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#21
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DG-300/303 owners...
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 |
#22
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DG-300/303 owners...
Short answer
The spar caps on a sailpane are often made of rovings. Rovings are long strands of, in this case, uni fiberglass (lots of it) postitioned at the thickest part of the chord, that extend from the spar roots to an area usually just short of the tip of a wing. For optimum strength they must be straight. If there are sections of the material that are not straight, a percentage of the spar strength is lost. On some DG300's some of the rovings were not laid in correctly, are not straight,so there have been reductions on the operating limitations of the sailplanes for safety reasons. Mike 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 |
#23
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DG-300/303 owners...
Mike wrote:
The spar caps on a sailpane are often made of rovings. Rovings are long strands of, in this case, uni fiberglass (lots of it) postitioned at the thickest part of the chord, that extend from the spar roots to an area usually just short of the tip of a wing. For optimum strength they must be straight. If there are sections of the material that are not straight, a percentage of the spar strength is lost. On some DG300's some of the rovings were not laid in correctly, are not straight,so there have been reductions on the operating limitations of the sailplanes for safety reasons. Also, see the pictures at the URL previously posted by Markus Graeber [shorter form below]: http://tinyurl.com/yoj2hl Jack http://groups.yahoo.com/group/dg-300 |
#24
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DG-300/303 owners...
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. |
#25
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DG-300/303 owners...
A short article with some additional information has been posted at the
AMS-Flight web site: http://www.ams-flight.si/ Marc |
#26
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DG-300/303 owners...
www.marskeaircraft.com/carbonrod.html
Another article on rovings vs Graphlite rods. The Genesis 2, LAK 17, LAK 19 and probably also the LAK 20 use the Graphlite rod design for their wing spars. I hope Jim Marske doesn't mind that I posted these but they are out on the net anyway. |
#27
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DG-300/303 owners...
DG just posted the preliminary TA which is awaiting approval by the
EASA (European FAA): http://www.dg-flugzeugbau.de/Data/tn-359-24.pdf DG also added some more FAQs at the bottom of the web page dealing with the main spar issue: http://www.dg-flugzeugbau.de/holm-dg300-e.html Markus |
#28
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DG-300/303 owners...
I wrote this short article on what I think the problem is, based on
the DG advisory and the photos at segelflug.de: http://www.hpaircraft.com/misc/dg300spars.htm It is only my guess at what the issues are, please view it with some skepticism. Please alert me to any factual errors. Thanks, and best regards Bob K. |
#29
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DG-300/303 owners...
On Fri, 06 Apr 2007 23:34:52 -0700, Marc Ramsey wrote:
You should take a look at this: http://www.dg-flugzeugbau.de/holm-dg300-e.html Marc Just finished reading this and the related thread. This is a nightmare for the affected owners and many others in the industry! However as a suggestion: Would it be possible to determine if any one aircraft has the defect by measuring wing deflection under a known load? From my understanding of the defect, the misaligned fibres would not only reduce the strength of the spar, but also it's stiffness. By measuring the deflection of a sample of wings under load and then opening them up to determine whether they have the defect or not, it might be possible to draw up a non invasive procedure which can be used to determine if the defect exists in a particular aircraft, and if it does, to what extent. Anybody from DG reading this? Ian |
#30
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DG-300/303 owners...
Ian wrote:
On Fri, 06 Apr 2007 23:34:52 -0700, Marc Ramsey wrote: You should take a look at this: http://www.dg-flugzeugbau.de/holm-dg300-e.html Marc Just finished reading this and the related thread. This is a nightmare for the affected owners and many others in the industry! However as a suggestion: Would it be possible to determine if any one aircraft has the defect by measuring wing deflection under a known load? I don't know about your deflection suggestion, but how DG handles this spar-cap situation for all of the DG-300 owners should have a significant bearing on whether, and how much, the market will discount all of their products in the future. A few dollars spent now to shore up what had been a good reputation can pay big dividends. We shall soon see if DG is to remain a desirable marque. Jack |
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