Well Bill, you already know my response: what you refer to as option 2
is worth it becuz it is stronger, lighter, and easier to build. I've
done it both ways and I'll never go back to option 1. I'd be interested
in others' experiences and comments. The difference between Mark's spar
and mine is that mine is built externally and fitted into the wing; the
spar goes thru the ribs, whereas in his design the ribs go through the
spar. His would probably turn out marginally lighter. After accounting
for the difference in joiner systems, my 2-piece 2M Dark Star wing
weighs about 10% more than Mark's Allegro-lite wing. Both will take the
fullest-pedal zoom your nerves can stand.
--- Bill Harris <[EMAIL PROTECTED]> wrote:
> Here are a couple of practical applications of techniques to
> strengthen wing
> spars, and some questions.
>
> You start out with a built-up wing. The spar is conventional and is
> made
> of 1/8"x3/8"x48" spruce top and bottom spar caps with 1/8"x 3/8"
> vertical-grain balsa shear webs. It is built around 1/8" balsa ribs
> with
> 1/16" balsa D-tube sheeting. You want to strengthen the spar so
> that
> stronger winch launches can be made; "full pedal zoom" launches
> would be
> dandy, but not necessary.
>
> Two approaches:
>
> 1. Strengthen the existing spar. You laminate carbon fiber laminate
> onto
> the spruce spar caps: .007" on bottom, .014" on top. Substitute
> 3/8"x3/8"
> vertical grain balsa core for the shear webs; as the ribs are glued
> to the
> bottom spar, this 3/8" wide balsa core is glued in between ribs. The
> top
> spar cap is glued in, and the high-stress areas (around the wingrod
> tubes
> and 6" out) get an additional shear web of 1/16" ply.
>
> The space between the ribs is wrapped with Kevlar tow (which is
> epoxied) to
> provide a "shear skin" and to provide "burst strength" as the spar is
>
> flexed. The leading and trailing edges are attached, and the D-tube
> sheeting is applied.
>
> Although somewhat heavier than the "stock" wing, this strengthened
> wing
> ought to be much stronger. Question is, how much stronger? Is it
> worth the
> effort?
>
> 2. Make a Balsa-Core Carbon spar, per Mark Drela's Allegro-Lite
>
http://www.charlesriverrc.org/articles/allegrolite2m/markdrela_allegrolite2m.htm
>
> or Tonly Estep's Dark Star
> http://www.mvsaclub.com/articles/dark_star.htm .
> You start with a 3/8"x3/8"x48" vertical-grain (end-grain) balsa core,
> glue
> on the carbon fiber laminate spar caps (.014" bottom, .028" top).
> Epoxy on
> a fiberglas or carbon cloth shear skin to complete the spar. The
> wingrod
> tube gets the 1/16" ply box, plus Kevlar wrap, as above. The ribs
> are
> attached to the spar and the LE, TE and D-tube sheeting are added.
>
> This ought to be stronger than the stock wing and lighter than the
> modified
> wing in #1 since you are not adding to the spruce spar caps. Question
> is,
> how much lighter, and would it be worth the effort?
>
>
> --Bill
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