SEN-414 May 7 2000
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News and Reports 2000 - First half
SCAT Electronic News 7 May 2000 Issue 414
Table of Contents
Looking for address - East
Cutting Ribs - Hinson
Victor's ribs - Fantham
Rib making techniques. - Blackam
Return of the Oppegard Rubber Stripper - Gelhar
Looking for address
Could you or one of the SCAT readers possibly furnish an e-mail address
for Henry Nelson.
Since I was the one that wrote the original article about using bicycle
spokes to hold rib blanks together, I will expand on the technique. I
make F1B ribs using this system. You do not have to just use bike
spokes, you can use 1/32,(1mm) or any size that suits. The holes in the
blank rectangles only needs to be about 1 to 1 1/2 inches (25-40mm)
apart. The holes should be in the thick part of the airfoil. The wires
are only used to keep the blanks from twisting out of position.
You need a template for the root, the dihederal break point and the tip.
I have both metal and plywood templates. Metal is best, but difficult to
make. Plywood (1/16-2mm) works fine if you harden the edges with CA,
then color the edges with felt tip marker so you will know if you sand
too far. I have a sanding block that has the rib undercamber on the top
and the rib upper surface on the bottom. I don't use this so much as a
sanding block, I use it more to hold the stack while sanding . The
sanding block is about 5 inches(125mm) in length. First cut away as much
waste as possible on the undercamber side, then sand with a small
sanding block until you have the underside down to the templates. (main
panel ribs are done as a group, then tip panel ribs.) After the bottom
is near completion, lay the stack on the airfoil sanding block and sand
the top shape. The sanding block is used really as a preshaped work
surface. I make all of my F1B wings using this system to cut ribs. I
hope this helps with your wing work. Good to see that there are some
that still make their own.
If the heading makes you think this is going to be a Ukrainian BBQ recipe,
your sense of humour is as warped as mine........
Jason Magill asked about the laminated wing ribs. Victor reads SEN so he
may well answer the question himself. I believe the method is driven by
his need to produce a lot of ribs in a reasonable timescale rather than by
it being a better way of making them. The presence of the glue adds weight
and, given the carbon caps, the glue adds little stiffness. To keep the
weight down, softer wood could be used but this would make the wood/carbon
bond weaker and make local crushing and distortion more likely.
I think one-piece wood ribs are best - and you need to look at the tedium
of cutting them out as therapy. Put on some nice music and think about how
well the model is going to fly as you work....
I use a wing set-up with the same undersurface shape right out to the tip.
This means I only need one under-surface form. The tips have forward sweep
at the trailing edge and this causes built-in wash-out. My whole former is
twisted to give wash-in on the inside of the turn - distributed over the
whole span. The rib blanks are all sliced full chord and then graded for
weight - lighter in the tips, heavier at the root and round the dihedral
breaks. If required, they are tapered by the 'sandwich' method - put a
template at each end of the stack and then carve/sand between them. This
can be done on the top of the blanks only, and supported on the
undersurface form, because the lower surface is already the correct shape.
The sandwich is set up so that the low side of the tapered ribs is the
right height for the spanwise position of the rib. When they are finally
glued into the wing, a long straight sanding block is used to sand off the
unwanted triangular edge that is 'too high'. I produce the nose ribs and
'behind the spar' ribs separately to get the optimum grain direction in
each and get more out of a sheet.
Rib making techniques.
Victor Stamov told me he uses his rib construction mostly as a way of
maximising strength in the rib and minimising wastage in construction. It
is no easier or quicker than other methods.
As I recall Victor laminates the ribs from 1.5mm sheet, the grain going
chordwise and enough layers to make the required maximum thickness. To
facilitate the ribs' grain following the curve of the undersurface
without any built in self-straightening stresses he half slices each
layer of balsa every 15 or so mm across the grain. Because there are
several layers overlapping this is no problem.
The balsa layers and the lower CF skin are glued together with epoxy in
one action (I think the lower CF surface is two layers .08mm
unidirectional CF material but you may be only able to readily obtain
0.12mm uni material in which case one layer would probably do). This
assembly is weighted or vacuum-bagged down on the lower surface mould.
Once this is hardened the upper surface camber is shaped and the upper CF
skin is wetted out and laminated (either a dense foam press or vacuum bag
will work OK). When this is complete the ribs are sawn off the resultant
block with a diamond saw.
An interesting aspect is that Victor uses a removable waxed insert at the
front of the block so that the CF overhang that goes over the wing D-box
assembly is included in the process. (Hope this makes sense).
With more traditional construction (by sanding a 'block' of rectangular
rib blanks), it's important that the grain of the ribs is along the thin
rear part of the rib for maximum strength. Also for best strength to
weight use no heavier than 8 lb/cu ft C-grain balsa (the C-grain helps
you get away with lighter timber).
Holding the rib blanks together while sanding is important and an idea I
got from somewhere is to glue the rib blanks together at front and rear
with cellulose glue (ie balsa cement or thick dope), then pin the
templates at each end, carve and sand the undercamber first, then the
upper surface, then the front, then the trailing edge. Another good trick
for this process is to use a form approximately the shape of the
undercamber and to rest the block on that while sanding the upper
surface. Either pop the ribs apart or, if they glued more securely than
that, use acetone to dissolve the glue.
Alternative to gluing the blanks together, you should make special pins
for this job from .015 inch music wire, sharpen the ends with a wetstone
and bend a loop in the opposite end. You can make these as long as
required to hold your ribs together. I use pins from either side of the
block rather than going all the way through unless there are only a small
number of ribs.
In this case after constructing the basic wing form (D-box/ribs/TE),
apply ALL the CF caps (yes, top AND bottom at the same time) with 24 hour
setting epoxy (make sure to sand each cap for best adhesion) and strap
the wing assembly down on the wing jig/form with rubber strip wrapping
over each rib. This way your wing will be really accurate (hope the wing
form is accurate first!).
Return of the Oppegard Rubber Stripper
I am pleased to announce the return of the Oppegard rubber stripper. Mr.
Oppegard has recently passed away, however, a family member is resuming
production of these fine tools. A new web site has been started.
Thanks for the recent donationd from Rey Mazzocco, Chris Borland
and the members of the SCAT.