by Ardis Almond
First, the background. My plane is N71AG. You might have seen it at Oshkosh
in the show plane camping area in the last few years. It is white with maroon
trim and the most noticeable differences is Val Bernhardt's drooped tips and
oversize wheel fairings. It also has a modified Cherokee 140 instrument panel
with Piper wheel controls.
When I built my wings in the mid 70's, a lot of people were experiencing problems
with leaks. I had the "white" wing panels as compared to the ones with the
"green" tint. As I understand it, these are different types of fiberglass
and the materials used for repair might be different. I overlaid the fuel
tank area with very light dynel cloth and finished it with fiberglass resin
and micro balloon slurry. After a couple of years I started getting bubbles
all over the top of the tank area and pinhole leaks on the bottom. The
bubbles were apparently coming from vapor that was actually separating the
paint from the fiberglass. I fought this for years. I patched small holes
with pro-seal and repainted the fuel tank area several times. I began
to get delamination between the wings and the dynel. Now I had a problem of
a small leak in one area that may travel several inches until it
found another pinhole in the dynel. This problem got worse and
whole sections had to be redone. I went through the procedure to
cut holes in the top of the wing and try to fix them from the inside. Finally, I had
a pretty sorry mess.
In the repairs detailed below, I used the following materials (all from Aircraft
Spruce except the Pro Seal type product from Seal Pack Company, 2614
S. Hoover, Wichita, Kansas 67215, 316-943-9489) :
Fuel Tank Poly Resin* 6060-5-1G-----------1 gal.
Polyester Resin Catalyst-----------------------4 oz.
Surface Curing Agent ---------------------------4 oz.
Feather Fill #401-----------------------------------1 qt.
Glass Bubbles -------------------------------------1 lb.
Standard Fiberglass Cloth #7533 60"------6 yd.
CS3204** Class B-2 Two-Part Polysulfide based Aircraft Fuel Tank Sealant------------1
gal. kit
*This is an isophthalic resin used for making fuel tanks. Iso
resin is highly flexible. Has excellent adhesion to metal, wood, concrete,
fiberglass and other "hard to adhere" surfaces. Compatible with most fuels
except gasohol. Iso is a wax-free resin and must be over-coated with Type "A"
Surfacing Resin to obtain a surface cure.
**I refer to this generically as pro seal. It is the equivalent of PRC 890
Proseal brand of sealer.
I knew I had to have a more permanent solution. I removed the wings and
brought them back to my shop in the spring of '96. I removed the
flaps and ailerons so that I had only the wing to work with. I peeled off
the old dynel layer. It came off very easily. I sanded, cleaned, and
inspected the wing panels. I patched a couple of places where the panel joints
were leaking. I used pro seal for that. I had also marked every pinhole
spot that was leaking. I used the thick pro seal and forced it into the pores
in those areas. I smoothed it right down to the surface. I again sanded
the panels. Then I squeegeed the wing tank area with the standard pro
seal (I originally thought I had used the paintable pro seal, but in checking
my records, it was the paste type). While it was still wet, I wrapped the
entire tank area with a very light fiberglass cloth. I overlapped the
cloth at the trailing edge of the wing. Next, after the pro seal had set up
for a couple of days, a very light coat of resin was applied to the cloth.
It was wetted just enough to leave the cloth fairly rough. The panels had
some areas that were "dished or slightly distorted. I filled these areas
with a very light weight micro balloon/ resin slurry and covered these (after
set up and sanding) with cloth. After this set up, I finished the rough
cloth surface with a resin and micro balloon "composite type" finish. I
then repainted the entire wing. It looked nice.
The theory behind this is that the pro seal would seal the gas everywhere
and would not delaminate as the resin had done. The cloth would add tensile
strength to the whole wing and help keep the joints from trying to delaminate.
I thought this was better than trying to cover the fuel tank area with metal
because of the difference in expansion rates and different modulus of elasticity
values. I used the Fuel Tank Poly Resin because I thought the strength was
sufficient, it bonded well, it was flexible, and it would hold up better
if fuel did get in contact with it. Believe it or not, the weight worked out
to exactly the same as the wings when they were new. I was very conscious
of keeping the layers as thin as possible and the slurry as light as possible.
When I first built the wings, I just had a very heavy layer with the dynel
and heavy resin.
It worked great. My plane has been flying now for 6 years with the repaired
wings. I have had only a couple of small bubbles show up on the upper
surface and no stain. I had one small leak appear at the spar inner
panel interface and that was easily repaired with pro seal. The last problem
was unrelated to the skin problem anyway.
If anyone wants to know more, I will be glad to help any way I can.
See also: Ken Strite's fuel cell
repair article.