Thanks for sharing. I will notify the Libelle owners in my club to have a closer look on their tail sections. In my opinion this type of failure can also occur by years of small movement/vibration (which occurs on flight) over time. A video of the repair, even with a sequence of photos, would be very interesting. The delamination is quite big, so it would be interesting how the repair masterplan looked like, with this restricted access.
The repair was very simple. The official repair scheme approval process is on the Streifly website that officially supports the Glasflugel range of sailplanes. See link www.streifly.de/TN%201-2005.pdf In that document, one of the approved methods is to use a manual written by Ugene Hanle. The English translation is below. www.dg-aviation.de/en/wp-content/uploads/sites/2/2016/12/flickfibel-e.pdf This manual is essentially the Libelle repair manual as its based on the Libelle. The thing i do differently (the manual was written in 1967!) is lengthen the GRP splices to the same as the ones in the Schelicher manuals. I prefer longer splices as its more forgiving of errors and actually stronger with less hard points. Do not use repair schemes in the FAA AC43. Although they are approved, they are all heavy and stiffer than the original structure. They are aimed more at airliners than sailplanes. One hand size hole each side of the fuselage to gain access, Abrade the broken frames away to almost nothing with 40 grit on a finger mostly (removing the crack). Make sure all broken GRP is removed and the area around it keyed up to accept the repair. Then reinstating the frames and patching the skins are all repair schemes taken from the manual. Note you cannot buy the original resin and there is a Streifly technical note to use a different resin see link here www.streifly.de/TN%202-2005.pdf After the repair it must all be post cured at 54 degrees Centigrade for 15 hours befiore you, fill and paint white. Being at the back end of the glider, i would also reweigh the glider to make sure the repair has not added too much weight.
I got one of those little inspection cameras for my yacht mast. 7mm dia, illuminated and dimmable, 10M cable, wifi, clip on hook, magnet and another thing all for a very small price. It should be able to insert a string/wire from one end of the fuselage to the other and inspect the whole thing with the camera on a flying fox set up, just run it along the bit of fishing line or similar.
This was really helpful as I'm about to do a pre-purchase inspection on a Libelle. Will you seal up those holes or leave them as removable inspection plates? Could this be spotted with an internal inspection (with or without the camera) or does it take reports of a problem to lead you to flexing it in such a way as to uncover the damage?
The holes are far too big to leave in place. They would substantially weaken the fuselage in a very critical area. Glasflugel approved repair schemes were written by the designers wife Ursula Hanle. They can be found here. www.dg-aviation.de/en/wp-content/uploads/sites/2/2016/12/flickfibel-e.pdf
As a rule as long as you can get a hand to the repair and ideally a camera to see what you are doing you do not need a bigger hole. Often though when time is money, cutting a bigger hole to speed up the repair is more cost effective. Dentists and gynaecologists would/are experts I have discovered! Here is the Libelle repair manual that gives the various splice angles abs repair schemes. www.dg-flugzeugbau.de/en/wp-content/uploads/sites/2/2016/12/flickfibel-e.pdf
I have an engineering degree, *_but I am not qualified to make an engineering assessment of this._* However, based on my understanding of composites, nothing one can do will restore the original structural rigidity of that empennage, following the cutting of those holes. Thus, tail flutter may be evident at much lower speeds. Fibres (fibers) carry structural loads in tension, and after a fiber is cut, the load path is permanently broken. The plastic resin is much less capable of carrying loads (roughly 1/40th that of carbon fibre (fiber)). I would estimate the longitudinal ultimate strength of the tail has been permanently reduced by at least half, after cutting those holes, and if someone told me the torsional ultimate strength has been reduced by 1/4th (or more), I would not be surprised. In order for a patch to make any real contribution to restoring the strength and rigidity of a GRP structure, and a patch can never equally absorb the strain of the original, uncut, structure, that patch must be much larger than the hole. The strain must be transferred from the original structure, via the plastic (which has a relatively weak shear strength) binder, to the patch's fibres (fibers), and then back out the the structure again via the plastic binder. *Again, I am not qualified to make an engineering assessment of this.* *If it were me, I would build some structural static load test articles that established some data by which estimates could be made.*
I'm not qualified either, but I know they typically grind a very shallow scarf angle around the edges so there is substantial overlap between the old and new fiberglass layers. I believe this helps transfer the tension. People have been doing these types of composite repairs for decades, they must know what they're doing.
@@FadoodleX I think that only shear can be the load transfer method, whether in tension or compression. If we made a butt joint with zero overlap, the interstitial plastic binder would be taking the entire tensile load, and the tension on the fibres (fibers) would be the result of the binder strain, transferred through shear, on to the fibres (fibers). As the deformation of the binder would be greatest at the end of the fibres (fibers), I would expect delamination between the binder and fibres (fibers) in that region (because the binder is a lot more 'stretchy' than the fibres (fibers)), especially after a number of cycles. Maybe I'll play around with this when it warms up. If I needed to repair something, in order to escape a predicament, I would make a full thickness sleeve 5-10 times the diameter of the hole, and that may be woefully inadequate.
@@DumbledoreMcCracken it just does, of all the gliders in the world at least one 1st generation GRP glider does a ground loop every weekend some where in the world. I'm sure it's going to be okay. (glider pilot).
A groundloop (or cartwheel) = when the Plane makes a 180° Turn (more or less) on the ground, most likely by one wingtip hitting the ground and beeing hold back by friction. A very dangerous situation in winch launch acceleration. High grass is therefore a no go, and super quick release of the winch cable absolutely necessary. Sometimes this "maneuver" is made intentionally, e.g. when outlanding on a short field, to make a 180° Turn before hitting the obstacles at the end of the field, by putting one wing on the ground with aileron and supporting the loop with rudder, on relative low speed, otherwise it is possible that the plane rolls over on the back (the high wing is accelerated and produces a lot of lift). Normally you would want to push the elevator forward to take pressure from the tail, otherwise the tail has resistance on the ground, an the 180° turn with the turn point at the main gear has a big momentum at the tail some meters away, therefore producing sideway stress on the rather "thin" (by Ø) fuselage at this point, shearing it off or like in this case making delamination. In my opinion this type of failure here can also occur by years of small movement/vibration (which occurs on flight) over time. Here is a video with a "kind of groundloop" (it´s more an impact). The sheared off tail section, can be seen @ 10:00 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-_zfuRb3csJc.html Another type of groundloop while landing an hitting a Corn field with the right wingtip with a K8. Notice the tail is in the air (a loaded K8 will have the tail always in the air, only when accelerating (start) coming down) --> no stress/damage to the tail has happened. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HT_9O45TFtQ.html And finally a wingdrop on a winch launch with a groundloop short to happen. A very dangerous situation, when the pull from the winch accelerates the upper wing --> lift and rolling the plane on it´s back, with no protection for the crew on impact (Plexiglass canopy). ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Ls_VIfxOV8U.html
