Graham,
I agree with all that you say, so unsure why you're not convinced.
The lift load is spread on the wings, but comes to a point at the lift
pins;
the stress path being either via the spar and root rib, and/or directly
through the wing skin to the root rib (assuming for the argument no lift
contribution from the spar pins).
I recall that when a Europa wing broke in the early tests, it broke
across
the D-box (LE of wing) adjacent to the rib (and was subsequently
reinforced
in that area).
Meaning that this part was overstressed, obviously, but the strain at
failure (being a material constant) was greatest in this part of the
structure (i.e. up to the point of failure, the area around the rib had
flexed more than any other part of similar 'I).
Duncan.
-----Original Message-----
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of GRAHAM
SINGLETON
Sent: 24 January 2011 23:20
Subject: Re: Europa-List: Re: Main wing bushings
Not convinced Duncan. The spar resists bending and the leverage is
enormous,
---From the centre of lift of the wing 30% span? The load on the root ribs
is
shear and some bending of the pins. Don't forget the lift load is a
spread
load whereas the spar bending of the tangs is a point load applied by
the
spar pins.
You are right about the stiffening of the fuselage side. We really
should
have flexible sockets at the LE as well as the TE.
I still believe the lift pins (in the root rib) should carry all the
lift
loads and the spar pins all the bending loads. That's why the tangs
needed
to be strengthened on the glider.
Graham
_____
From: Duncan & Ami <ami-mcfadyean@talktalk.net>
Sent: Monday, 24 January, 2011 21:58:21
Subject: RE: Europa-List: Re: Main wing bushings
<<.. would stiffen the structure in such a way that it could lead to
overloading of the forward lift pin >>
Ian,
How does that work, if the lift pins are designed to carry the lift
loads in
the first place, there being no greater load available?!
Actually, it was stiffening in bending that might restrain rotation of
the
lift pin that was the issue, resulting in an additional bending load
(not
lift load) being applied to the forward lift pin. The LAA were offered a
spherical socket that would articulate and remove the bending load on
the
pin, but declined to accept that.
However, you are on right path, The earlier discussion in this thread
presumes that the spar flexes, as it indeed doe. But actually the spar
(loaded in this direction) is the least flexible part of the whole
system.
What about the forward and aft portions of the root rib? Let's say these
ribs have equal section and 'I' as the spar, but being laid-up at +/- 45
degrees have x1.41 the elasticity of the unidirectional spar (in the
direction of principal stress), added to which the length of these
flexing
root ribs (between lift pin and spar) is longer than the offset between
lift
pins and spar pins. So, as the root ribs will flex more than the spar
and it
follows that the "leverage" effect between the longitudinal offset of
lift
pin and spar pin centres is removed, or reversed to the extent that the
spar
pins share some lift load.
As you say, the flexing of the fus side also contributes, albeit this
had to
be stiffened-up to prevent pin disengagement.
Previously I have put a small finger down one of the (1/2") spar pin
holes
with the wings rigged, while someone else rather violently loaded the
wing,
albeit not even near to 1g. But there was no hint of the "leverage"
effect
or the spar hole flexing downwards relative to the pin hole in the seat
back.
Rgds.,
Duncan McF.
-----Original Message-----
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of G-IANI
Sent: 24 January 2011 12:34
Subject: RE: Europa-List: Re: Main wing bushings
In support of Bud and Pete's notes I can confirm that flexing of the
fuselage is important. The LAA had considerable concerns that the tail
dragger conversion would stiffen the structure in such a way that it
could
lead to overloading of the forward lift pin.
Ian Rickard G-IANI XS Trigear, 300hours
Europa Club Mods Specialist
e-mail g-iani@ntlworld.com
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