Belly skin/Lower Wing Skin deformation??

crabandy · #1 03-09-2015, 05:57 PM

I'm not exactly how to word this, RV7 with 95 hours currently doing the first condition inspection. Not sure how I didn't notice before, but while running wires through the outboard seat ribs up top I noticed that the belly skin was arched up and would "pop can." Underneath the airplane between the front spar and rear spar, it appears the flange of the outboard seat rib rivet flange has been tweaked upwards. Now that I'm looking for it it is very noticeable, both left and right sides are the same.
Right side, you can see the curvature of the skin reference the ruler on the outboard seat rib bay.

Left side...

My non-engineering simplistic thoughts may attribute it to the weight of the wings while on the ground?

I don't have negative G systems and the airframe has only been to -1 G for testing, ideas?

ChiefPilot · #2 03-10-2015, 06:26 AM

If it is the weight of the wings doing this, it'd certainly be easy to prove. Does it go away if a wing is lifted off the ground? If so, then that's a bad deal - something is amiss. Something to be checked, for sure, but seems unlikely.

What does the seat rib itself look like - any sign of deformation?

BillL · #3 03-10-2015, 07:27 AM

Talk to the Vans guys, but I suspect they will say no problem. It looks like some very minor geometry in building allowed the actual wing installation to compress the extended lower fuse skin from the building phase. Or, it was jacked up with a pad and stretched the skin?

It still will carry the +g loads when flying and -g loads it was not (minor) structural anyway. Just my opinion, so when you talk to vans, be sure to ask them to discuss with engineering so you get that depth of response.

Larco · #4 03-10-2015, 07:59 AM

If I were looking at this I would remove the screws from the overlap of the wing and fuselage skins and see what happens with the fuselage skin. Is it possible that the skin overlap screw holes were drilled while the wing was out of position and now have loaded the bottom skins???
Out on a limb here but this looks weird to this multi time builder.
Larry

crabandy · #5 03-10-2015, 01:52 PM

Thanks for the replies, I spoke with Ken at Van's who basically said no problem. Like BillL said probably geometry mis-alignment when built.

I did watch it as a partner lifted a wing, no change. I removed the screws from the bottom belly/wing skins and the arch/bow in the bottom skin went away. All the holes/dimples/screws still fit very well, nothing I could see moved. I reinstalled the screws and everything fit as I remembered when I assembled it a year ago.
Same vantage point but after removing and re-installing the screws.

Is it possible that when flying the wings bend, the screws hold the belly/wing skin joint in the new position and that new position provides just enough tension at rest to cause the belly skin to bow/arch up?

BillL · #6 03-10-2015, 02:08 PM

Good to hear you got this sorted out. Just for grins, I would be sure all the wing spar bolts are in place ( not just the 4 big ones) and torque checked again. I know this is a pain, but the fact that it seemed to change from first assembly, it is just prudent.

JonJay · #7 03-10-2015, 02:26 PM

I agree with Bill.
Also, I noticed in the first few years of flight, the wing overlap screws would take some tightening up. Not a lot, but I could get a 1/8 turn or so on most of them. In the last few years, they are not taking anymore tightening.
Why I no longer can take up any slack may or may not have anything to do with it. Just another data point.

crabandy · #8 03-10-2015, 03:40 PM

Quote:

Originally Posted by BillL

Good to hear you got this sorted out. Just for grins, I would be sure all the wing spar bolts are in place ( not just the 4 big ones) and torque checked again. I know this is a pain, but the fact that it seemed to change from first assembly, it is just prudent.

Actually they're easy to get to at the moment, one of the last items on my condition inspection "to do" list. All the bolts are there including the AN4's in the service bulletin, I didn't have my build manual with me to get the torque values.

I've had a wrench on just about every bolt/nut on the airplane, more than a few took a quarter turn or more. Starter solenoid lug had a loose nut that looked tight, 1 bolt holding the horizontal stabilizer to the longeron took at least 1/2 a turn, the engine sump bolts all took 1/4 turn etc. I know all of these were torqued before my first flight. None of the rotating type assemblies were loose, just the stationary ones.

Thanks again for the replies!