Oscar,
Don't forget also that the UK Piets (and mine) have a full "D" section from
the top of the front spar over the leading edge to the bottom of the front
spar. Makes for a really strong spar..
Peter
Wonthaggi Australia
http://www.cpc-world.com
-----Original Message-----
From: owner-pietenpol-list-server@matronics.com
[mailto:owner-pietenpol-list-server@matronics.com] On Behalf Of Oscar Zuniga
Sent: Thursday, 19 November 2009 2:42 PM
To: Pietenpol List
Subject: built up spars
Thanks, everyone, for the info and leads on the
built-up spar. Of the articles posted on the
Westcoastpiet site, note carefully the information
given in the addendum to the article that ran years
ago in the old EAA Design Manuals (which I am very
fortunate to have at least some of). Referring to
I-shape compared to box beams, there are distinct
advantages. The UK built-up spar with its C-shape
as a compromise between the I-beam and the full box,
has some of the advantages of both. Note, however,
that the I-shape is inherently more resistant to
having the web buckle, which is why the UK spar has
vertical stiffeners between the capstrips at each
rib position.
To take the I-shape to what I would call "elegant
overkill", consider the spars on Mike Cuy's wing.
Refer to the sketches and photos of his wings and
spars that Mike provided on the Westcoastpiet site.
In essence, since Mike's spars use a solid 1/2"
spruce web, he has the best of all worlds in that
he has essentially a 1" square top and bottom flange
to take the tension and compression loads and a solid
1/2" web of spruce for shear. With that thickness
(as opposed to, say, using 1/8" or even 1/2" ply for
the web), his spar does not require stiffeners to
provide buckling resistance for the web. Note the
photos of Mike's wing before covering... there are
no vertical stiffeners at each rib. It is hell for
stout and, as I say, "elegant overkill". It could
doubtless be engineered to a slimmer profile but it
is clean and effective. However, there is still the
cost of that spruce in straight, clear lengths.
For us cheapskates still trying to use thinner and
less expensive stock and either scarfing or laminating
things so we don't have to use long expensive pieces of
clear wood, and trying to use plywood for the web rather
than spruce planks, the approach becomes one of
engineering a built-up section with equivalent strength.
This is where I am right now, and Jim Markle knows why ;o)
Meanwhile, 41CC soldiers on... putting grins on faces
all over south Texas as the C75 begins to break in and
pull stronger with every passing flight.
Oscar Zuniga
Air Camper NX41CC
San Antonio, TX
mailto: taildrags@hotmail.com
website at http://www.flysquirrel.net
