Written 1999 by Erik Dahl Christensen. Will be edited sometime 2004

  Three plan sketch of Trinitus placed on WWW.AERODESIGN.DE

  Three plan sketch of Trinitus placed on WWW.F3J.DK

Here is a link of  a very nice looking vacuum pump system - Thank you "Eccoy"

And here is my page with Eccoy's advice and links to other vacuum pump system building advicees.

Here you can see a list of my sources for supply.

A bit more than a year has gone by since the design days and we have hopefully all become wiser, smarter not to mention younger and more pretty. Until now we have build  a total number of about 30-35 airplanes, and it is not all of them that are flyable today – some of them has been the subject of our learning mistakes. This is unfortunately the price you have to pay when getting into a completely new field. By now we have though learned to build them strong enough to cope with even the strongest F3B demands, without having a wing loading the size of a space shuttle. This means that we can build the models so strong that we don’t have to worry about launch in high winds (more than12 m/s) and still keep the weight around 2,2-2,5 kg. This gives a wing loading around 33-35 g/m² without ballast. This is very satisfying. At the same time we have optimized the building process so we have to be as little time in our workshop as possible – and this is what this article is about. I do realize that it is very few of you that ever get to build your own gliders in a mould but for you who don’t, it may give a slight idea about what is inside these plastic pigs. For those of you who get to the molding sometime, it may save you some time and frustrations about the fruitless mistakes that we have already made. This is not only our hints ands tips. We have been in contact with among others Martin Webershock, Peter Huberts and some from the Swedish F3B team, and they have all given us some very helpful hints along the way. Thank you.

Step by step I’ll tell and show how we make a carbon wing for a glider. I’ll also tell a little about the mistakes we made and how we solved the problems. I do hope that the pictures will be of help to this text. I’ll start with the first thing you see – the paint, which is also the first thing to do, and then work my way into the wing, in the same order as we make it. Please forgive my not always correct English and the fact that Denmark went metric years ago – ”stones per cubic yard” gives just as much sense to me, as ”rød grød med fløde” is hard for you to say correct. Have fun.

The paint:

Gel coat or two-component car paint? Some of us use gel coat and some use car paint. Some have even used spray from cans but I don’t know exactly how to work with it.

Gel coat has the advantage that it should give a slightly elastic surface and besides you can pint the moulds and wait almost forever before you make the wing and without having to spray something on the dry surface. The disadvantage is that you have to find the right gel coat and according to the gel coat instructions you have to paint rather thick layers (min ½mm – that’s a lot!) to be sure that it hardens. In practice you can paint thinner layers but you’ll have to experiment, because a very thin layer with the amount of hardener that the instructions say, can take 1-2 months to harden. The gel coat we have used should have 2% hardener, but we used 5%. And gel coat is cheap.

Car paint makes a hard surface, but it is impossible to get epoxy to stick to dry car paint. This is great on the outside, but on the inside it is nice to have the paint to stick to the wing. If done wrong  (have been there) all that happens is that most of the paint stays in the mould or just can be peeled of the finished wing. This is not funny if you have spent time making a great color scheme  - it’s not funny even with a simple paint scheme! It is though not difficult, and it can be done right in (at least) two ways:  

1.      Paint the mould and wait about 3 hours before you start laying the cloth into the mould. This way the paint hardens so much that the cloth structure cannot bee seen through the paint, but not so much that the epoxy don’t stick to the paint. This is a rather easy solution, if you have the patience to wait. Claus and my workshop is though not in our backyard and our families like to se us at least once a week, so we found another solution.

2.      We paint the moulds and go home. The day after (or more as we like) we spray on a thin layer of ”transparent filler” that acts as a glues between  the paint and the car paint. Then we make the wing. This works perfect. 

Since I’m Danish I have not given any product names. I suggest that you contact a car painter in your neighborhood and get some products like those I have mentioned but in the brands of your country. 

Molding the skins - protect you self:

Choose which lay up you want. This also determines partly the models weight and what purpose you can use the model for. Before you start, make sure that everything is ready. ¾-1½ hour working time is not much if you have forgotten to cut the cloth or something else. First cut you cloth 25-50mm oversize. It is very important that the threads on the main cloth are 45 degrees to the span wise direction, since this is the way to make torsion stiffness. The skins make the torsion stiffness and the carbon main spar makes the bending stiffness. Your support material (the balsa or rohacell in the skins) shall also be prepared, because we make the skins in one process. In the beginning we used a PVC material called Herix (green), but we think balsa is easier to control and with balsa we can use a suction cloth which in German is called ”Abreiss gewebe”. Abreiss means tear-, and gewebe is German for cloth, so Abreiss gewebe is tear cloth in English. With tear cloth we can remove some of the redundant epoxy from the inner cloth and from the carbon main spar.

A few words to explain what tear cloth do. It is a nylon material which means that epoxy can’t stick to it and it is a cloth which means that there are many tiny holes in it. When you put it on some wet epoxy with paper in the other side, the paper will suck in redundant epoxy through the holes away from the material, and since tear cloth doesn’t stick to your epoxy material, you can tear it away when the epoxy has hardened. At the same time you remove the paper and the redundant epoxy – this is weight saving. tear cloth can also be used as hinges. If you place a 15-25mm wide piece of tear cloth along the hinge line between to layers of cloth, the nylon will be glued to the glass or carbon cloth with epoxy through the tiny holes. When the wing is finished you can cut the rudders on the side that shall not be the hinging side, and with a straight ruler as control, you can move the rudder up and down until the cloth breaks. Since tear cloth is untearable it will not break, and you have a perfect hinge. This method has been used on a lot of plastic pig at the same time as others has used silicone hinges. Both works well.

Our balsa is 1,2mm thick and comes in 250 x 1700mm size. Actually we order the size we want. It is of course ordinary balsa sheets which are glued together with rabbets and it is very light balsa. All of our oversize sheets weight around 43-47 grams which corresponds to a normal sheet (100 x 1000mm) weighing 10 grams. Cut the sheets so they match the wings contour minus 6-10 mm at the trailing edge (this is where the wing is very thin). We have made a very accurate template the size of half a wing. It is made of 1mm steel plate and made on a machine I don’t know the name of, but the machine used the same CAD drawing as the positives were milled from so it is very close to the right shape. In this template there are holes at the correct places to mark the support material, and since the holes are not exactly on the same spots on top and bottom due to the surface curving more on the top surface than at the bottom, we have 2 templates. Cut a slash in the sheets where you have determined you want your carbon main spar and in the size you want it. Don’t remove this slash until the balsa is in the mould – you can keep it in place with a few pieces of tape. This makes the big sheets easier to handle. Our main spar slash is 45mm wide at the root, 10-15mm wide at the wing join and 4mm wide at a distance of 1350mm from the root. Cut another slash where the flaps/aileron hinges shall be. On both sides of this hinge-slash you shall trim the edges (look at the drawing of the wing cross-section). If the airfoil you use is so thin that there is not sufficient room for the servos, remove some balsa to make room for them. The trailing- and leading edge shall be trimmed (look again at the drawing) 

When you know the wing area and the weight of the cloth you want to use, it is time to mix epoxy. We use the thumb rules that say that for 1m² cloth weighing 100 g/m² you shall use 100 grams of epoxy.

I build my wings in many ways:  

1.      Glass wings with either 1 layer 105 g/m² or 2 layers and 49g/m² on the outside, and 1 layer of either 25 or 49 g/m² glass on the inside of the support material (balsa)

2.      Carbon wings with 1 layer of 25 g/m² glass cloth (not diagonally) and 1 layer 93 g/m² carbon cloth diagonally on the outside and on the inside 1 layer 25 g/m² glass (not diagonally)

Our wing is 64 dm², so in the glass wing we use 105 times 0,64 = 67,2 gram epoxy and in the carbon wing we use (25+93) time 0,64 = 75,5 gram epoxy for each layer outside the support material. We haven’t come to the support material, main spar or inner cloth yet. To be sure that the leading edge is perfect we fill the most curving 2-4mm of the leading edge with a mix of epoxy and micro balloons. For each half of the leading edge we use 15-gram epoxy. The brushes and the paint roller take some epoxy too (15-20 gram) and it is impossible to get all of the epoxy from the mixing pot so we add another 15 gram. Which gives us the following numbers:  

• For the outer part of a glass skin we mix (67,2 + 15 + 17,5 + 15) 115 gram
• For the outer part of a carbon skin we mix (75,5 + 15 + 17,5 + 15) 125-130 gram

(se different lay-ups for more and newer details)

Mix you epoxy in anything suitable – we use milk bottles made of cardboard, cut to size. Take 15 gram in another container and add about 1-1½ dl micro balloons to the leading edge, mix it and pour the mixture into a small plastic bag (size 4-6 l) and cut a small hole in one corner. Now it is time to pretend that you are a confectioner and place the mixture along the leading edge – afterwards you even it out with you thumb. Now you place the first (or only) layer in the mould.

Here you can see a picture of what it can look like when I'm ready to put a wing in vacuum,   The glass cloth is being wetted.

If you use more than one layer wet them one by one, since the following layer then will be wetted from both sides. Make sure the cloth doesn’t wrinkle, and pour some epoxy along the span on the middle of the mould, and wet the entire cloth with the paint roller. When the first layer is wetted, you continue with the second layer (any surprises here?). There is a distinct difference between glass and carbon cloth. Glass is white un wetted, but gets clear/see-through when wetted. Carbon is black at any time of the day, wetted or un wetted so it is important to calculate how much epoxy you shall use and not use more than that. If you can remove any epoxy it is wetted (check the entire wing). You shall put so much pressure on the putty knife that you don’t move the cloth.

  Carbon cloth being wetted.

Three pictures showing with text what a glass/carbon center D-box skin looks like

If you use carbon cloth, cut the leading edge clean with a scissor, since carbon/epoxy is rather stiff and gets very hard when hardened. If you make a glass wing you can wait till you are going to close the moulds.

Now you put the support material in the mould, and adjust it so it is flush with the leading edge. Hold it in place with lots of small heavy things you have laying around.

Now it is time to measure you carbon tows and lay them flat on an plastic covered table. For each part of the carbon main spar inclusive the inner cloth, we use about 130 gram epoxy. This is a pure experimental result. It is important that you use carbon tows that are pre-wetted (about 1% epoxy). Pour epoxy over the tows and press epoxy into them with the roller.

Do NOT try to check each tow to see if they are wetted – they are. Now you place the tows in the slash in the support material. Start with the longest tows and place the longest tows in the middle and make sure that they are a bit more straight that you can make them. Place all of them side by side until the slash is filled and the rest on top of this layer or wetted tows. With the roller you gently press them into the slash, to make sure that they are into the slash without (to much) excessive air.

Next you place you inner cloth over the support material and wet it with epoxy. Place extra pieces of glass where you want to place your servos. We also place a piece of 50mm wide carbon cloth over the center of the wing, where the wing will be fastened to the fuselage.  

At last you place you tear cloth over the mould, some tissue from the kitchen above the carbon main spar, and with a ”joint filler gun” you place acryl joint filler on the sides of the mould all the way around, and place and fasten a piece of thin plastic over the mould and make sure that it is tight. The few un tight places, you will discover when you connect you vacuum compressor.

Hopefully you have 0,05-0,2 bar inside the bag. Now you do the same with the other half of you wing mould, and within a total of 2½-3½ hours you have a wing mould set under great pressure

.

Assembling the wing molds:

The day after the skins are theoretically hardened, but to be safe we wait another day before we continue.

The first thing you do is to remove the plastic and the joint filler on the mould sides. Then you carefully remove the tear cloth by tearing at an angle close to 180 degrees to the direction it originally lay in, i.e.. along the span wise direction, to be sure that you don’t tear the wing out of the mould.

Now you do hopefully stand in front of 2 perfect wing skins with a perfect surface for gluing. Now we cut the trailing edge at 10mm oversize with a knife and ruler, and the leading edge we clean with a firmer chisel, so the leading edge is flush with the mould edges.

This shows the leading edge cleaning process on the V-tail

This makes it easier to clamp the 2 mould halves tight together in a short while. To be safe we sand a bit on the skins all the places where we are going to glue.

Now it is time to remember ALL the things that go inside a hollow molded glider wing. It may be a god idea to make a check list because it can be difficult to place anything inside the finished wing.

What goes in a wing is the following:

Material / Things:                                                                         To be glued with:

Webbing                                                                                      epoxy/microballon

Foam list with carbon sock in front of the hinge line                         epoxy

Foam list with carbon sock behind the hinge line                              epoxy

Alu. plate for fastening the wing to the fuselage                                epoxy/flock

Extra fastening material                                                                 epoxy/flock

Extra + extra fastening material                                                      epoxy/microballon

Foam airfoils with glass sock at the wing joints                              epoxy

Brass tubes at the wing joints                                                        epoxy/flock

Foam webbing with glass sock at the tip                                        epoxy

Carbon tubes at the wing joints                                                     epoxy/flock

Glass sock to secure webbing and carbon tube                              epoxy

Closing of the ailerons and flaps                                                       epoxy/microballon

Perhaps servos and wire                                                                epoxy/flock

Assembly of leading and trailing edges                                         epoxy/microballon

Wetting out the webbing to make sure the balsa doesn't "steel" epoxy when assembling

This is the new way (summer 2004):

The webbing is 165 grams carbon on both sides of 5mm depron, The ribs are the same or 80 gram glass on 3mm depron.

Have a look at the enlarged picture with inserted text.

To close a wing I use a total of about 200 gram epoxy.

It is time to close the mould.

Lift the empty mould on top of the filled one and let it settle. Clamp it together on the middle and work your way towards the tips. When almost done, nail the guide dowels / tillers into the holes and clamp the mould halves together with all your strength. We use one clamp for each 250-350mm on each side. It is very important that the mould halves are as close as possible, because otherwise you will get a thick trailing edge, or a wrong (=thick) leading edges. Since the leading edges consist of 2 layers of outer skin it is impossible to make it perfectly sharp, but around 0.1-0.3mm will do.

Now you can breathe again and start dreaming about the perfect painted and ornamented very light and very strong beautifully assembled wing you can take out of you wing mould in 48 hours.