Duncan and Graham,
As for the wing failure point, the idea was to have the actual wing
break before the attachments. Duncan is right, the failure occurs
outboard of the root rib.
I asked why not an articulated forward socket and a proper attach point
with a pin, however a pivoting pinned fitting would then imply that the
forward pin if articulated without a pip pin would slide (not good), as
the pin slides under bending and the forces are not translated due to
the slipping pin I guess. The articulated forward attach was dropped.
I guess since the forward root pin is in shear only, and as long as the
tolerances during wing construction are met, the pin stays fairly
straight, and I am sure under extreme loads, the front pins are pushed
into the socket and with the stiffener added, flexing and failures are
non existent.
The carbon fiber MG spar flexed under load and the more brittle carbon
shear web cracked near the outboard wing pin. So the idea Dave and I
talked about was simple, there is a bit too much taper in the spar tang
and the height of the spar needed to be increased or said another way,
the spar now has less taper. It is now massively strong.
I prefer the common way of distributing that stress concentration at the
rib where the upper skin can collapses under the rib bending, and that
is put in a span wise beam or spar to distribute the point load on the
rib. The attach point is a horizontal bolt or pin in a proper spar.
Also I prefer the beam to go through the fuselage caring the load. Of
course it is that much more stuff to build.
That said, I am amazed this little airplane has soldiered on without
that forward socket ever failing, or the pins. I guess it all works
together without the fancy computer programs or stress analysis
diagnostic programs or even my original skepticism. The combined
attachment points keep things intact extremely well. I was impressed by
the latest glider wing tests done by the factory. On the first test the
rear pin broke because they grabbed a plain steel one used in shop
alignment work and it snapped like a twig. The rear root pin was
replaced and one of the guys in haste put a common bolt in the main wing
and it sheared like butter. Once all the proper pins were correctly
installed (and much egg removed from faces, I'm sure}, the wing with the
spar mod went to nearly 9 G's, finally failing in the leading edge D
tube area when the upper skin flexed until it collapsed. Not a bit of
damage or problem with any of the sockets or the proper wing pins or
bushings.
So, it works brilliantly and is extremely tough at current load limits
imposed. I like it.
Bud
----- Original Message -----
From: Duncan & Ami<mailto:ami-mcfadyean@talktalk.net>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Tuesday, January 25, 2011 5:10 PM
Subject: RE: Europa-List: Re: Main wing bushings
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
To: europa-list@matronics.com
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>
To: europa-list@matronics.com
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
To: europa-list@matronics.com
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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