By Verne Koester
OVERVIEW
The method I'm going to describe utilizes Dean Pappas' (Flying Models Magazine)
concept of tying in all the hard points of a structure together to spread
the load over as wide an area as possible and involves the traditional plywood
plate, two shear webs, two false ribs, and four carbon fibre spars. Sounds
heavy doesn't it? Well, odds are, you've already got most of these items
in your wing. We're just going to tie them along with the wing tube spar
into one, very strong internal structure. If the materials are chosen wisely
and the adhesives used sparingly, the structure won't be any heavier than
what you've already got.
MOUNTING PLATE
The mounting plate you see in figure 1 should look pretty familiar except
for the hole and slot shown at the leading and trailing edge. Most kits
call for 1/4" ply for the plates but you can save a few grams by using
3/16" ply instead. We want the grain in the plate to run from the leading
to trailing edge to take advantage of the extra strength provided by running
the grain parallel with the forces being applied. The slot at the front
and rear edge of the plate houses the two shear webs. The hole at the edge
of the slot is used to fish a pintype jig saw blade through so that the
slot can be cut. Prior to starting on the plate, you have to decide what
material to use on the shear web and then find a jig saw blade that will
cut the slot to the proper width. I use .028 carbon fibre, but 1/16"
ply would suffice if you can find a jig saw blade to cut a wide enough slot.
Take your pick and then experiment with various jig saw blades on some scrap
plywood until you find the one that gives you a slot with a good fit to
the shear web. Here's the procedure step by step:
1. Cut a retract mounting plate out of 3/16" or 1/4" 5ply with
the grain running from L.E. to T.E. Size the plate according to the retract
used (usually about 2.5" X 2.5").
2. Drill the holes for the blind nuts that hold the retract but don't install
the blind nuts at this point.
3. Drill the holes at one end of the shear web slot that will accommodate
a jig saw blade.
4. Clamp a piece of wood to the table of your jig saw to act as a fence
and cut the shear web slots. The slot should run parallel to and 3/16"
away from the leading and trailing edge of the plate. The ends of the slot
need to be about 1/4" from the ends of the plate as shown in Figure
1.
5. Glue a piece of dowel in the holes used to start the jig saw blade in
the slots and sand flush when dry. Use a Zona saw to extend the slot through
the dowel.
PREPARING THE WING CORE
Now that the plates are finished, we can use them as templates to cut the
shear web slot into the foam core. Start by cutting the pocket in the foam
that the plate fits into. Sam Turner's KISS foam cutting tool is the best
tool I've seen for this job. The common mistake I see, and used to make
myself, is to cut the pocket parallel to the surface of the foam. If you
do that, you're transferring the dihedral built into the wing to your mounting
plate and the end result is that the struts won't be perpendicular to the
ground not good. Every kit I know of with foam cores has the dihedral built
in. So if you have the core resting in the top shuck with the bottom of
the wing facing up, cutting the pocket parallel to your workbench (not the
surface of the core) will ensure that the struts end up 90 degrees to the
ground. If you're using strips of ply or whatever to guide your cutting
tool, simply shim the strips off the surface of the foam to make them parallel
to your bench and the angle will be correct. All of this assumes that your
retracts rotate a full 90 degrees. The 85 degree variety are designed to
allow you to mount the plate even with the surface of the wing. The downside
to this type of setup is that the slightest amount of wear and slop will
allow the wheel to get out into the slipstream in the retracted position
which can cause trim problems. The secondary benefit to using 90 degree
retracts with mounting plates offset for the builtin dihedral is that the
wheel is driven deeper into the well when retracted. As the retracts wear
and loosen up a bit, they'll still stay inside the well. Once the pocket
is cut, use a tiny sanding block to square up the rounded corners left by
your cutting tool. It's really important to make sure that the mounting
plate fits absolutely flush to the foam for maximum shear strength so don't
omit this step. I made up a dinky little sanding block out of scrap 1/4"
ply and sticky back sandpaper that measures 1/2" X 3/4" that works
great. Now you can use the plate itself as a template to cut the shear web
slots in the foam. With the plate temporarily installed, use one of those
real long XActo blades to cut the slots all the through to the top surface
of the wing. Use a slicing motion as much as possible to avoid tearing out
beads of foam as you go. After they're cut, you can use a nail file to open
up the slots to the width of the shear webs.
CUTTING THE SHEAR WEBS
Cut a piece of .028 carbon fibre or 1/16" ply to the width of the slots
you cut into your mounting plates and long enough to go all the through
to the top surface of your wing and then some. We want the grain to run
vertically, or from top to bottom in the wing. Temporarily install the web
into your mounting plate so that it's flush to the top of the plate and
install the whole assembly into the wing.
Mark the excess that protrudes from the top of the wing, remove the assembly,
and cut the shear web to size. It's not a bad idea to mark each shear web
so that you get it back into the slot it was sized for when we glue everything
in. Set these aside for now.
FINAL POCKET PREPARATION
Install the blind nuts into your mounting plate with CA and paint the flanged
area of each nut with lipstick. Put the plate back into the pocket, press
down to transfer the lipstick marks, remove, and countersink the foam to
accommodate the blind nuts. This can be done with sharpened brass tubing
or one of those grinding stones that came with your Dremel. This step ensures
that you get maximum strength from your shear webs by keeping the plate
and web assembly absolutely flush to the foam.
FALSE RIBS
We still haven't installed anything permanently yet, right? Next we're going
to prepare and install a pair of false ribs for each wing panel out of 1/8"
lite ply. The first thing you need to know is that this stuff really isn't
1/8". It's actually a lot closer 3/32" so check to make sure.
The outboard false rib runs all the way from the leading edge of the wing
to about 1" behind the wing tube spar. We'll start by making the templates
we're going to use to hot wire the slot in the foam. I use cardboard that
I get from a local drafting supply outlet. This stuff is about the same
thickness as the cardboard that comes inside of a dress shirt and is glossy
on both sides. It cuts real easy with an Xacto knife and a straight edge.
Lacking that, 1/32" ply should suffice. Leave one end open to start
your wire at the leading edge and attach identical templates to the top
and bottom surface of the wing. I use push pins to hold them in place. Make
sure that this slot is positioned to intersect the outboard 1/8" of
the retract plate and cut the slot parallel to the root rib as shown in
Figure Three. You might want to test your template on some scrap foam to
make sure the width is right. Too wide requires a bunch of that heavy epoxy
and too narrow will push your leading edge apart when you glue the rib in
so get it juuust right. Go ahead and cut the slot in both panels. The KISS
foam cutter handles this chore with ease. We're going to use the wings as
templates for the ribs and they're in a really fragile state with the slots
you just cut, so be gentle! Slide a piece of 1/8" lite ply into the
slot and use a pencil to trace the airfoil as well as the notch where the
retract plate goes. Paint the end of a wing tube socket with lipstick and
push it in up to the rib to mark the necessary hole. Remove the ply and
cut out your rib as accurately as possible. We want to minimize any overhang
so that we don't build a twist into the panel when we glue the rib in. Once
you've got the ribs cut, tube hole drilled, everything sanded and ready
to go, place a sheet of wax paper in your top shuck and glue the rib in
with slow epoxy. Clean off any epoxy that oozes out and use strips of masking
tape to keep the slot from spreading apart. Set the panel into the top shuck
and press down on each side of the slot to make sure you're not twisting
the panel, lightly weight it down, and allow to cure. Once it's fully cured,
sand the rib flush to the foam. I lay a strip of 2" wide masking tape
about 1/4" away from, and parallel to the rib on both sides to protect
the foam. The inboard rib is prepared and installed exactly the same way
as the outboard rib except that it ends right at the opening for the retract.
I heat up a long Xacto blade with a torch to start a pilot hole for the
hot wire in the foam. Once that's done, we can install the plate and shear
webs.
FINAL ASSEMBLY
Glue the shear webs into the retract plates with 5minute epoxy and use acetone
and paper towels to remove any excess glue. Put some modeling clay or candle
wax into the blind nuts from the back side to seal em from epoxy. Mix up
a batch of slow epoxy and liberally coat the shear web slots, shear webs,
false rib notches, and the plate itself and install the retract plate assembly
into the wing as shown in Figure Three Carefully wipe and squeegee the excess
epoxy off the wood and foam and allow to cure. Once cured, trace the retract
flange onto the plate and glue in balsa filler blocks to bring the rest
of the plate up to the surface of the foam. Sand the balsa flush to the
foam using masking tape to protect the foam as mentioned earlier. Figure
Three also shows the optional use of carbon fibre and where it could be
tied into the structure for additional strength. This was the setup I used
last season and will use again this year. Only the bottom side of the wing
is shown for clarity. The top side is simply a mirror image of the bottom.
Essentially what we've done here is to tie the retract plates into the false
ribs, tube spar, root rib, carbon fibre spars, and the wing skins. In other
words, all the hard points are tied together ala Pappas and if these plates
move, there'll be a lot more to worry about than landing gear!
see ya......Verne
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