On Apr 9, 9:10 pm, "ContestID67" wrote:
After reading all this I am still unsure what is wrong. Can someone
paint (or draw) me a mental picture on what was manufacturered
incorrectly? Also, what is a roving?

Finally, I assume that only DG-300/303's that say ELAN on them are
affected.

- John
This is copied from a LAK 17 site. I think it is dated approx.
1996 Note that when you have ripples in the roving it degrades the
compression as a bend can't take compressive forces as well as
something absolutely straight.


THE SAGA OF A CARBON SPAR DESIGN BY JIM MARSKE (1996)

When I went about designing the wing spar for the Genesis 2, I tried
to find design strength values for the carbon rovings to be used in
the spar caps, the manufacturer's data sheet claimed 310,000 psi
tensile strength but gave no compression data. A call to the
manufacturer produced a claim of 100,000 psi in compression. Talking
with others who have constructed carbon wing spars, I was advised to
be careful as such values cannot be obtainable and to back off to
90,000 psi in tension and 60,000 in compression. To satisfy myself, I
had several test strips made up of hand laid-up carbon rovings and
sent them to an independent test laboratory for strength evaluation.
The results of the five test samples were disturbing. In tension the
values ranged from 152,000 to 190,000 psi. In compression the values
ranged from 48,000 to 74,000 psi. The reason why there was so much
scatter and the values lower than expected is that it is almost
impossible to lay the rovings down without zigzag waves every few
inches. You can only pull the tow till its shortest filament pulls
tight and the rest lay in small waves. As a result of this condition,
and to acquire the necessary strength, the G1 prototype spar is fairly
heavy and very stiff. To prove the spars strength, we static loaded it
to +5g's and -3g's. Wing deflection, measured at the tip, was 25
inches at+5g's.
We needed carbon that was stronger and more consistent in strength
values, Therefore, we looked into pulltruded carbon rods, Samples were
ordered and examined, but waves were found in those filaments also, At
about this same time, I found a brief article in Sport Aviation
concerning a carbon rod expounding very straight filament alignment. A
sample rod was obtained and after examination of the filaments, we saw
that they were indeed very straight. Furthermore, the manufacturer
claimed a tensile strength of 315,000 psi and a compressive value of
200,000 psi, which was impressive. When bending the 1 /8 inch diameter
rod to first sign of fracture, it was the tensile filaments that were
failing one at a time. In addition to the high strength properties of
these rods, the automated manufacture of the rod controlled the resin
content, which is not possible in a hand laid-up situation, and
dimensions of the rod assured consistent strength properties.
The next step was to prove that the rod would function as a spar cap,
and that it would carry the required loads without delamination. A
Genesis spar segment of the aircraft center section was made and
tested in the Sportine Aviacija laboratory. No failure occurred during
a load sequence to a load limit of +8.3g's. This load represents an
aircraft design load of 1200 pounds, times a safety factor of 1 .5 as
required by JAR-22. Impressed with these results, the load was
increased past the required load limit of +8.3g's to +1 g's without
incident. Going to +10.5g's we reached the maximum output load of the
test machine, and again no degradation of the rods was observed. This
load was nearly twice the required spars design load of +5.55g's.
Satisfied with the static results, we did not however have a history
of dynamic cyclic endurance testing for this particular rod. Since the
majority of the rods do not span full length of the spar we had
concerns as to what would happen at the end of each rod end in the mid
section of the spar where a stress riser may occur. So we embarked
upon a cyclic endurance test at an elevated load to force an early
failure.
The first run was a 4g positive loading. We hoped for 5,000 cycles but
stopped at 10,000 cycles. We then increased the load to 6g's expecting
a failure in a few hundred cycles; we stopped the test at 5,500
cycles. The test spar was then inverted in the fixture to apply
negative loading. The test lab director insisted that we start at -3g.
We started at -4g's and ran for 5,000 cycles. No degradation was
noted. To finish the test we repeated the static loading test again.
One cycle to +8.33g's and two cycles to -5.33g's. Again no degradation
was visible.
So we asked Klemas, Sportine Aviacija's chief engineer as to just how
many flight hours all this cyclic testing is equivalent to, Klemas
gave me a report on recorded accelerations made on one of their
LAK-12's during 50 hours of flying, which included towing, takeoffs,
landings and ground handling. The accelerations were all counted and
grouped together to form a 50 hour flight period. The cycles were then
multiplied by 200 to find the life of 10,000 flight hours for the
LAK-12.
THE SAGA OF A CARBON SPAR DESIGN continue
This data was then transferred into chart form. I overlaid the Genesis
data on the same chart to obtain a comparison. A diagram of the
results appears below.

pictu Genesis spar test results compared to the LAK-12 spar test
results.



After completing a quick calculation, which still requires further
evaluation, I feel that we have acquired an excess of 5,000 flight
hours (probably 6,000 hours) in positive loading and an excess of
10,000 flight hours in negative loading. I understand that a survey of
various glider clubs around the world responded to an inquiry as to
the maximum flight hours that had been accumulated on any of their
gliders. Only a few gliders had accumulated near 5000 flight hours.
However one Australian club reported nearly 6,000 flight hours.
G2 WING SPAR DEVELOPMENT
As mentioned previously, the Sportine Aviacija facility has an
extensive engineering test lab and experienced engineering staff.
These capabilities in combination with our own engineering efforts
have produced some amazing results in the area of the wing spar
development for the G2.
For example, the main spar on the G1 prototype was constructed in the
usual manner using hand laid-up carbon fiber roving. However
laboratory tests have shown that there is plenty of room for
improvement in this process. So we decided to look into using
prestressed carbon fiber rods as a replacement to the carbon roving
used in the wing spars. These rods alone are five times stronger than
conventional hand laid-up carbon roving.
Using the Sportine Aviacija test lab, we prepared a sample for cyclic
fatigue testing and took it through 10,000 cycles at +4 g's, 5,000
cycles at +6 g's and 5000 cycles at -4 g's. Then as required for
Jar-22 certification, we loaded this same spar sample twice for 10
seconds (once for 3 seconds is all that's required) at +8,3 and -5,3
g's and experienced absolutely no degradation or failure whatsoever,
We then took it to 10 g's and also experience no degradation or
failure. Certification test results like this are practically unheard
of in sailplane development, but that's not to say they shouldn't be.