11-Dec-18 - 03:59 PM

Thermal Circle Text - Thermal Circle 03

Written by Hayden Daley - 03 December, 2007.
Welcome to this edition of the soaring circle, in this column is constructing semi kits, models with a fiberglass fuselage and foam core wings. Semi Kits provide good value however require a fair amount of construction, this Soaring Circle contains techniques that I have used constructing these models and techniques used by friends to complete these models. Semi kits normally don’t contain wood and other parts for completion, keep this in mind especially for larger models when purchasing material to complete these model's can be expensive.

When constructing foam cores wings you will need to have a way of holding the wing skin to the core as the glue dries. When laying up semi kit wings, a "press" really helps to make accurate strong wings and to provide even pressure on the wing skin as epoxy dries. To make a wing press is not difficult, your local wood supplier can cut wood to lengths and your local hardware store can provide bolts to be used to apply pressure to the wing skin. The wood that I used for my press was MDF fibreboard and my press measures 40 cm wide by 2.5 metres. It will cost between $60 and $80 to buy wood for the construction of a press, use 1/2 inch thickness for models under four meters, 3/4 inch thickness for bigger models. To make the press overlay two pieces of fibreboard for top and bottom of the press and drill holes every 30 or 40 cms in the fibreboard to allow for bolts used to apply pressure. When the holes are drilled your press is finished. To apply pressure to wings, put the "laid up" wing in between each side of the press attach the bolts and tighten them by hand till tight, then using a spanner tighten the bolts, one and a half turns, this is plenty of pressure, you must be careful not overtighten or you will crush wing cores, destroying the wing.

When making wing skins you will find most of the time that balsa wood is the stuff to use. There is other alternatives but I recommend the use of balsa wood, it is light, easily sanded and the only downside is long lengths of balsa wood for big models are hard to get. Friends have used other wood for wing skins, obechi was used on the popular Multiplex kits, 0.4 millimetre plywood and timber veneer used in furniture have also been used and is available in large thin sizes. Don’t feel you have to use balsa for all models, shopping around can lead to different, usable options.

When using balsa wood for larger semi kit skins you will have to splice and glue wood together to make the wing skin. When making skins tape the wood together to the width required. This is done by laying the balsa wood flat and running tape down the seam . When joined it can be turned over and opened up to allow for either PVA or wood glue applied along the join, put back on the bench to dry. You only need to tape one side and do not remove the tape until the wing dries and is pulled out of the press, the tape will prevent epoxy seeping through and gluing the foam jacket to the skins. Splicing the wood with tape will allow for width, for extra length you will also have to splice, however this time it is better to overlay the wood and cut 45 degree angles into the wood, the over lap is to allow for perfect matching of the join.

To attach the wing to the fuselage you will need a wing joiner and joiner box. For making wing joiner your local aluminum supplier can supply suitable aluminum for the task. Circular aluminum rod and pipe in many different lengths and widths can be purchased for a couple of dollars and are great for joiner boxes. I have tried lots of ways to make joiner boxes and the most suitable is to make square joiners approx 20- 30 cms for models up to four meters and forty to 50 cms long for models up to five meters. Make these boxes from ply wood 2 to 5 millmetres depending on strength required, the width of the core. Constructing these boxes entails making four rectangular pieces to make the box and support the aluminum by using square pieces to support the aluminum, four of these small square pieces is usually enough to support the aluminum tube on smaller models and six or more on bigger models. You then need to make a root rib, this is used to "face" the root of the wing with plywood. The root rib allows for incidence pins and for the joiner to be supported and also the wing skin's should overlap this root rib. When making the joiner box the aluminum has to protrude outside the box into the root rib, it helps with supporting the joiner tube. When finished cutting the small square tube supports and have drilled these pieces to the width of the aluminum so that the tube is supported down the joiner box and can't move. Using long cure Epoxy, glue three sides of the unit and the rod support pieces as well as the aluminum tube together and clamp all but the top piece in place as the epoxy dry's. You will have when dry an open joiner box and making sure all is glued appropriately you can now add epoxy to the joins or if worried about strength, and it adds a lot of weight, fill the joiner box with epoxy, and finally put the top onto the joiner box. Checking alignment and making sure the joiner box is the correct width of the core to allow the carbon fibre to bond to the joiner box. When laying up a wing with joiner boxes, cut the foam away from the core, glue the joiner box in place first and lay up each side of the wing. For alignment of the joiner box in the wing cores sit the two wing cores side by side with the rod into the tubes and using a felt tip pin mark out the area and carefully cut with a long blade making sure to get a straight cut. It is critical for strength that the carbon fibre tape, top and bottom contact the joiner box.

When making wing skins you must make sure you have the right width balsawood to match the core. To wide or to narrow and the wing will not line up with fuselage root. For spars I have had a lot of success using carbon fiber tape as the spar. When laying up the wing use an appropriate width carbon fiber tape, 4 to 6 oz tape and 1 to 2 inches wide is plenty on each wing skin. Carbon Fibre tape is available in long lengths from specialist composite suppliers. Using epoxy wet out the wing skin and making sure that there is adequate resin, too much is a lot better than a dry lay-up with wings also do not put epoxy on to the foam core, lay up the wing skins only.

When laying wings seven day cure epoxy is the best cure time and provides for plenty of working time also having a helper when laying up large wings greatly helps the process. You will need mixing tubs, brushes or roller to apply the resin and rubber gloves for protection from epoxy contamination. When the carbon fiber and balsa is "wetted out" place the wing skin on to the core and align the wing skin accurately to the core. It is okay to overlap wing skins if you are worried about alignment you can cut the skin to size later. When "wetted out" flip the wing core over and place into the "jacket" the core came with. Same with the top wet out the skin and lay up the carbon fiber tape with epoxy until wet, put onto the skin, put the skin onto the wing and your wing is laid up. Place the laid up wing into the jacket and put the top piece of the press in place, do up the bolts by hand until tight. Being careful not to crush the wing, tighten the bolts one and half turns, this is plenty of pressure and allow to dry for at least three days or longer. Unless you are going really fast or making a competition model it is fine not to have a full spar, foam is fine between carbon fiber tape.

If you wish to add a shear web you can achieve this by laying up one side of the wing, and cut a slot through the core for vertical grain balsa, then lay-up the other side. Also when using this style of lay-up of wings the carbon fiber is providing the strength and the skin is really just providing torsional rigidity. Do not get too concerned about the wing splice reducing the strength of the wing. If you wish you can run the carbon fiber too three quarters of the wing this is okay but weaker, and in this case splicing for strength becomes important. And if you require more strength you can recess the carbon into the core and use more layers of carbon fiber to add strength. By using "T" aluminum available in various widths you can create a sanding tool, this is done by gluing glass paper to aluminum and sanding the core to recess the carbon cloth. Recessing cloth is really only required when more than one layer is required. When adding more layers reduce the length of carbon tape down the wing to allow for weight and even flex especially on long wings.

For semi kits with gull wings like the Reiher vintage sailplane or the Model Flight Pirat semi kit which has a flat center section and kicked up tips, making these angular joins strong can be difficult. For these type of wings it is best to lay up the wing panels and join after they are dry. Then cut a dihedral brace from plywood to the appropriate dihedral and width of the wing. Chamfering each wing , creating the correct angle with a sanding block cut a slot for the wing brace. When epoxied in place this will give appropriate dihedral know you can lay carbon fibre tape in a "cross" shape, along the join of the two wings and along the spars, over the top of the wing. You can then fill the weave with car body filler available from your local automotive store, to hide the reinforced join. If you are worried about strength add more carbon fibre but be aware that as more is added it his harder to hide the "width" of the reinforced join on the wing.

When preparing a wing for paint you may have heard of the technique of "glassing" and painting wings, laying up light weight cloth to allow for painting. You can achieve the same result, a sealed surface to paint, by brushing epoxy on to the components and sanding, albeit without a strength gain. It is however way easier to get a good "neat" result. When dry a light sand and you will have an almost ding proof component and you can prime and prepare the surface for paint. There is no need for epoxy to be applied to a film covered wing unless you wish to make it more "ding" proof. When painting models I have had good success with the Kill Rust brand of epoxy spray paints and giving the component a light sand and applying a clear coat. With clear coat's you can make an old surface look like new just by applying the clear coat, do not be afraid to use this technique especially on less than perfect spray jobs.

For the trailing edge you can get really narrow and strong trailing edges by laying up lightweight fiberglass cloth along the trailing edge, on both sides of the skin. Lay up the wing and with the cloth cut to length and size. One to two inches is plenty, and you will have a really strong, durable, trailing edge. When sanding trailing edges, long sanding blocks, and "t" shaped aluminum is suitable for constructing sanding blocks for sanding trailing edges, long lengths can be cut to size by the supplier and are approx five dollars to purchase a 2 meter length.

When dry it is time to cut flaps and ailerons into the wing. To do this use a pen or similar and measure and mark out the ailerons and flaps. Using a razor saw cut out the marked out surface making sure you are very careful with alignment when cutting. You will then have to again cut the aileron and flaps to allow "capping" with balsawood. At this stage you will have to decide whether you wish to use tape or normal hinges. With tape hinges, sand an angle at forty give degrees to allow "top" hinging of the surface with tape. If using normal hinges, mark out the center of the capped surface along the full length and sand a "arrow" shape. Forty five degrees each side of the line. You can use cyano hinges for these models but by aware the cyano and foam don't go together well, make the balsa caps bigger for cyano hinges or use foam friendly cyano for the hinge join. Next up is the capping of the wing cut out. Again cut away from the wing the appropriate length and width, attach the balsawood, PVA or epoxy are suitable, and use pins to hold all this in place while drying. Finish by sanding neat using a sanding block.

When making control horns PC board used in electronics make cheap and suitable control horns. Using a triangle shape and rounding the PC board after it is cut. Control horns can be constructed that have massive strength and cheap cost to the size required. To cut PC board use a scroll saw or metal bandsaw blade in a bandsaw. Mark out the PC board to the required shape with a pen and cut. Drill holes in the horn to allow for clevices and for the epoxy to flow through. Cut a slot into the finished surface and glue the horns in place underneath.When making pushrods threaded rod is the stuff to use and attach clevices onto each end.

For wing tips and tail plane tips square balsa blocks roughly shaped are the way to go. I recommend gluing on tip blocks with epoxy, it easy to shape these pieces when attached to wing with a sanding block. I have heard of people laminating tip blocks with sandwich of balsa, ply then balsa on bottom for strength. If you are worried about wing tip strength then use this technique.

With leading edges these are best done as the last part of construction. Glue a square piece to the front of the wing, epoxy and so can be used and mark the center of the wing section chord line the full length of the leading edge. This is to allow a reference when sanding the leading edges to shape. This helps with accuracy and if you want perfect leading edges you can make templates of the leading edge from plywood. Long sanding blocks are most useful with this task and you must use the line to get an accurate result.

For models with spoilers I have found it is easier to add these controls after the wing is laid up. To achieve this cut a rectangular "box" from the wings and line with balsa wood, "inset" the box lowered to the height of the spoiler and lower the spoiler in and glue in place, you can then cap the spoilers with balsa wood and with bottom, again cap the rectangular box with balsa wood and, when dry sand the bottom to match the bottom skin. If you wish to put spoilers in to cores before they are covered you can use Glad Wrap used in cooking to cover the spoilers and protect them from being ruined by epoxy when the wing is laid up. By accurately measuring before wing skinning, you can cut the right spot for spoiler actuation.

When making holes in the fuselage for incidence pins and so on make a template from cardboard of the root of the fuselage to wing join and use these templates to drill holes for the wing joiner and the incidence pins, guaranteeing an accurate result. I cannot stress enough the importance of templates when making semi-kits. For things like fuselage formers for radio gear and when reinforcing the fuselage for retracts, make a template first, it is so much easier to make accurate parts and drill accurate holes.

For retracts in modern aircraft also make a template for the gear doors and using a rotary tool cut the doors of the fuselage, you only get one shot at this so be careful. Generally tape hinges are suitable for landing gear fuselage doors, however you can use a silicone hinge, tape the doors in place, run silicone along the doors inside the fuselage and believe it or not this style of hinge retracts the doors. Other wise use rubber bands attached to the retract, these will hold the doors shut when the gear is retracted. For retract mounts make these as strong as possible and make the retract removable. These surfaces are highly stressed on aero tow take off, I suggest making the mount from plywood and fibreglassing the area, these are real weak points and for trouble free operation need to be strong. Carbonfibre and Kevlar are suitable for this task if in doubt of the strength of the mount. Model Flight sell suitable retracts for modern semi-kits that have good operation and are strong.

When making tail planes you will need to make a smaller press for the task, to prevent the crushing of tailplane cores. For this press you do not need a great width for the press halves. Again overlap the wood and drill the press holes, four holes each side is plenty. Make the tail plane skins and for the mounting points to the fuselage cut squares from the core and use plywood glued into these rectangular areas to be used as crush free mounting points. For the fuselage use plywood and "blind nuts" in the elevator fuselage area for the mount. For alignment drill and recess the area, and with the help of a second pair of hands you drill the mount in the fuselage, or mark out the fuselage with a felt tip pen and drill the mounting holes. If you are worried about the tail plane strength reinforce the tail plane with carbon tape using the technique of laying up the carbon on the tail plane skins. For elevator control, I prefer this servo in the tail it is best to make a mount for wood and epoxy the servo in place using the mount with a direct link. Rudder cores also need skinning for strength it is best lay up light weight quarter ounce cloth on the rudder trailing edge and use epoxy. When dry attach balsa to the front and sand a semi circle shape, using hinge attached to a rectangular shape and glued in place with epoxy, after the rudder is complete, or glue the balsa rudder mount to the fuselage and attach the rudder afterwards. Pull-pull systems are useful for rudder control although pushrods could be used to actuate this surface.

When finishing semi-kits depending whether they are modern or vintage, paint, fabric and normal film covering are suitable for the task and also many semi- kits particularly the Rosenthal brand require reinforcement in high stress areas, carbon fiber and Kevlar are suitable for this task, the tail plane area and fuselage boom are particularly problematic areas on these large models. Epoxy resin should be used for all fuselage reinforcement.

With scale models, decals cut from vinyl make the detailing of these models easier than painting. Stripes, numbers aircraft designations are all easily made, your local sign writer will be able to help with this. With canopies I have had a lot of success screwing canopies to canopy frames. This allows for replacement canopys to be screwed in place in case of a disaster and when making canopy frames again use templates for an accurate frame to fuselage match. Graupner make scale instruments in different sizes and these also can be purchased from Model Flight. A scale cockpit with good detail really makes a scale model look good when finished.

Finally when finishing models in fabric or with covering always cut the fabric or film to the required shape before attempting to cover. Do this by making templates from paper or similar and by cutting the Film to size you will get a lot better result. Marking out accurately is vital for well constructed semi-kits, pens textas and markers are easily obtained and will really help with these models.

In regards to most of the equipment and tools I have used, pretty much everything was purchased at the popular hardware store Bunnings for a very reasonable cost. Bolts, all nuts, and suppliers, check the yellow pages for a store near to where you live and scroll saws allow for the cutting of wood with a very cheap replacement blade cost. I wish you all the best and any questions in regards to semi-kits email at soaring@newlitho.com.au.



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