Glass spar-carbon skins question

[Brad[_2_]]

Anyone out there know if there have been any sailplane wings made that
use a glass spar and carbon skins?

Cheers,
Brad

[[email protected]]

On Sep 1, 4:35 pm, Brad > wrote:
> Anyone out there know if there have been any sailplane wings made that
> use a glass spar and carbon skins?
>
> Cheers,
> Brad

As carbon fiber has a higer stiffness than glas fiber the skin would
carry ony the load and the glass spar would ony adding wieght and
cost.
The spare in a glider wing will carry the bending load, the skin will
carry the torsion load. With the higer stiffness of the carbon fiber
the skin will carry torsion load as well as the bending load. That
will lead to a overloaded skin befor the spar has a change to carry a
significant amount of the bending load. If the skin is thickend up
enugh the spare becommes useless.
So you can build a glas spare with a glas skin, a corbon spar with a
glas skin, all carbon, and wings withoud spar in glas or carbon were
the skin carrys all the load.

[Bob Kuykendall]

> Anyone out there know if there have been any sailplane
> wings made that use a glass spar and carbon skins?

What that other guy said.

As much as you'd wish otherwise, stress follows the path of greatest
stiffness, and not necessarily the path of greatest strength.

So if you independently sized the carbon skin for panel stiffness,
ground handling, and torsion loads, and sized the fiberglass spar for
bending loads, you'd almost certainly find that the spar is so limber
that the skins would kink and buckle at loads well below a reasonable
design limit.

Of course, almost everything is possible. If for whatever reason you
really really had to combine carbon skin and fiberglass spar, you
could always just size the spar caps to the stiffness of the skin.
However, in order to achieve the necessary stiffness, the spar would
have something like six times as strong as it otherwise needs to be,
and commensurately heavy.

I ran up against a similar issue with an early design sketch of what
became the HP-24. I proposed a wing spar based on Graphlite pultruded
carbon ribbon, with PVC foam ribs and a bonded aluminum skin ala
HP-18. However, Jim Marske set me straight on that idea, calculating
that in order to be stiff enough so as to not kink the aluminum wing
skin within the design load limits, the spar would need four times the
material dictated by strength alone. And even then the skin/spar bond
and the fatigue life on the skin would be marginal.

Thanks, Bob K.
http://www.hpaircraft.com

[Eric Greenwell]

Bob Kuykendall wrote:
>> Anyone out there know if there have been any sailplane
>> wings made that use a glass spar and carbon skins?
>
> What that other guy said.
>
> As much as you'd wish otherwise, stress follows the path of greatest
> stiffness, and not necessarily the path of greatest strength.
>
> So if you independently sized the carbon skin for panel stiffness,
> ground handling, and torsion loads, and sized the fiberglass spar for
> bending loads, you'd almost certainly find that the spar is so limber
> that the skins would kink and buckle at loads well below a reasonable
> design limit.
>
> Of course, almost everything is possible. If for whatever reason you
> really really had to combine carbon skin and fiberglass spar, you
> could always just size the spar caps to the stiffness of the skin.
> However, in order to achieve the necessary stiffness, the spar would
> have something like six times as strong as it otherwise needs to be,
> and commensurately heavy.

If the wing was a fairly thick, say 20% or so, could the spar be stiff
enough without much excess strength? I realize that a thick wing is an
unlikely choice for a glider, of course.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[Brad[_2_]]

On Sep 6, 8:39 pm, Eric Greenwell > wrote:
> Bob Kuykendall wrote:
> >> Anyone out there know if there have been any sailplane
> >> wings made that use a glass spar and carbon skins?
>
> > What that other guy said.
>
> > As much as you'd wish otherwise, stress follows the path of greatest
> > stiffness, and not necessarily the path of greatest strength.
>
> > So if you independently sized the carbon skin for panel stiffness,
> > ground handling, and torsion loads, and sized the fiberglass spar for
> > bending loads, you'd almost certainly find that the spar is so limber
> > that the skins would kink and buckle at loads well below a reasonable
> > design limit.
>
> > Of course, almost everything is possible. If for whatever reason you
> > really really had to combine carbon skin and fiberglass spar, you
> > could always just size the spar caps to the stiffness of the skin.
> > However, in order to achieve the necessary stiffness, the spar would
> > have something like six times as strong as it otherwise needs to be,
> > and commensurately heavy.
>
> If the wing was a fairly thick, say 20% or so, could the spar be stiff
> enough without much excess strength? I realize that a thick wing is an
> unlikely choice for a glider, of course.
>
> --
> Eric Greenwell - Washington State, USA
> * Change "netto" to "net" to email me directly
> * "Transponders in Sailplanes"http://tinyurl.com/y739x4
> * "A Guide to Self-launching Sailplane Operation" atwww.motorglider.org- Hide quoted text -
>
> - Show quoted text -

Thanks for replies on this guys. It was kind of an academic question,
as well as a practical one. We have a source of fiberglass laminate
pultrusions that are suitable for spars, and the price is quite
attractive, but the geometry of the wing and hence the spar requires
using different methods. The graphlite rods make the most sense and
the spar made using these is limber enough to allow a fore-aft bend to
hit the high points of the wing. Plus, carbon fiber is now getting
more available and prices are becoming more reasonable. I like the
idea of carbon skins and carbon spars, my Apis is built that way and I
rather enjoy how easily it is to rig.

Brad