Martin Wrote:
>Mark Drela posted some fine information regarding spar design and testing.
>Mark's approach is suited very well to wood wings, but I see some
>limitations to its usefulness for composite skinned wings.
I disagree, I think it is very well suited to composite construction. The
main spar is an excellent means for assuming all of the bending loads, and
composite skins on a 45 degree bias provide all of the torsional rigidity.
This is the common method of most composite wings, including hollow core.
>
>Mark's method has the spar taking all the shear and moment forces. This is
>fine for wood wings since the balsa, obechi, or ply skins provide very
>little bending strength contribution to the wing when carbon reinforcements
>are incorporated. That's due to the high modulus of the carbon compared to
>the wood...that means the carbon will have taken the entire loading and
>reached its failure stress before the wood components have strained enough
>to provide significant bending resistance.
The same is true when using carbon spar caps and composite skins. Unless
you use the same modulus carbon skins as you are using in the spar you end
up with the same result of the higher modulus spar assuming most of the
bending loads.
>
>For composite wings, we have to have a minimal amount of fiberglass or
>carbon in the skins just to provide enough strength to the skin to prevent
>finger denting of the foam core, resistance to hanger rash, or provide
>enough rigidity to molded wings skins. In addition, the torsional rigidity
>of the wing generally cannot be attained with only a strong spar, so some
>sort of skin is necessary. Using Mark's methodology will work well, but it
>might be a little heavier.
I think this method is one of the lightest methods. The spar is located in
the thickest part of the wing, which is the optimum location to maximize
bending moments of inertia (squared function), hence is the optimum place
to put all of your material designed to resist bending (The spar cap) The
"I-Beam" structure is a very effective/weight efficient means for resisting
bending. It is also amazing how well fiberglass/carbon skins on a 45
degree bias resist torsional loads. I personally feel this type of
construction is simple and benefits from composite structutral properties.
In other words you use a good high modulus carbon as spar caps to resist
bending loads, and as Marks analysis showed it is amazing how light you can
make the spar, and then use a lightweight glass/carbon skin on a bias for
torsion. You get the best of both worlds in a simple process.
If you had to incorporate skins to resist bending, you would have to orient
the fibers spanwise, which would result in poor torsional rididity.
>
>Because of this basic need for the skin on a composite wing, it may be less
>efficient to beef up the spar so greatly when the skin strength can be
>incorporated into the equation. A stressed skin design should be a more
>efficient use of materials in our wings. Unfortunately, stressed skin design
>is not so easy.
Use the spar for bending and the skins for torsion. Very effective and
efficient. The spar place is in the optimum location for bending, the
skins best suited for torsional rigidity.
>Many of you have seen or used Joe Wurts' wing design spreadsheet. That makes
>designing the wing skins easy, but the aspect that isn't incorporated into
>that spreadsheet is the shear and buckling design. It can be fairly easily
>ignored if you choose a high strength foam like spyder foam and also
>incorporate some sort of limited spar for shear in the core. This works
>fine, but it can be a little heavier than desired.
>
>I see the design of our sailplanes continuing on a path toward lighter
>airframes with lower camber wings to help the penetration capability.
>
>In the past 14 years that I've been involved with sailplanes, the prevalent
>camber percentage on our wings has dropped by about 1 percent(4 to 3). I
>think that many of us will be running aircraft with 2 percent camber in a
>few years if we can get the weight down to match the airfoil's Cl range. But
>we still will have the need for strength since the planes are still going to
>go through a workout on launch. We need to push the envelope to lighter
>airframes for this to be possible. Design methods that promote strong, but
>heavy, aircraft make it harder.
>
>Molded composite wings are a great way to go, but they are out of the reach
>of the common person due to the great effort in time and money. I believe
>that foam core construction will be the most available technology for most
>of us. There is research going on now that hopefully will make a difference
>in how we design and construct our wings.
>
>We have all benefited from contributions from brilliant and (or) dedicated
>individuals like Selig, Garton, Drela, Wurts, Hazel, etc...
>I fully expect that more will come from the likes of people like these and I
>think that improvements to structures and the resulting weight savings are
>the next step.
Gavin
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