composite
construction of aircraft
by Tom Wright of Advanced Composite Tech, Somerset PA
About the Author:
Thomas J. Wright is the owner of Advanced
Composite Technologies and has been custom building aircraft since
1982. The company employees six fulltime staff members. They are
responsible for building over 30 aircraft of various types, currently
completing their eighth Velocity.
I find that building anything is a series of assembly and disassembly
steps. It seems to proceed like this: make the part, see how it fits,
temporarily fasten the part in place, determine how to make it fit better
and what to do to it next, unfasten and remove, go modify it and try again
until it is acceptable. To be able to assemble and disassemble quickly we
sometimes use “CECOS” but they are not always the best tool, especially if
there are nuts, bolts, and nut plates involved. So to that end I have
found a very quick and inexpensive device that allows assembly of a nut
plated part without using your good aircraft-quality screws and bolts. Go
to your local hardware store and find thumb screws one inch long, with
fine threads in each size you will be using, i.e. 8-32, 1/4-28, 10-32.
Also get knurled thumb nuts in the same threads and hex nuts all in fine
thread. For those of you who don’t know about threads, there are two types
of threads that are used on fasteners: national course (nc) and national
fine (nf). Aircraft bolts, nuts, nut plates are fine. The two thread types
are not interchangeable with each other. Continuing at the hardware store,
purchase a dozen or so thumb screws, thumb nuts and regular hex nuts, and
a can of red spray paint. Since these are all temporary fasting devices
that we don’t want to leave in our completed plane, we will spray
everything red so we don’t forget to remove. Install the thumb nut onto
the thumb screw now when you have to attach something to a nut plate. For
example, on an aileron hinge, simply place the screw through the hole and
turn into the nut plate until it gets tight, then turn the thumb nut tight
against the skin drawing the nut plate tight against the underside and
through, holding the part in place. The red nuts can be used anyplace in
place of your aircraft nuts; they spin on easily with fingers and are
easily removable, for temporary fitting purposes. If you can’t find this
hardware at a local hardware store, try a really big international company
Mc Master Carr, Cleveland, Ohio – they have everything man has ever
invented.
Again at the hardware store, usually in the paint
department, you can find sanding blocks that are made of spongy foam with
abrasive surfaces of various roughness/grits. These are excellent to use
for sanding and forming various concave edges, etc. For straight surfaces
I use 3M adhesive-backed paper for use on air boards: obtain a good
straight piece of 1x3x12 and screw another 1x3x12 right down centre to
form a “T”. Countersink the screws, then use the adhesive-backed paper on
the flat surface (it glues itself). Use whatever grit you want. When
finished, remove the paper in one piece with a putty knife. Cut in half to
make two pieces 3” x 6”. Fold each piece in half with the sticky backs
against themselves. Now you have good pieces of really tough sandpaper
that will not wrinkle when you’re trying to use it. It’s rigid enough to
get into thin, tight areas like the door gap space and make a good sand
line.
To save one complete cure-sand-fill cycle, here’s a trick
I’ve been using for years. After the parts have been laminated and
partially cured, apply your micro fill (the glass on the part must be
stiff enough that it does not distort from the application of the filler)
and squeegee. Another way to fill glassed surfaces is to sprinkle micro
powder directly onto the glassed surface while it is still wet. The micro
will soak up the excess wet epoxy. It becomes a thin film of filler that
chemically bonds to the glass. Spread it very thin, only enough to fill
the small valleys in the glass. Let cure then sand as you would ordinarily
do finish.
Tom’s Technical Tips: Part 2
As a builder of many types of aircraft, I have discovered many little
tricks that make working and building more enjoyable and less
frustrating.
To clean the sticky residue from masking tape of duck
tape from parts without scratching the surfaces, try a product called
"Opps," available at Wal-Mart. Simply wet the area, let stand a minute or
two and wipe off. Using a paper towel wetted and placed over the gooey
stuff also helps. It even works on Plexiglas, however the paper towel will
make fine scratches, so use a soft cloth towel for
Plexiglas.
Speaking of acrylic, why not protect it from the start.
As soon as you unpack your new parts, protect the Plexiglas by wrapping it
in your best bath towels.
Buy a product called "Spray Lat," from
A/C Spruce - sign shops also have it. Paint at least four dry coats on all
surfaces of your windows and windshield. Put at least six coats on the
inside of the windshield, the thicker, the better. Allow to dry between
coats. If you apply this product too thinly and try to peel it off, it
will tear away in small pieces and you will have an almost impossible job
to remove the film on your windows. Painted thick (four to five coats), it
is strong enough to prevent itself from tearing, and stays in one big film
which is easy to tear off in one piece, especially on the inside of the
windshield after the panel is in.
To trim your windows to size,
follow the instructions in the plans. After they are to size, peel some of
the Spray Lat from the edges all the way around, approximately 2". Apply
two layers of duck tape over the Spray Lat surface with the duck tape to
within about 1" of the edges. This becomes the bonding surface. Sand with
80 grit and install per plans. You can even clecoe these in if you are
very careful when you drill the holes. Use nothing bigger than 3/32
diameter for the initial drilling, then absolutely no holes bigger than
1/8", and use a Plexiglas bit. To keep the clecoes from sticking in the
epoxy, dip the ends in vaseline prior to installing in the holes. It makes
for easy removal after cure. Install clecoes from the outside so you don't
pull the Plexiglas away upon removal.
Do not use WD 40 or any
silicone based product any place on the surface of your airplane. The
fibreglass, even though cured, still absorbs the silicone. Nothing will
ever stick to the surface, including your epoxy and glass laminations or
fillers.
Use duck tape on surfaces to prevent laminations from
sticking to the surfaces. You can even make you own fibreglass angles and
various shapes. Simply buy angle the size you want, apply duck tape, glass
over it with four ply and after cure, remove. Now you have fibreglass
angles to use for you needs. Another way is to nail some 1x3 wood furring
strips together and duck tape them to use as a mould to make fibreglass
parts, angles, flats, etc. You can use your scrap pieces of bid for
this.
Use Permatex Antiseize to install together any aluminium parts
that must fit tight, especially elevator tube plugs which fit tight. They
never seize. This prevents the aluminium from galling into each other and
allows you to rotate the plugs freely for drilling.
Use containers
from grocery items for parts bins to sort and store items. Cut the top off
of any gallon jug, clean it out, and you have a container to mix large
quantities of epoxy or filler, especially big laminations, wing skins,
etc.
You can use a hole saw to drill a washer shaped piece out of a
plastic jug. Use as a wear washer between the aluminium seat hinges. This
prevents the aluminium pieces from wearing against each other as you move
the seat back.
Need to find a big wrench for the nose gear nut?
It's a 1 13/16 hex nut and that size open end wrench is very difficult to
find. Buying a complete set in that size is big bucks. McMaster Carr has a
vast assortment of inexpensive wrenches for less that $20. each. I
purchased a 30 degree angle thin-head short handle P. N. 5414A52 and it
works great. Just make a pipe extension to use for tightening the elastic
stop nut. It's really tight, but has to be to keep the nose gear fork at
he proper tightness and prevent shimmy.
When designing panels that
are somewhat complicated or difficult to visualize, instead of using the
actual foam to cut up and maybe get it incorrect, try using cardboard
pieces first as a temporary pattern before cutting the actual parts to
size.
To determine the correct length to make those hoses: Most of
us don't want inexpensive fuel and oil lines in our expensive airplane, so
we tend to go way over the actual requirement for hoses and they can get
expensive. In order to prevent having a line made too short or too long at
the hydraulic shop, obtain some inexpensive vinyl tubing from a hardware
store. Slip over the the fittings you need to connect. Route the vinyl
tubing over and around to obtain the best routing, even slipping over the
fittings. This is to simulate the actual hose in place. Mark the hose for
length and application. Now take this hose to the hydraulic shop where you
get your hoses made up, and have the more expensive hose made exactly like
your vinyl hose. They will fit every time. The hydraulic shop can help you
select the correct type for the application.
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