Discus CS grounded in France

"The next time someone sneers at a wooden sailplane, remind them that it is
made of a unidirectional reinforced laminated composite material consisting
of micro-tubular fibres embedded in a long chain polymer matrix and having a
near infinite fatigue life."
-Unknown-


The wooden rotor blades on a Bell 47 helicopter (i.e. MASH) were "on condition"
where as the metal blades are life time limited.

This is a troubling report. I've read both the French and English
version of the grounding order: the English version is an accurate
translation. No mention of high time or turbulence. One finds
oneself drawn to the conclusion that some sort of construction defect
played a role. Tales of third party reports of insufficient resin are
troubling and certainly deserve a more complete explanation,
recognizing, of course, that liability is still being considered by
lawyers. And recognizing too that there is such a thing as too much
resin in composite aircraft construction: you don't just ladle on as
much epoxy as gravity will hold in place if you are trying to find the
best balance of strength and weight. But if a given factory is
producing parts that fail in flight this certainly calls for more than
hints of workmen drinking too much beer and inspectors abdicating
their responsibility. Let's get serious with this, shall we? Does
anyone have more than suspicions of beer drinking and third party
negligence?

A classical composite is reinforced concrete. Steel reinforcing along with
concrete makes a composite structure. Concrete has fair compressive strength,
3-5,000 psi is generally used, with tensile strength of around 100-500 psi,
miserable. When reinforcing steel, 40- 60,000 psi is added, and suffucient
bond provided (like joining together two composite structures in a spar/skin
joint), the steel takes the load in tension while the concrete takes the load
in compression. (Steel rods in compression are OK only as short lengths before
they buckle)
This is a simple example: where a single bar of steel is in the bottom of a
simple concrete beam and none is in the top. Loading produces tension in the
bottom of the beam while compression is in the top. Each material then uses
its best property to provide a composite structure superior to each
individually.

Bruce Patton
(A civil engineer to pay for soaring)
96S, aka, JJ, " Tinman"

In article ,
says...
Now I am confused, Eric wrote
In composite construction, there is no "glue": the resin permeates the
other material (glass, carbon, Kevlar, etc), and the material
properties depend on this. There are layers of the fiber, but they
aren't glued together like the wood layers in plywood.

How is the fiberglass layer bonded to the foam core, if it isn't *glued* with
the resin? I thought you were making the distinction between several layers of
fiberglass cloth, as in a fuselage and fiberglass cloth/ foam core/ fiberglass
cloth, as we find in the wing.

It's my understanding that "composite" generally means the fiberglass
and epoxy itself. No core is needed to have a composite structu
just look at the fuselage construction of many fiberglass gliders.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

As I write this, I am looking at a cross-section of an ASW-22 wing, taken from
a ship that has met with misfortune, It has styrofoam dams at the leading edge,
on both sides of the upper spar cap and at the forward edge of the drag spar.
These styrofoam dams were set just high enough to allow for a 10% excess of
glue (epoxy resin, chopped fibers, micro-balloons) that were trawled in from
the edge of one dam to the edge of the other dam. In this way, the construction
crew was 100% sure that the proper amount of glue had been spread along the
spar cap. When the upper skin was mated, all voids were filled and the excess
10% of resin was forced out each side into the styrofoam dams.
The inside of this wing is a work of art. I would expect nothing less from all
sailplane manufactures.
JJ Sinclair

(JJ Sinclair) wrote in message ...
As I write this, I am looking at a cross-section of an ASW-22 wing, taken from
a ship that has met with misfortune, It has styrofoam dams at the leading edge,
on both sides of the upper spar cap and at the forward edge of the drag spar.
These styrofoam dams were set just high enough to allow for a 10% excess of
glue (epoxy resin, chopped fibers, micro-balloons) that were trawled in from
the edge of one dam to the edge of the other dam. In this way, the construction
crew was 100% sure that the proper amount of glue had been spread along the
spar cap. When the upper skin was mated, all voids were filled and the excess
10% of resin was forced out each side into the styrofoam dams.
The inside of this wing is a work of art. I would expect nothing less from all
sailplane manufactures.
JJ Sinclair

It is interesting that you mention dams on BOTH sides of the upper
spar cap, as the AD for the Duo Discus had an inspection of the rear
side of the upper spar cap only. There must be an assumption that if
epoxy resin oozed out the back it equally oozed out the front side.
How good is that assumption? If it is possible to have voids where
there is no bonding of the cap to the shear web it must also be
possible to have a partial bonding of the upper aft corner of the web
but not the top and front side. How strong would this spar be? Would
it fail after 1000 hours and only in "extreme turbulance"? Why
wouldn't they have checked both sides of the spar?

"George William Peter Reinhart" wrote in message ...
Yep,
Makes you wonder doesn't it?
YUP.
Where are the cut offs for High Time and "severe " turbulence that would
make it not OK to fly?
How about 1000 hours and/or 5 years.
Currently flying a well aged Nimbus with over 1000 hours on it.
No problem as long as you don't feel any "bumps" in the air. Or you
could invest in a Ballistic Recovery Chute System.
Should I be worried about my personal safety because of the airframe hours? YES
Are there some days I just shouldn't fly because the "turbulence" might be
"too high"?
YES, DEFINATELY. Don't fly on days when the air is "bumpy". Cool
winter mornings when the air is smooth should offer the safest
"gliding" and don't bank in turns steeper than 45 Degrees. Tell your
tow pilot to take off slowly.
All advice carefully considered. Even advise which is recklessly given?
Cheers!


Guy Byars wrote in article
...


I heard it was a high time glider in severe turbulence.

And that makes it ok?

In article ,
(Slingsby) writes:

the AD for the Duo Discus had an inspection of the rear
side of the upper spar cap only. There must be an assumption that if
epoxy resin oozed out the back it equally oozed out the front side.
How good is that assumption?

I think SH has you inspect the aft face of the spar because the front side has
a foam dam to help keep the bonding paste from squeezing out. So if there is a
void at the aft side then the void may or may not go through to the front side.
If there is no void it highly likely that there is enough bonding paste all
across the bonding area.

Now I if you don't like the word "likely" when applied to this particular
process then you should not fly any composite glider because such assumptions,
based on tests and real world experience, are used through the construction
process. 100% inspection of every bonded joint is impossible given the
constraints of manufacturing of gliders as it is done now. Perhaps these
incidents will cause the LBA and manufactures to re-think the inspection
standards. And likely raise the price of an already costly toy.

Robert Mudd

Now I if you don't like the word "likely" when applied to this particular
process then you should not fly any composite glider because such assumptions,
based on tests and real world experience, are used through the construction
process. 100% inspection of every bonded joint is impossible given the
constraints of manufacturing of gliders as it is done now. Perhaps these
incidents will cause the LBA and manufactures to re-think the inspection
standards. And likely raise the price of an already costly toy.
Robert Mudd
************************************************** *********************************
W. Edwards Deming the father of statistical process control showed
that a 100% inspection system will regularly miss 20% of the defects.
The goal of manufacturing and design should be to develop a process
which minimizes the likelyhood of defects, especially catastrophic
defects.
The DG website has a very detailed explanation of how a wing is
constructed and I assume the technique is similar for all German
designed gliders. Spar CAPS are formed with carbon fibre rovings and
are inspected in minute detail for voids and any cap which fails
inspection is immediately cut in half to preclude its inadvertant use.
As the upper and lower wing skins are moulded, the spar caps are
glued in place so that they are well bonded to the skin. So far so
good, the top and bottom of an I-beam shaped spar are well built and
in place. Next the shear web, which is also well built and inspected
is glued into the bottom part of the spar cap. Gravity holds the glue
in place and the surface is well lighted and clean so the craftsmen
and inspectors can see what is happening. The wing spar is now an
upside down T shape. Later on in the process the top surface of the
wing is glued to the bottom half forming both a wing and a complete
spar. It can be SEEN that the exterior surfaces of the wing are well
mated because epoxy oozes out of the joints. It can only be ASSUMED
that the upper spar cap is well bonded to the shear web because the
glue joints cannot be seen. Assuming you have a good spar because of
German craftmanship works because German craftsmen are quite good, but
it is not a well designed manufacturing process. Building a complete
spar outside of the wing and then bonding it to the upper and lower
surfaces strikes me as a process much less likely to lead to a
catastrophic failure. In the case of the Duo Discus and probably the
Discus CS, failure of the spar is more of a process defect than an
inspection defect.
The price of these costly toys may indeed go up but something bad has
just happened to the value of Shemp Hirth products.

"I assume the technique is similar for all German designed gliders."

Very bad assumption. Not even the same for all products from each mfg.