{"id":13483,"date":"2016-12-02T14:11:08","date_gmt":"2016-12-02T13:11:08","guid":{"rendered":"https:\/\/www.dg-flugzeugbau.de\/en\/?page_id=13483"},"modified":"2017-10-13T14:25:45","modified_gmt":"2017-10-13T12:25:45","slug":"manufacturing-a-sailplane","status":"publish","type":"page","link":"https:\/\/www.dg-aviation.de\/en\/library\/manufacturing-a-sailplane","title":{"rendered":"Manufacturing a Sailplane"},"content":{"rendered":"
During my soaring holidays I am often asked to describe how the manufacture of a high-performance sailplane is carried out. I’ll tell a tale out of school, and you will see that it has nothing to do with industrial manufacturing processes, but is good German manual craftsmanship.<\/p>\n
Here is a somewhat simplified description of building a wing. It is built from the outside in. For that we use four large molds per sailplane, the right and left wing upper surfaces and lower surfaces. When a new wing is started the workday of the lacquerer begins at 6 AM, when the first of four UP-gelcoats is sprayed in. That is later the outer coat, which is the first thing not too thinly applied to the mold.<\/p>\n
Unfortunately we have to use polyester lacquer, because a PU lacquer, while it would better combine with the epoxy applied later, it would also form droplets on the release agent which has been applied to the\u00a0 mould, just like the water in a car wash.<\/p>\n
At 7 AM, when the actual wing workers arrive, the lacquer is partially dried and quite sticky. Epoxy is then rolled onto this surface and a thin fiberglass layer applied and pressed into the epoxy. The main function of this thin layer is to prevent the structure of the following layers to show through the wing surface. Some manufacturers skip this part of the process, but after a few years a fine diamond shaped structure becomes visible. That we do not want.<\/p>\n
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After rolling, the thin fiberglass layer becomes almost invisible, because of the saturation with epoxy. The bond between lacquer and fibers is “wet on wet” so that it can never result in the lacquer peeling off – at worst you could get hairline cracks due to rapid and extreme temperature changes during wave flying.<\/p>\n
Next a layer of carbon fiber fabric is put in, which account for most of the strength of the wing surface. The direction of the carbon fiber is diagonal for greater torsion strength. This fabric requires a heavy use of epoxy, about 250 grams per square meter, and is also very expensive. A wing of 11 sqm contains about 46 sqm. carbon fiber. In addition we need 11.5 kg of epoxy.\u00a0<\/p>\n
Foam is put on the wet outer layer of the carbon fiber fabric. The foam core is made from carefully cut and plates of PVC foam of about 6 mm thickness, which forms the center of the sandwich construction. The foam is carefully prepared , tapered towards the rear, with cut-outs for the dive brake boxes etc. and a machined bed for the spar cap. Furthermore the foam is perforated with a needle roller to better absorb the epoxy. This prevents delaminating.<\/p>\n
The spar caps, which must be able to absorb the extremely high tension and compression forces due to the bending of the wings, have been pre-manufactured with the help of a small tooling machine from hundreds of carbon fiber rovings in a special mould. The quality requirement for the spars are very, very high. A single air bubble can condemn a spar cap, and to be sure that it is not inadvertently used again, the quality controller takes his diamond saw and cuts it in half.
\n A lot of labor and material has gone to waste, but fortunately this is a rare occurrence.<\/p>\n
The foam core is then fixed, the spar cap put on it, and the inner carbon fiber fabric is layed in, – again diagonally, so that after curing a stable, pressure resistant sandwich is formed. This is followed by a layer of peel ply and a perforated foil pre-epoxied together with the inner carbon fiber fabric. Foil and peel ply are throw-away parts which are later removed, giving the inner wing surface a rough surface, which makes the gluing of the inner parts adhere better.<\/p>\n
Now an absorbent cloth is applied to soak up any extra epoxy and to make the removal of air easier, and a plastic foil is used to cover everything, and it is taped to the mold. A few plastic pipe stubs are put in and sealed with plastilin. These are used to suck out the air so that the vacuum forces the entire construction evenly against the mold. At the same time the mold is heated with water so that the wing can cure over night – warm hardening.<\/p>\n
In the meanwhile it’s now time to go home. Breakfast and lunch breaks on these days are not dictated by the clock, but must wait until certain processes are complete.\u00a0There can be no interruption of the process.<\/p>\n
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When I was “new” I had an interesting experience: I watched a foreman cut the previously mentioned carbon fiber\u00a0 fabric off a roll apparently without a pattern or tape measure. He then put it into the mold, where it protruded considerably over the edges. This struck me as irregular, using guesswork as a measurement. I was about to go over and asked him to be more careful with material that costs Euro 35.00 per square meter, when he took a roller and began to press the material into the mold. This caused the protruding material to progressively to shrink towards the middle, and especially in the nose the protruding material disappeared one centimeter at time. I was astonished to see that about 10 minutes later the protruding excess had just about disappeared, leaving only about 2 cm of excess material. I went up to him and said: “Unbelievable, how you worked the material. Ten minutes ago I was going to reprimand you for wasting expensive material, but now it fits properly.”\u00a0 He said: “Mr. Weber, I have been doing this for 25 years.”<\/p>\n
The next morning the absorbent cloth and the perforated foil are removed. The parts for the wing parting device, if ordered, are\u00a0 built in. The two spar caps per wing half are stiffened by a shear web, which is similar to the double-T-bars used in steel construction. The finished spar extends from the wing tip to the wing root and into the spar end – in a 18 m wing about 9.30 m long. The shear web is quite massive, so it can with-stand the shear forces. At the main pin, for instance, the bushings into which the main pin is inserted, must with-stand forces up to 14.5 tons ! The finished shear web is put into the upper wing shell and epoxied to the spar cap. For wings with parting the receptacle for the outer spar is set in. The spar cap of course ends at the parting and an extra spar for the wing tip is installed.
\n As mentioned before, we manufacture the spar caps separately and glue them into a 3 mm deep cut out in the foam core.<\/p>\n
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There is another method. You can lay the rovings for the spar cap “wet-on-wet” on the outer layer, and you will get a spar about 6 mm higher than we do. The advantage is such that because of the greater height material can be saved, because a higher spar has a static advantage. In very thin profiles, such as the DG-600 one cannot achieve the required stiffness otherwise. This method is also less expensive. The disadvantage is that after a few months the outline of the spar becomes visible on the upper wing surface. Take a look at various sailplane types, and you will be able to see the spar outline on the wing surface.<\/p>\n
That is why we opted for the more expensive but better method of gluing the spar cap into the foam core and fix it with the inner fabric.<\/p>\n