my recommendation for an improvement. I think regardless of exhaust, there is a tendency for that panel to oil-can. Even more so on the RV-7, I've seen it.

If retrofitting such a stiffener to a completed fuselage, what would be an effective method of dimpling the skin? The skin is too thin I guess for machine countersinking, and the pulled rivet tool dimplers make a dimple that is a bit on the shallow side.

dpansier · #9 02-10-2021, 07:53 PM

Question: Would an angle similar to Carl’s extending between the bulkheads and pro-sealed to the internal fuselage surface solve the oil canning issue?

DanH · #10 02-10-2021, 09:31 PM

Quote:

Originally Posted by Scott Hersha

Your configuration - IO-390, Hartzell BA prop? That configuration is probably not that common...

Kinda doubt the specific engine and prop choice is a factor. I was thinking more about other folks with exhausts which point mostly rearward rather than down. The best clue is probably the exhaust deposits. Not oil, but a deposit which feels like fine sandpaper.

Something is working that skin. If the cause remains a mystery, I'll just proceed with a stringer and monitor it going forward. Carl, Steve, thank you for the suggestion.

Quote:

Originally Posted by dpansier

Question: Would an angle similar to Carl’s extending between the bulkheads and pro-sealed to the internal fuselage surface solve the oil canning issue?

Me like. Should work fine for the skin, and it's easy to fab a good tie to the bulkheads.