Our First Wing Skin!
The picture above shows the first of 10 wing skin parts we will be making in order to build our wings. Here is what went into making this part….
Plugs, molds and wing skins
Since October, we have been working on making molds for our wing skins. We decided to create a “plug” of the wing cross section and use that to make an upper and a lower wing skin mold. The plug is about 6 feet long and is a very accurate representation of our wing’s airfoil. We got help from our fellow EAA 663 member Jack Burke and his homemade CNC machine. His machine accurately cut 8 ribs matching Chuck’s design. In early October, about the time we moved into the new hanger, we glued together the ribs and spars to form the plug’s skeleton.
Three views of the framework for the wing plug. The 8 ribs were cut on Jack Burke’s CNC machine. We then glued and nailed wood spars onto the notches in the ribs. The plug is now ready for us to glue on a plywood skin. (October 19-21, 2009)
Next we glued a thin birch plywood skin over the framework. In order to get the plywood to bend around the leading edge without cracking, we soaked about 12 inches of the plywood in an ammonia and water mixture to soften the wood fibers. Other than a small mishap in which Darryl’s wood, cardboard and polyethylene sheet trough decided to come apart, the set up worked pretty well.
Our makeshift water trough Getting ready to glue the plywood onto the plug frame. (Oct. 22, 2009)
containing ammonia and water.
(Oct 21, 2009)
Part way there! (October 22, 2009).
The plug with top plywood skin drying with strap clamps keeping it on tight (October 22, 2009).
Bottom plywood skin glued in place (October 29, 2009).
Here is the plug after we have put a layer of
fiberglass over the plywood skin. February 2-10, 2010)
After fiberglassing the plywood skins on the plug, we found that the surface was not nearly as perfect as we had hoped! There were lots of peaks and valleys which we needed to fixed. We first tried a plaster-like product called “ProForm”. We figured that it would be easy to apply and sand down. Unfortunately, the plaster did not stick to the fiberglass. When we started sanding, the plaster just chipped off. So we removed all of the ProForm and went to “plan B” which was to cover the entire plug with epoxy and glass microspheres (also known as “micro”).
Chuck squeegeeing on ProForm plaster and Darryl inspecting the dried plaster (February 10-11, 2010).
Because of the cold weather and other outside activities (like the holidays and Darryl and Renee buying a new house) we did not accomplish a lot on the plug between November and February. We did, however make the seat back bulkhead for the fuselage (see below).
Seat back bulkhead, forward facing side. Seat back bulkhead, aft facing side.
Completed seat back bulkhead with cutouts (November
Completed seat back bulkhead with one luggage cover
in place (November 30, 2009).
Getting back to the wing skin plug, we spent most of the month of February coating the plug with micro and carefully sanding to get as smooth a surface as possible.
Here is the plug after we have covered it with a slurry of micro. We spay painted
the micro surface with green paint so that we could see where the low spots
were as we sanded it down. Note the 7 foot long aluminum extrusion that we
used to sand the surface.
We did several coats of micro, followed by a lot of sanding. After a few cycles of this, we got smarter and began to use bondo get the final low spots filled in. By the middle of March we were pretty happy with the overall shape of the plug, so we began applying a “high fill” primer on the surface and carefully sanded to get the final surface we wanted. This did this step several times for both the top and the bottom sides of the plug. Here is what the plug looked like at this stage:
Plug with surface primed and sanded (Mar 18, 2010). Plug ready to be waxed (Mar 26, 2010).
Here is Chuck looking pretty pleased with the wing plug. We are
now ready to make the top and bottom molds for the wing skins
(March 29, 2010).
On March 29th, we made the bottom mold. First we clamped a very straight piece of wood onto the leading edge of the plug. The wood allowed us to create a flange on the leading edge of the bottom mold. When we make the top wing skin mold we will make a matching flange on it so that the two skins will mate precisely when we put the wings together. I will go into this later on in more detail.
Wood strip clamped to the leading edge. This will create a flange on the mold.
In order to make the bottom wing skin mold, we started by covering the bottom surface of the plug with a gel coat layer, which is made up of resin, Cab-O-Sil (a powdered glass material) and pigment. The Cab-O-Sil makes the gel coat layer very stiff, so that it doesn’t drip or flow. We let the gel coat cure for about 2.5 hours and then covered it with seven layers of fiberglass.
Chuck spreading the gel coat layer with a squegee (March 29, 2010).
Bottom wing skin mold after the seven layers of fiberglass had
been applied on top of the gel coat. Note that we used a straight
piece of wood to create a flange on the leading edge. The flange
will be important later on when we make the upper wing skin
mold March 29, 2010).
Here is a picture of the mold after it has been trimmed and placed
back onto the plug (March 31, 2010).
The next step took some thought on our part. Basically we needed to put a backing on the flange and also make sure that the flange edge had a clean and sharp corner with no gaps or pockets. To do this we attached the mold to the plug by inserting screws all around the side and back edges to make sure it would not shift. We then applied strips of fiberglass onto the back side of the leading edge flange.
Next we clamped the wood backing on top of the wet fiberglass and then filled in the gaps with a mixture of cotton fibers and resin (also known as flox)..
Wood backing clamped onto back of Adding more flox to fill in the gaps on
flange. the wood backing.
Since the mold was still pretty flexible, the next step was to make it as rigid as possible. We did this by adding foam stiffeners going lengthwise and chordwise while the mold was still attached to the plug. We fiberglassed over the foam which resulted in a very stiff and inflexible mold. Here are a couple of pictures of this process:
At this point, we were able to start making the top wing skin mold. Basically, leaving the bottom wing skin mold attached to the plug, we turned over the plug and then fabricated the top wing skin mold the same way we did the bottom.
Here is a picture of the top wing skin mold. The leading edge is towards the
left and the trailing edge is on the right side of the photo. There is a flange
on the leading edge that was created by presence of the flange on the lower
wing skin mold which we left in place when we made the top mold. Notice the
bottom wing skin mold in the background (April 28, 2010).
After finishing the layup of the top skin mold. we removed both molds from the plug. We now concentrated on the finishing of the bottom wing skin mold. First we needed to attach a wood extension to the leading edge flange. This would allow the fiberglass of the leading edge to be supported. We would then trim most of that away. Here is what the extension looks like:
Here is the mold with the wooden extension screwed into place. The mold has been coated
with 3 layers of wax.
Before we tried to make our first lower wing skin, we made two test parts that were only 10″ wide. From the tests, we concluded that we would do the skin in two separate vacuum bagged layups. The first would consist of the two outer layers of fiberglass and the 1/4 foam core. After curing, we would then do a second layup in which we apply the single inside layer of glass. It would be a bit more work and would use more materials (peel-ply, release/cauls and polyester fiberfill bleeder), but would give us more consistant results.
Vacuum bagging the first bottom wing skin. This was actually the second lay-up
where we have applied the inside glass layer over the foam. The wet fiber glass
is then covered with Dacron polyester cloth (peel-ply) followed by perforated butcher
paper (“release/caul”). Polyester fiberfill (“bleeder”) is then added on top in order to
absorb the excess resin. The entire mold was then enclosed in a polyethylene bag
and vacuum applied. You can see some of the resin being squeesed out along the
edge nearest you in this picture.
The first look at our wing skin! The release/caul and polyester fiberfill bleeder have been
peeled back to expose the peel-ply and inner skin. The peel-ply is fairly easy to remove
leaving a nice clean surface which is ready to be bonded to the ribs and spars (June 8, 2010).
Here is the first lower wing skin sitting on its mold. It is starting to look like a wing!
(June 8, 2010)