Dear Gordon, thanks for keeping us attached. The video is not that clear...if I am right the cracks are in flight direction, and there are many of them, is n't it? Mostly the quite thin 1e layer of the composite, say an 80 grams 0/90 glasslayer is only affected. The second layer underneath , mostly a 300 grams +/-45 layer is not affected. Can you specify more precise what the case is here?
Thanks for this information. At 3:40 we see in section that two thinwalled " half moons " transfer the differential torsion loads from left to right elevator. By deflecting the rudder different pressure exists left and right on the vertical tail, and hence the elevators see different pressure on their undersides. These differential loads travel through this drilled tube, which gives stress probably just above the fatique boundary for this metal. Conclusion : I doubt it is a ground loop problem, more a forgotten, or overseen load case. But thanks for the pictures anyhow. ( I engineered for Lange Flugzeugbau, espec. the Antares)
10:27 Quote " It doesn't meet really modern health and safety regulations" but the Aerodux resists 24 hrs of boiling water... The best wood glue ever. Second best is the certified L285/H285 resin system, can also be used as sealant on wood surfaces. It penetrates deeper in the wood compared to aerodux, but is less viscous, hence requires ore care in joint accuracy. A very usefull video. Well done G. Macdonald.
The picture at 0:27 shows the effect of Kaurit glue failure. This particular glue uses formic acid as hardener. In time the acid dissolves the wood and the gusset plates on the rib joints shear off. Otherwise said: the cellulose of the wood is decomposed by the acid. The adhesive on itself is 100% OK, it is the adjacent wood that shears off. Evidence: A 1957 Ka6br showed ample tack between the plywood gussets (rondelles) and the ribstringers; they just fall off like paint flakes. The Ka6 rib stringers are in principal 4 * 7 mm in section where the 4 mm value is in contact with the gusset plate (half rondelle), hence this is a minimal connection area. I studied the fractures in detail. Always the dyed acid-side (pink or purple) of the joint fails. Minute wood residues stay behind on the glue, which on itself is intact. This fracture picture strongly points to wood deterioration by acid. Well done G. Macdonald.
Had that happen on several occasions to our Juniors. Cause is usually a heavy landing with torsional load, or a groundloop. Usually it is not just this bulkhead, but the ones other further aft too. Removing the rudder often reveals damage in de glue joints at the lower end of the aft bulkhead and vertical spar too. A substantial repair to do, I may add.
The large number of gliders that BGA programs have found to be not airworthy and not economically feasible to repair is astounding. I'm afraid that many wooden gliders in other countries will be given inadequate inspections and will continue to be flown until we have more accidents.
Somebody is pushing at the top of the fin (about 10kg) to put some torsional load on the fuselage. Often the only way to spot a delaminating is to see it moving.
I've enjoyed my flights on K6s, Ka7, Ka8 an K13 and we also have a private Ka4, Baby and Prefect still flying. Your work helps us keep these gliders safe and flying! Very informative and clear presentation Gordon, thanks as well from the Dutch glliding community!
I was flying gliders in the 1990's. We had mostly plywood and doped fabric gliders. It was the rule on DI's that one drew a pencil-line at the farthest extent of a crack, and dated it, so that the rate of crack propagation could be seen. One day, I grounded one of the gliders, only for someone to remark "but it's been correctly dated". I countered "but it's an EIGHT FOOT LONG CRACK! Suddenly, those present looked appalled, because they'd managed to over-look this, secure in the knowledge that what was required had been religously observed!
The Astir I am told did fly once. The elevator damage was hidden by Mylar and they did not put any stress on the tip of the tailplane to see the flexibility. There were no external cracks at all. But a thorough pre flight inspection would have found the damage. ASW24 and Nimbus 3 both flew with damage because the owners and engineers did not see spot it. In both cases there were zero cracks on the outside. The ASW 24 also passes a tap test. Only by comparing it to another ASW24 that had a stiffer fin was the damage found. These videos are real world broken gliders, that professional sailplane repairers have sent me, to help educate owners and engineers on damage that has been missed.
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
Thank you, Gordon. I have added your video to my collection of your very helpful videos. My LS6 has had several unexplainable gear collapses: one during a takeoff roll, another during an out-landing, and still another during the latter part of the landing roll (immediately after applying the wheel brake at about 3-4 knots groundspeed). As it turned out, the over-center adjustment was only about 1 mm. The factory specifies 5mm. My speculation is that with time, things change. Thank you for the "grabbing paper" test. Raul Boerner
Another great video. I think your content is very very valuable for gliding community. If I will ever get my glider here in UK, I know to who I have to talk to!
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.
Wait what ? How are you going to bail out established final in a glider ? Or you mean he could have zoomed high with his excess speed, then bailed out ? Really ?
The pilot was recovered to 2000 ft above ground after the flutter occurred. He was not near the ground when it happened. He immediately slowed down to less than 50knots, and evaluated the handling before continuing his now slow final glide back to his home airfield. But the fin could have fallen off at any moment, a bit more yaw, roll or pitch load (or a combination of all 3) in the wrong direction would have broken what little skin was holding it on. In my 40 years as a repairer I have seen pilots stay with broken gliders after inflight problems or collisions. 2 I know did not survive. After recover back to level flight, that decided to land, but the tails failed in circuit, too low for bailout in a rapidly accelerating/tumbling glider with no tail. Personally, based on the UK law of averages of emergency parachute failures verses trying to land with unknown damage after a collision or failure. The parachutes offer a better chance of survival on average.
