-----Original Message-----
From: Q-LIST@... [mailto:Q-LIST@...] On Behalf Of
Robert X Cringely
Sent: Saturday, December 19, 2009 2:36 PM
To: Q-LIST@...
Subject: Re: [Q-LIST] Re: carbon

What are the constraints on this design? On most composite designs the
deciding factor in how many layers to use on the wing skins, for example, is
hangar rash. Yes, hangar rash. Less glass could always be used but it would
be damaged too easily. So we sit around speculating about the implications
of changing materials on a design that's
overbuilt in glass OR carbon. What is the history of structural
failures on Quickies? Zero. Change it to carbon and it will still be zero.

Bob


On Dec 19, 2009, at 1:50 PM, "rick_nordgarden" <grnordgarden@...>
wrote:

--- In Q-LIST@..., "Robert X. Cringely" <bob@...> wrote:

An ALL-CARBON Q1 should carry its loads exactly like an ALL-GLASS

Q1.

This is an example of how risky an untutored "common-sense" approach
to engineering can be. Make two identical foam wing cores, then skin
one with fiberglass and the other with carbon fiber. With the same
number of layers of cloth of the same weight per square yard the
carbon-fiber wing will be stiffer -- and therefore weaker. Uniformly
stiffening a wing's skin alters its spanwise load distribution,
shifting load away from the tips and toward the centerline; the
wing's load-carrying capacity and its g-limit at a given load are
both thereby reduced. Stiffness and strength are not necessarily
complementary properties; to a great degree they're antagonistic.
This problem can be overcome by altering the number and/or weight of
the plies when switching materials, but that means re-engineering
the structure -- the fuselage and bulkheads as well as the wing-
skins and spar layups. Before you can build a *safe* all-CF Quickie
you'll have to design one.

Rick Nordgarden
Council Bluffs IA
Dragonfly MkIIH under construction

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