Struggling with tank skins
 

I am building the ranks for my 4. I have the ribs clecoed to the rear baffle, that assembly is on the wing and I have the top of the skin match drilled. I have been trying to bend the skin over the leading edge with cargo straps and I need so much force on the straps that it is crushing the inboard rib but still not lying down on the ribs on the bottom surface near the leading edge. I have 4 cargo straps.I bent a radius into the skin as per the manual. With no straps and the skin clecoed on the upper surface of the ribs the skin sticks out a bit below horizontal. But to get it to lay down with straps alone is impossible. I tried slipping some 2x4s under the straps and it helps, but the skin won?t lay down tight just aft of the leading edge on the lower surface, like the first 1 or 2 holes. I know that if I match drill everything else with that gap I won?t have a good airfoil shape. The gap is about 1/8?. The straps are tight like a guitar string and things are bending and distorting. I know something is not right.

This is a 30 yr old slowbuild kit, as I assume all the 4s are. I know 1000s have been built successfully so I am not sure what I am doing wrong. The wing skin is from flat stock. The only radius in the LE was put in by me. I don?t know if skins from the factory have the radius pre-bent. I am pretty sure that the bending, or lack there of, is the problem but I am afraid to push down more, creat a crease and scrap the skin. Any advice greatly appreciated. Any pics on a build log of how to strap down the rank skins, or pics of how pre bent tank skins should look prior to installation also appreciated. I really want to finish the rank before the end of Christmas vacation.

Check ribs?
 

Agree that something is not quite right, it should not take that much pressure on the cargo straps. Perhaps it is due to the shape of the nose ribs, I recall needing to spend some time tweaking the flanges and fluting carefully to get parts to fit. My 6A is similar vintage and wings are the same as -4 I think, but I can't remember if the tank skin was already pre-curved by Vans. I used the plastic strapping that Vans supplied to hold on the skin, from memory it was assembled in a cradle off of the wing.

My old, old RV-4 project came with factory pre-bent LE and tank skins.

Looking at the edges of the skins, there were sharpie markings apparently to be used in the bending brake. If I remember correctly at least 4 or 5 such marks.

https://drive.google.com/open?id=1ho...D4EpwYc3-uyf23

Picture of LE skin, but may give an idea of the number of "bends" the factory used.

Sorry can't be of more help.

Finn

Added: I did my drilling on the spar with spacer blocks between tank baffle and spar, main skins already drilled and clecko'd, and was able to tighten the 3 or 4 cargo straps against 2x4 blocks on the rear spar. I did the LE skin before the tank.

Ok that’s interesting. I am doing it the same way, with spacer blocks. It must be the LE bending. I am not sure how to bend it more without creasing it. Thanks for the info.

To me it sounds like your skins are too far forward on the top, moving the sharp radius of the skins beyond the sharp radius of the ribs.

Are you using the proper size spacers behind the tank baffle? I cut several blocks out of MDF and had to experiment with different sizes to get the skin just right.

Same here.
Try to prebend the skin, the closer the spacing the better, right and left skin at the same time. A deluxe way would be to fabricate tank rib form blocks outta wood and form the skins over them using the straps.
And check the the thickness of the skin, just wanna be sure it is of the correct 0.32 and 2024-T3 gage...

Thinking about this a bit more, it probably is the LE bending. The factory bent those in my kit, like the other poster said, by making a number of smaller bends close together, presumably on a bending brake. Can you get access to a bending brake? It will probably be a slow and iterative process to make successive bends so as to match the LE rib radius.

This pic shows the amount of leading edge pre bend from the factory with my RV 3B wing kit. It played quite nicely without undue force. If (as you say) you are starting with flat stock then a substantial pre form of the leading edge will make it whole lot easier. Hopefully your predrilled top holes with still line up.

Russell

The trick
 

Should have drilled the bottom skins first.


Cm

On my RV-3B, I used Howe Fittings, the wooden gadgets, to help with the shape.



These changed the angle of the tiedowns and gave a better shape.

If the leading edge portion of the rib starts to crush down, you can use supports like these. These were on the outboard part of the wing, but the same would work for the tanks.



The threaded rod kept the ribs in position.

Dave

Yes I had to experiment with that.

Dave thanks for the pictures that?s awesome. Holds the ribs in perfect alignment to.

Thanks Russell. My skins are only at 90 deg. That?s the big difference. I wrestled with them some more and I think I have it now. Thanks and thanks to all who responded especially with pictures. Makes a huge difference.

Look up (vacuum forming leading edge skins) on YouTube. There are all sorts of videos on doing it with a shop vac.

One more tip
 

I did the leading edge skin first.

With the W-423 strip in place, you have a reference for the tank skin. Not sure how it would work the other way.

Finn

New RV4 tank skins
 

Worse case scenario I have a set of RV4 tank skins and rear tank bulkheads I would part with if things dont work out.