The craziness continues!
My continuing flirtation with Aviation:
I've purchased a 90% complete Mitchell U-2 Flying Wing!
(Which in Aviation math usually means, 90% is left to be done.)
Manufacturing a new front Deck
Ok, some apologizes are in order - it turns out that I am not good about taking the camera to the airport with me. Exactly one week after we got back from Oklahoma I moved the bulk of the Aircraft to a "community" hanger at Mack Mesa Airport (CO7).
The original deck was unfinished and the shape was no longer rounded - it had a bunch of flat spots between sharp bends - totally unacceptable. I had initially planned to use the existing front deck in my garage as a plug in order to produce a mold for a new fiberglass deck but then I discovered that it was just didn't set correctly on the bench, so I had to change my plan and I re-installed the deck on the fuselage in order make sure it was stable and squared up to the plane.
The first layer of dry wall compound is dry. I will have several trips to the airport to sand and fill until the shape is correct. Then the plug will be sealed using micro, sanded smooth and a final coating of epoxy will be applied.
|Finally finished with the
dry-wall compound. Next step was to apply a thin (and sloppy) coat
of micro to fill the porosity of the compound. Looks like it will
take more than one coat of micro to fill all the scratch marks left in the
One of the local mechanics walked by and asked "What's this process?" The dry-wall compound had convinced him that I might be nuts to use such a product on a flying part. I assured him I was just making a plug to use to build the deck and he seemed to be relieved.
|After several days of
filling, sanding, waxing and buffing, I think the front deck plug,
although not perfect, is as ready as it is going to ever be. I have
decided not to make a female mold but rather I will cast the front deck
skin directly from the plug.
The deck skin layup will be one lamination of 10 oz fiberglass and 2 laminations of 7 oz Carbon fiber. Formers and stringers will be needed to achieve the proper stiffness.
I have found that carbon fiber cloth is very squirrelly and will deform badly when handled after being cut. But applying painter's tape and then cutting the cloth through the tape helps the cloth hold it's shape. The excess painters tape will have to be trimmed away later!
|Sorry but I couldn't get
photos of a couple of the steps - couldn't touch the camera with epoxy
After cutting the fabric it was saturated with epoxy and then covered with a perforated plastic film.
Here we are covering the perf sheet with an absorbent cloth (wicking layer).
|Once the wicking layer was
in place, a top sheet was taped down to seal the layup in an air tight bag
and the vacuum pump turned on.
The vacuum removes air from between the laminations of the layup and also helps remove any excess epoxy.
Here you can see the excess epoxy coming up through the perforations into the wicking layer.
|After the layup begins to cure but not harden (called the "green" or "B" stage), we removed the layup from the bag and placed it on the front deck plug. Then it is cut and pressed approximately into it's final shape.|
|The new front deck will be
allowed to cure for about 24 hours. After being cured it will be cut
down to the final size. There will be some sanding and filling with
clear epoxy before the Marine varnish is applied to protect it from UV
(No, I don't know if that brush ever did slip off the leading edge!)
After it cured we found several flaws - so this part is a reject (an expensive reject).
|Back home in the garage:
We learned enough from the first attempt to decide that we could make the second attempt at home in the garage.
|See the "funny"
ripple along the right edge of the carbon fiber? That is the worst
one but there are others - hence the rejection.
But we decided that we can use this plug to form a shallower front deck which is actually more like the original drawings.
So on to the second try!
|First thing we did was to extend the front and back of the form by 2 inches at each end.|
|The final part - ready for sanding and final coats of automotive Clear Coat after the formers are epoxied in place. There were some minor problems during the vacuum bagging steps but the part is marginally acceptable.|
|First we made a composite sandwich from 1/4 in. thick foam covered on both sides by a lamination of 7 oz fiberglass. Then we cut the formers from the sandwich and sanded them to size. Before the formers are installed approximately 1/4 in. of the foam is removed from the edges of the sandwich. The resulting groove is then filled with a mixture of resin, cab-O-sil, micro- balloons and flox. The mixture prevents delimitation of the sandwich and adheres the two parts together.|
|The front and back formers
are clamped into place while the epoxy cures. After the epoxy cured
the clamps were removed and the nose and it's internal former were mated
to the front of the plane and epoxied together. Finally, all that is
left is the final finish prep and spray painting with Clear Coat.
This is the end of the manufacturing for the cowling (front deck).