11-Dec-18 - 04:20 PM

Thermal Circle Text - Thermal Circle 16

Written by Hayden Daley - 08 September, 2008.
Welcome to this edition of the Thermal circle. In this edition we will take a detailed look at pressed composite construction as used in early 1980’s f3B models before molded model came to the soaring scene. This issue will detail the techniques used to construct these models but to use these techniques you must have a fuselage and foam cores to use in construction. Blue foam is better than white foam for these models however white foam can be used if sanded. The first molded models were pressed form constructed and petrol and acetone were used to make tho foam removable. The techniques that will be described here are not meant to be made hollow molded. It takes a lot more composites to make old style hollow molded models, and this column is intended to be used as a reference only.

The first part of construction is to make an oversize template from cardboard of the foam cores to be that are to be used as wings. The reason for these templates is to cut the fibreglass cloth to the appropriate size and to waste as little material as possible when cutting the cloth to make wing skins.

You will need to purchase a sheet of suitable size and width glass because this is the way the skins are laid up. The glass must be cleaned and a mould release agent must be used so that the skins do no stick to the glass. Then you must lay up the wing skins. The lay up for most models is 0.8 oz finishing cloth and 8oz woven cloth over the top of the finishing cloth. Weigh the cloth and use 20% more epoxy by weight than the weight of the cloth. You will need swipe cars, 7 day cure epoxy and gloves to lay up the skins. You must not use carbon fibre tape on the skins as they are to stiff to be pressed properly and that is how the spars are made.

With the appropriate size cut fibreglass cloth lay this onto the glass, finishing cloth first and 8oz cloth over the, and now to laying the epoxy. You need to pour the epoxy onto the skins as evenly distributed as possible and use swipe cards or credit cards to smooth the epoxy evenly and you must have a wet lay up because you can sand the skins later once they are dry. With skins wetted out you must now leave the skins for 7 days to cure and; using a paint scraper the skins should release but remove the skins very slowly to prevent damage. Then you must lay up all four skins and it will take 28 days to properly construct the wing skins for these style competition models. Do not use more or heavier cloth or you will have to do a wet lay up to produce the wing.

The next part of construction is to press the completed skins onto the foam cores. Press construction and wing joiner construction was dealt with in an earlier soaring circle; and most presses are made from MDF fibreboard because it is affordable, also on this website you will find out how to construct presses and wing joiner boxes. With the skins and joiners in place you must know cut the skins 1-2 cm oversize so the can be accurately trimmed once the laid up wing is completed. Now to laying up the wing. You must not brush epoxy onto the foam core but onto the 8oz side of the fibreglass cloth. The next layup is the 8oz carbonfibre tape, which is the spar wetted out on the wing skin. The skins must then be place on the core as accurately as possible and the layed up wing needs to be placed in the foam jackets as supplied with most foam core models; and then be pressed. You must then use your press with even pressure and allow to dry for 7-8 days. You can then remove the pressed wings from the press for the next part of construction. If you wish to use spruce or other wood as spars you will need a foam cutter to cut the slots and PVA and seven day epoxy are suitable for gluing in spars.

The oversize wing skins need to be trimmed to size, both the leading and trailing edge. Trim the skins as close as possible and use a sanding block to make the finished skins as close to perfect accuracy as possible. It is now time to glue the leading edges on , epoxy and PVA are suitable for this task and then sand the leading edges as accurately as possible to the wing section profile.Reference lines on the leading edge help with this task. Once the leading edges are sanded you can use thinned epoxy or layup lightweight cloth on the leading edges and fill with body filler as used in car repairs and once dry sand as accurately as possible the leading edge shape.

You will know need to cut aileron and flaps from the wings and the best way to do this is mark out the areas to be cut with narrow felt tip pens and use long length rulers to mark out as accurately as possible. If you make a mistake methylated spirits will remove most marked out, pen errors. The best way to cut the flaps and ailerons is to use a razor saw and make one cut as slowly and as accurately as possible. You must then decide what size flaps and ailerons you would like and take the cut surface and cut in half to be used as aileron and flaps. Most people use balsa wood on the flaps and ailerons as used for example in the Southern Sailplanes style of model such as the Eclipse and Ricochet and use tape as the hinges. However the ailerons flaps and wing areas must be have balsa attached as foam is easily damaged and not suitable as attachment point. The wood is easy to sand to shape and helps with torsional ridgity, and when the surface, wings flaps etc are finished brush thinned epoxy onto the balsa, sand the surface, use body filler to fill any dings and prepare the model for paint. When painted the wing can be tape hinged and the wing is completed. If you have used other techniques that you have used in construction of these models please feel free to donate text to soaring@newlitho.com.au and I will post them on this website.If you wish to use servos in the wings the boxes and wiring must be put in place before pressing the wing.

By "pre-making" wing skins before pressing you guarantee the quality of the surface on the outside of the core. To complete a "wet layup" you will need vacuum bagging equipment to complete the task. This added expense can be recreated by pressing wing skins and with the outer surface quality guaranteed you can add more carbon, triangles of carbon, spars etc under the surface. Also any disaster will not result in the loss of wing cores which can occur with wet layups and vacuum bagging. Essentially the techniques described in this column are not expensive and will allow composite models to be built on a budget.

That it for this issue, happy safe and successful soaring. Hayden Daley September 2008.

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