Message-ID: <FDD045B6B83FD311A9880090273053E20B644E@CDIM-PTS-MAIL>
From: "Rumburg, William" <wrumburg@cdicorp.com>
To: "'jerry@mc.net'" <jerry@mc.net>, "lancair.list"
	 <lancair.list@olsusa.com>
Subject: RE: LNC2 SB050-0999
Date: Thu, 14 Oct 1999 14:05:09 -0400
Mime-Version: 1.0

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> When you say full-width do you mean this......a full-width piece of
> phenolic applied ONLY TO the trailing edge of the BOTTOM cap of the spar?
> If this is so what would be the length what would be the height, how
> thick,
> how many bids, would it go all the way to the main mount phenolic and
> tied-in there?  Lots of questions here I know.
> 
	Jerry et all -
	More correctly, I should have written "...a SINGLE, full-width
secondary attach point...".
	The single, full-width attach block would attach to both the upper
and lower spar caps. It would be at least 3" wide (centered about the
overcenter link bolt) and at least 1" deep. Fabricate it from 1/2" thick
phenolic sheet, bonding two pieces to obtain the 1" depth. An important
technique while fitting it is to temporarily reinstall the gear mounting
bracket and use it to precisely position your new secondary attach block. If
it's not installed to mate precisely with the gear mounting bracket,
permanent and detrimental force will be applied to it.
	A Navy composites technician once told me that the ideal thickness
for a bonding agent such as Hysol is ten-thousandths of an inch and that the
military uses a ten-thousandths thick cloth to establish that. In our
amateur-building world, we "goop it on" and get away with it because of
cotton flox, which imparts a lot of strength in relatively thick bonds
because of random fiber distribution. Therefore, fit the phenolic between
the upper and lower spar caps so that there is only about ten-thousandths of
an inch for Hysol. As it turns out, a bonding agent such as Hysol has
maximum strength in the shear plane (no pun intended). 
	Use simple, rectangular, three or four bid layups to lock in the top
and bottom. Attach them on the inboard, outboard and forward sides,
contacting at least 1" on the attach point and 2" onto the spar caps, where
possible. As always, small mico fillets are necessary for maximum strength
at the joint.

	Bill Rumburg's course in Secondary Attach Point mechanics...  
	A casual observation shows that both shearing and tension forces
must be resisted by the current secondary attach point design. 
	The shear forces result not only from the obvious gear loads
transmitted through the attach bracket, but also from the substantial force
applied by the hydraulic cylinder in the down locked position. The shear
force from the hydraulics can be alleviated by lowering the down-lock
pressure to a value a lot less than the specified 500 psi, which is
unnecessarily high. The "screen door" springs are themselves quite capable
of snapping and holding the overcenter link in the down-locked position. In
fact, they are all you do have should your hydraulics fail. A civil engineer
could analyze the vector(s) for the force(s) applied to the secondary attach
point using the technique of "virtual motion", i.e. freeing one point at a
time while allowing others to remain pinned. I'm sure there are components
in both planes (again, no pun intended), so as to twist the attachment as
well as shear it. Both tension and shearing forces are substantial, not only
during landing, but also when rocking the aircraft (that's what broke one of
my attach points three and one-half years ago before it even flew!). My
design (as well as the SB "fix") turns these tension forces into compression
forces, exerted on either the upper or lower spar cap.
	Reread this and you're a graduate of Bill Rumburg's course in
Secondary Attach Point mechanics....Whew!
	Bill Rumburg
	N403WR  (Sonic bOOm)

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