On the wing skin of the Wizard,why do you use 1.5oz. E-glass instead of 1.4oz S-glass?

From : Don Stackhouse

Paul, nice to hear from you again! Excellent question as usual. In this particular case, the S glass doesn't really provide a significant advantage.

The main requirements for the wing skin are acceptable dent resistance, and sufficient torsional stiffness to prevent flutter.

1.4 oz. S-glass will be slightly thinner than 1.5 oz. E glass. This means that it's flexural stiffness will not be significantly different than the E-glass, despite it's slightly higher tensile modulus. Since much of the dent resistance is determined by the foam characteristics, the overall effect on the subject of dent resistance by switching to S-glass would be negligible.

The difference in weight would be minor, probably slightly better with the S-glass, but not enough to be significant in the finished weight of the model. The S-glass cloth I've seen tends to have a coarser weave than the E-glass, which would tend to require more epoxy to get a good surface finish. It's possible that it could actually come out heavier as a result.

The torsional stiffness with the S-glass would be significantly higher, but since the torsional stiffness of the E-glass is already adequate to avoid flutter, the potential improvement from S-glass is unnecessary.

On the negative side, the S-glass is harder to find, can be more difficult to work with (getting a high-quality trailing edge might be particularly difficult), and is substantially more expensive than E-glass (though still generally less than Kevlar, and much less than graphite). Its extra strength can't really be applied to an advantage in this situation, so the few if any benefits from it in this particular application simply don't justify its expense.

In general, we've found that when you truly need the performance of the more exotic composite materials, it's frequently in a situation that also benefits from the use of unidirectional materials, such as fuselage stiffeners and wing spar caps. The best applications for the exotics in cloth form seem to be in cases that require high torsional stiffness in a highly loaded structure, such as the skins and/or shear webs of high aspect ratio open class or giant scale wings. Conventional E-glass is usually the best all-around choice for the relatively wide wings of HLG's.

Don Stackhouse @ DJ Aerotech