Wing construction with
Polyurethane Glue
by Ed Vollmer

Elmer's Pro Bond Polyurethane Glue

Every so often there comes into our experiences something new and exciting
that gets the juices flowing again. In years past it has been iron-on
films, CyA adhesives, "reliable" radios, etc. The latest item to hit my
"exite" button is Elmer's Pro Bond Polyurethane Glue.

I have just finished skinning the wing on my Lanier Giles 202 using this
material. It was used initially to install the spars in the spar slots and
seemed to work so well that I did some experimenting on scraps to see how
it would perform on the sheeting task. I used varying amounts of glue and
varying amounts of water to determine the amounts of each that worked best.
Even though the glue will foam up by itself, it seems that water aids this
process. I found that stringing beads of glue approximately 3/4" apart and
spreading it to a very thin film on the sheeting with a plastic scraper
worked quite well. Because of the foaming action, it does not take much
glue to generate a very good bond. The water was misted on the foam wing
with an atomizer. The amount of water was just enough to wet the surface
well but short of beading up. This worked so well for me that I decided to
install the cowl ring into the fiberglass cowl with the glue also.

There is a small weight savings in using this type glue over using epoxy
in that not nearly as much is required to do the job. Because of the
expansion of the glue it is not necessary to use as much glue to fill any
voids and assure a good joint. Also, because of the expansion, the wing
core does not need to be sanded as smooth, but just freed of any high spots
with a long block sander.

The process is as follows:

SPARS
As is usually the case with spar slots in foam wings, the slot was rather
oversized compared to the spar. This usually causes us to fill the void
with epoxy, adding unnecessary weight, just to get a good attachment. The
foaming expansion action of the PolyU glue is tailor made to solve this
problem.

These processes are done to one side of the wing at a time. The wing was
placed in one of the core shucks and a bead of PolyU glue was placed in the
bottom of the each slot. The spar was heavily misted with water and placed
into the slot. A strip of 1" masking tape was then placed full length of
the spar to confine the expansion to the slot. A strip of waxed paper is
then placed on the wing and the top shuck placed atop that. This assembly
is then weighted down much as is done with an epoxy sheeting process. I use
steel weights that evenly distribute the load over the assembly. The
combined weight is approximately 150 lb.. Because I am in no hurry, I leave
this to cure overnight.

Upon removing all the weights, shucks, etc. I found that the glue had
filled the voids and even sent "runners" into the pores of the foam itself
for about 1/2 inch. This gave additional strength to the bond of the spar
to the foam.

SHEETING
Applying the sheeting on this wing was made easier by the fact that it has
open bays with capstrips. This allows the sheeting to be applied in smaller
pieces than a fully sheeted wing. The individual sheeted areas were applied
one at a time. The sheeting was positioned on the wing core and held in
place by masking tape. The tape was located such that it prevented the
expansion of the glue into areas it was not desired. The sheet edge at the
spar was taped full length to prevent the glue from entering the bay area.

After locating the sheeting it was folded back over the tape and PolyU
glue applied as noted earlier and spread to a very thin film with a plastic
spreader. Water was then misted on the foam core where the sheeting would
lie. As soon as this was done the sheet was folded back into place, waxed
paper separator installed, top shuck installed and the assembly weighted as
noted earlier. Again, because I was not in a hurry , this was allowed to
cure overnight. There is probably some reduced amount of time that could be
allowed for the cure, but I will let that be someone else's determination.

After the large sheets were in place the capstrips were installed with
Gap-Filling CyA that does not attack foam. The bays were then lightened by
"troughing" or "pocketing" out the individual bays on each side of the wing
to leave a 3/8 inch thick web in the middle of the wing. The web left in
the wing affords some amount of transverse strength remaining in the wing
while at the same time lightening the wing and preventing the covering from
contacting the foam core.

The "troughing" was done with a Weller soldering gun to which #12 or #14
copper wire from household "Romex" wiring was installed. The wire was bent
to the desired shape to just fit between the capstrips to the desired depth
and ledges bent in the wire to ride on the capstrip. The capstrips were
protected from the heat the wire with two layers of masking tape. The web
is formed by allowing the lower wire cutting edge to raise by laying the
gun at a progressively decreasing angle from vertical as the cut is made.

The remaining leading edge and end caps were then applied and the wing
sanded for covering.