Hi Wes,
I am in the process of building a Majestic. Mark Drela has
provided a series of suggested modifications for the Majestic, one of
which is to add carbon fiber to the spars. For this 110-in.-span
plane, he recommends tapered carbon fiber for the top of the spar.
The suggestions are similar to those for the Bubble Dancer.
Details are available on the Allegro web site.
http://groups.yahoo.com/group/Allegro-Lite/
For the Majestic, the prelaminated tapered unidirectional
carbon fiber strips are 0.060-in. thick and 0.5-in. wide at the center
tapering to about 0.014-mils tick and 0.25-in,. wide at the tip.
I purchased 48-in.-long 0.5-in.-wide strips. These don't go
quite to the wing tips. For the last segment of the tips, I used
0.007-in. unidirectional carbon fiber.
The bottom spar cap is under tension and needs only half the
carbon fiber. For the bottom spar cap, the prelaminated tapered
unidirectional carbon fiber strips are .030-in. thick and 0.5-in. wide
at the center tapering to about 0.007-mils thick and .25-in,. wide at
the tip.
For a 2-m-wingspan plane, all of the thicknesses can be
approximately halved. For one of my 2-m kits, I purchased 48-in.-long
double tapered carbon fiber. Look at the details for the Allegro
Lite:
http://www.charlesriverrc.org/articles/allegrolite2m/markdrela_allegrolite2m.htm
I have used end-grain balsa shear webs
between the spar caps and wrapping with Kevlar tow in a manner similar
to the Allegro construction. Also, I have used carbon fiber
carry throughs as spar joiners.
Mark Drela has pointed out that the spruce
or basswood spar caps normally supplied with a wooden sailplane kit
add nothing to the strength if carbon fiber is used. He
recommends substituting hard balsa instead. What I did was to
cut pieces of hard balsa the same width and thickness of the supplied
spruce spar caps. Then, lengths and widths of carbon fiber were
cut to match the length and width of the balsa pieces. Because I
used carbon fiber carry throughs, I sanded slight depressions in the
balsa where the carry throughs would eventually go. Before
vacuum bagging, I slid pieces of plastic sheet into the depressions so
that the carbon fiber would not be bonded to the balsa in those
areas. The carbon fiber was vacuum bagged to the balsa
with epoxy. Lacking vacuum-bagging equipment, a series of
weights could be used to press the pieces together while the epoxy
sets. After lamination is done, the edges of the carbon fiber
are beveled so that the edges won't cut the Kevlar tow. I have
also tried using Kevlar thread, but I prefer the tow.
Because the carbon fiber will add
significant thickness to the spar caps, you need to either deepen the
spar-cap notches in the ribs or you need to thin the spar caps.
I found that the thinning was easy to do. The balsa spar caps
were laid on the bench with the carbon-fiber side down. Pieces
of the original spruce spar caps were laid along each side.
Using the spruce as a sanding stop, I sanded down the thickness of the
laminated spar caps with the sanding bar running transverse to the
strips. This procedure makes the final laminated spar cap
dimensions exactly equal to those of the supplied spruce spar caps.
Having done this, the assembly of the wing is just as if the original
spruce spar caps were used. I used Tite Bond. CA can be
used, but this adhesive adds weight and is more brittle than
construction with Tite Bond (aliphatic carpenters glue) CA
really soaks into end-grain balsa.
Regards,
Dick
Building a new woodie RES and this time I want to reinforce the top and
bottom spars with carbon fiber. I've been looking over the CST and Aerospace
Composite Products web sites ending up with more questions than answers. I
know I should use unidirectional carbon fiber, but what's easier to use:
tapes or tow? What thicknesses are recommended? I don't have a vacuum
bagging system, so I'll have to use old fashion phone books to weight the
stuff while the epoxy dries. I did find an informational article by Bob
Vixie which helped out a lot, but he didn't talk about thicknesses. Anyway,
what does everybody recommend out there.
Wes Gibson
AMA 607029
LSF 7533
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Dr. Richard C. Williamson Phone: 781-981-7857
Room C-317 FAX: 781-981-0122
Lincoln Laboratory Email: [EMAIL PROTECTED]
Massachusetts Institute of Technology
244 Wood Street
Lexington, MA 02420-9108
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Dr. Richard C. Williamson Phone: 781-981-7857
Room C-317 FAX: 781-981-0122
Lincoln Laboratory Email: [EMAIL PROTECTED]
Massachusetts Institute of Technology
244 Wood Street
Lexington, MA 02420-9108
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