"BALSA FREE" C/L RACING W INGS

Transcription

1 "BALSA FREE" C/L RACING W INGS or "a different approach to make a wing using a mould and (almost) no balsa" by Enrico M AULETTI Jan st PART THE MOULD (first part - upper mould) "Balsa free" wing means (for me) a glassed foam wing made using two devices: - a two parts mould - a simple vacuum bagging system. In m y opinion, there are several reasons to think about a "balsa free" wing: 1. Possibility to reach always the same result: 1.1. Profile: if the mould is OK, then all the moulded wing will be OK 1.2. W eight: using "constant density" materials as foam and fibre glass, it will be possible to have (almost) the same weight 1.3. Stiffness: using "constant properties" m aterials and a standardised procedure, the final result will be always the same 2. Possibility to reduce working time (accepting to spend time in the mould preparation) 3. Possibility to avoid balsa dust and lot of "sanding hours" My final weight target for a STR wing is something around g (without internal bellcrank and lead-outs) here after I'll explain why I think this is possible: WEIGHT DISTRIBUTION wing surface [dm^2] 12 fibre-glass cloth [g/m^2] 42 no. fibre-glass cloth layers (upper and lower wing surface) 2 total fibre-glass weight [g] carbon rowing (leading edge) [g] 10? foam density [g/dm^3] 30 average foam tickness [mm] 4 foam weight [g] 14.4 epoxy resin [g] 40? TOTAL WEIGHT [g] About glassed foam wing and vacuum bagging: I read a lot on Internet about how to produce foam glassed wing for different type of model glider these information I collected convinced me that it's possible to follow a similar method for C/L racing wing too so I decided to have a go! (one of the more sophisticated, but really interesting web page: Here after, a section of the mould, the fibre glass clothes and the foam sheet during the construction (of course the same job has to be done for both the moulds).

2 The basic idea is to use 2 fibre glass cloth layers (one at 90, the second at 45 for torsional stiffness) for each wing surface (top and bottom); with the vacuum bagging technology is possible to obtain 2 separated shells, one for each mould, with a very low resin quantity (that means "light and stiff"). To fill the space between the 2 shells, after the curing of the epoxy used to produce the 2 shells, two 4 5 mm thick high density foam sheets are glued to the internal part of both the shells (still inside the moulds); these foam sheets are not shaped to any profile (just cut with the top view shape of the wing), but just pressed on the internal surface of each shell by the vacuum. After curing it is possible to sand (or to cut with an hot wire) the foam in order to have a flat joining surface between the 2 parts of the wing; as reference the flat division surface of the mould is used. Before joining the 2 parts together it could be possible to put internal leadouts, but for the moment I think to make the first test with external controls and bellcrank. At least for the leading edge is possible to make a groove in the foam and to fill it with carbon rows before to join the two shells. In this first chapter it's possible to find some pictures and descriptions about the mould construction this is where I'm now in other words: it's possible to follow step by step what I'm trying to do, but I'm not sure at the moment that it will be a success of course, I hope so! This is my first attempt in making such a big mould so surely there will be things that can be improved! Note: Of course a trapezoidal wing top view can avoid the necessity of a master wing like these: it's possible to cut a master using foam, tem plates and an hot wire or, m ay be, it's possible to avoid any mould. This is my STR (you can find it somewhere in the cl_group model plan collection): As you can see it's trapezoidal and it's really "different", but it's the best model I never had, believe me! (I fly C_L racing profile models since 1978 I stopped my activity for ~15 years and now I'm back since 4 years 7 years of racing with any kind of model) The reason why I decided to use a "traditional" wing shape is only a "marketing reason". "Down under" someone made this model and another guy said him: " why you decided to build this model, it's so different, unusual try a Challenger, this is really like a F2C model!", so if I'll have in the future any change to sell som e wing (but this is absolutely not my first and only goal; I'm just thinking / hoping to supply some RTF wing in order to catch some newcomers or "returnees") I need a "traditional" shape that looks really like a F2C model something like this "Russian wing"!

3 Picture 1 - The basic idea is to produce a two parts mould (upper and lower m oulds) using a "commercial" wing as master. The two moulds will be divided along the l.e. and t.e. The green hollowed sheet is 30 mm thick high density foam that I will use as division surface. The grey part is the base to support the m aster wing. The first problem to solve is how to align the master wing in order to have the l.e. and the t.e. at the same height of what will be the division surface of the 2 moulds. Picture 2 - The 3 "things" here are M 3 screws with a small balsa block glued on the head. They are inserted inside 3 holes and it is possible to adjust their height screwing or un-screwing the relative nut. Adjusting the height of each single screw is possible to align the wing. Picture 3 - W ith the 3 "things" ("aligning screws"?) inside the holes, using double face sticky tape, the master wing is fixed on them (at this stage no alignm ent is done). In this way it will be possible to extract the wing plus the 3 "aligning screws" without loosing the relative position and the alignm ent (if necessary).

4 Picture 4 - To check the right alignm ent and height of the l.e and t.e. I use a simple device made with a pin fixed on the top of a small foam block (the small foam block is a piece of the hollowed 30 mm high density foam sheet so the pin is at the same height of the division surface). All around the wing perimeter, adjusting the "aligning screws", the l.e. and the t.e. m ust be at the same height of the tip of the pin. Picture 5 - This is the picture of the master wing after the alignm ent. The wing and the 3 aligning screws are in position on the support (the grey part in Picture 1) as the foam sheet too. The hole inside the high density foam doesn't fit perfectly the shape of the wing, but is tight enough to allow a supplementary alignm ent check. Picture 6 - This is another picture of the master wing after the alignm ent. A s you (perhaps) can see, the upper surface of the foam sheet is coincident with the trailing edge, being the "division surface" for the m oulds. But only 3 supports are not enough to avoid deform ation of the master wing during the mould construction this is the second problem: how to support the wing in a better and well distributed way in order to avoid as much as possible any deform ations?

5 Picture 7 - In order to have a better and well distributed support of the master wing I use around 50 M4 screws. They are screwed trough the support from the bottom surface (see next picture). Picture 8 - Each screw has a nut glued with cyano to the bottom surface of the support. Picture 9 - Looking from the upper surface of the support it's possible to have an idea about the distribution of the 50 screws in respect the shape of the wing. In order to not damage the master wing, on all the tips of the screws a small plywood part is glued on (see next picture).

6 Picture 10 - Now the wing plus the 3 "aligning screws" are again on the support. After this is possible to adjust the height of each "supporting screw" in order to have a contact between the tip of the screw and the lower wing surface (of course it must be possible to reach the screws from the bottom of the support). Picture 11 - The only difference between this picture and pictures 5-6 is what it's not possible to see! Now the wing is evenly supported by the 50 screws adjusted at the right height. Picture 12 - As I wrote, the hole inside the high density foam doesn't fit perfectly the shape of the wing. In order to make the mould is necessary to seal with som e putty all around the m aster wing perimeter. The putty must be coincident to the division surface; I use a sharp knife to cut the putty all around the wing perim eter. To have a good sm ooth division surface I put all around the wing som e adhesive tape.

7 Picture 13 - This is the wing ready for the gel-coat application, but remember the mould releaser and hope it will work correctly!!! Picture 14 - Around the wing I put a frame made with 20x20 mm wood. I fix it on the foam using 8 nails (just to avoid any movement) and then I add 2 other wood pieces in order to lim it the area where the gel-coat is applied. The gel coat I use is ARALDITE with HV 2419 hardener. After 2 days, over the gel-coat, I apply a layer of fibre-glass mat with epoxy resin. Then, after other 2 days, in order to have a good support for the mould, I fill the frame with plaster. The 2 white arrows indicate 2 tubes I put in contact with the wing surface; blowing some air trough them I hope to be able to have back my master wing at the end of the m ould construction. Sorry, no picture here (I'm using a traditional cam era and the film was over at this point, but I promise to continue my reportage ) Now (12 th January 01) the first mould is made: it looks great! Of course the wing must stay inside the mould to proceed correctly in the construction of the second part of the mould. In the next days I'll proceed in the same way to produce the second part of the mould then I hope to be able to take out from this "sarcophagus" my precious master wing stay tuned and "may the fortune be with me"!!! As usual: English is not my mother tongue at least I hope you'll understand the m eaning of what I wrote let the pictures help you! Enrico Mauletti A bsolutely no copyright: I'll be glad to know that this has been of some help for someone at least to avoid to repeat my mistake