(frequently asked questions)
The purpose of this page is to try to answer those frequent questions that I receive. Hopefully it will save you time and ensure that I give consistent answers. If there are other questions that flow from these questions please ask.
Q1. What size carbon spars, trailing edge etc. do I use in my model?
A1. e-mail me the details of your model and I will advise. It is easier to do one-off quotes rather than try to compile a complete listing. Some basic sizes and suggestions are included with the Hi-Tech Materials stock list.
The block of questions and answers below are taken from a requirement and questions of one of Free Flight Supplies customers:-
"I am ordering the "Millennium Flashback" plan as I think it will be a good start to re-enter F1A. However, I am thinking of modifying the structure to be a closer starting point before eventually moving up to the all composite D-box bunt structures that seem so prevalent today (things have changed over the last 15 years!).I was thinking of going to a carbon fibre tube spar arrangement (8 mm in the centre panel and 6 mm for the tips). Since the span is relatively short, and the section quite thick and would carry less load, my thinking is that this would be fine for a zoom launch model."
Q2: Am I thinking right
A2: If you try the tubular spars remember they would not be any stronger than the existing structures. They would be stiffer but they could split if too higher load was applied. They would be OK for the zoom launch but not for a bunt.
Q3: Can the tube spars be used to house the wing wires? Seems very convenient, if so, what size wire would fit the inside? If no, how would a structure be formatted to transfer the load to the spar?
A3: Yes you could put a wire in the tube but the dia of the tube is greater than the wire needs to be so it would need to be sleeved down. There could also be a potential problem of splitting. The spar would need to be bound with Kevlar thread where the wire was installed.
Q4: I am assuming the spars are "butted," thicker or stronger on the side the mounts close to the wing canteens. Am I thinking right?
A4: I assume that you are comparing tip and centre? If so OK.
Q5: What size of carbon trailing edge material would be appropriate?
A5: TE size of 3.5 x 0.8 mm, however this assumes that you cap fully. a carbon te dictates that you use rib caps and extend the caps over the te.
Q6: What size of cap strip material should be appropriate? (I plan on using 3/32" thick ribs -- first time doing this type of structure ) I figure thicker would be better the first time as hands could be a little clumsy. FYI: I plan on using a hard balsa leading edge.
A6: With carbon rib caps the cap carries the load so 1/16" thickness will be fine and caps to that width of 0.01 mm - 0.015 mm thickness.
Q7: What would be the sizings for a similar structure for the stab? (tube spar, TE, cap strips).
A7: Carbon tube spar 4.5 mm or 5.0 mm dia, ribs 1/16" same size caps as the wing and a te of 1.5 mm x 0.6 mm
Q8: It seems like plastic coverings are very much in vogue these days. Sure seems real slick I think the air would like something a little rougher and tissue/dope like. Are there alternative coverings that offer this type of surface but with the weather proofing of modern covering at a reasonable weight?
A8: For the wing of an F1A the Polyester tissue is a good choice. The tail I would still suggest Mylar of 10 micron thickness
"I know this is a lot of questions. But thanks in advance. Once I get this figured out, I would like to order the lot enough for two models."
I have based the above comments on the fact that you say that this is your first try at a "modern" structure. With practice you will be able to refine the above.
Q9: What are the advantages of pre-cured materials as against laying up from tow?
A9: The pre-cured materials, although more expensive, are produced under controlled conditions in that the mix of carbon and resin is optimised and hut cured in an autoclave. This will produce a consistent product and a better strength to weight final component. The hot cure will prevent problems of softening when the model is subjected to high temperatures on the flying field.
Q10: Will post curing of cold set resin help?
A10: Post curing of cold set resin will help. Components can be post cured in a hot box on in the airing cupboard. Note this must be done straight after the resin is cold cured if it is to work. Problems come with post curing of lay-ups that are part of a mixed material structure.
A11: What is best matrix to use, epoxy or polyester resin.
Q11: Although more expensive always use epoxy as polyester will produce soft components. You can check what has been used in pre-cured materials by cutting with a saw. Polyester resins when sawn will produce a smell epoxy will not.
Q12: How can I tell the quality of pre-cured materials.
A12: Quality is not as easy to check as with traditional materials, no such thing as quarter grain carbon! Bright shiny material suggests access resin in the lay up. Drop a piece on a concrete floor, good material will "rattle" and bounce.
Q13: What is a pultrusion and what are it's benefits.
A13: Take a look at Maarten van dyk's web site, Maarten produces all Free Flight Supplies pultrusions. Go to Maarten Vandijk web page for manufacturing details.
Q14. What is the point of mixing C-fiber and Kevlar in "mixed cloth". Price perhaps? As they have differing stiffness I would have guessed that the less stiff material is redundant.
A14. The idea is to get the best of both. Carbon is stiff but can be brittle. Kevlar is unbreakable! So that was the idea.
Q15. Flat carbon fibre tow, What does "flat" mean, I thought that tows were untwisted "ropes" of carbon fibre strands and as such without form until impregnated with resin and cured.
A16. Tow is flat and the fibres should run straight with no twist. Twist would both damage the fibre as well as reduce the potential effectiveness.
Q17. Unidirectional carbon cloth. What does this mean. In order to be "cloth". I assume that there must be a warp and a weft or it is simply unidirectional fibres that fall to bits when handled.
A17. It is a fine cloth with the occasional fine cross loop that holds it together. Care is needed when it is used.
Q18. Carbon fibre sock. If this is preimpregnated, how does one organise the resin cure. The only prepregs that I know about require carefully controlled heat for the cure.
A18. The sock is a tube of unimpregnated carbon woven from carbon filaments.
Q19. Tubular carbon spars. Are these simply parallel tubes to the dimensions given, or have I missed something.
A19. Yes they are parallel tubes.
Q20. If I understand correctly you are saying that your precut carbon fibre spars can be assembled into, say, an I or C section with adhesive. What adhesive gives a good enough joint. I have always thought that a fully moulded spar was the only way to use the full potential of carbon fibre.
A20. Carbon will stick with either epoxy or cyano. The strength will come as you say from the moulding or the material. The way that the spars are used is as two flanges - one at the top of the wing section one at the bottom. The space between is filled with a full width web of vertical grain balsa. To stop the two shearing apart the spar is encased in carbon sock or closely bound with Kevlar thread.
Q21. I am considering running a strip of carbon fibre along the bottom spar of a A/1 towliner. The spar is a piece of 1/16"x1/4" spruce, flat side down. Last time - quite a few years ago - I used Cyano with spruce spars in a wing, but I was not at all happy about the wood-to-wood bond using the Cyano so I have not used Cyano with spruce since then. So, what adhesives should I use with carbon fibre material and spruce?
A21. First you must put the carbon on the top not the bottom! The bending loads dictate that the top spar has to have the strength. Use epoxy rather than Cyano. The reason for this is that the materials that we use for spars be it wood or carbon are least resilient in compression. The top spar compresses and the bottom is stretched, try pulling on the ends of a bit of 1/8" square then pushing on the ends of the same piece. The upper flange of the beam has to be of a larger cross section than the lower to resist bending. Flat flanges webbed apart are best. The further apart the better, however the wing section thickness dictates the distance. The web has to be secured to both flanges otherwise the spars will shear out under bending loads. Webs should have a vertical grain to resist the tendency for the bending of the top spar to compress the fill. One serious mistake is to make flanges of different materials that have different properties. Use the same material for top and bottom, just use a larger cross section on the top flange. The joints can be made with either cyano or epoxy When complete the assembled spar must be bound with Kevlar thread and sealed with cyano to finish.
Q22. I want moulded carbon fibre stab platform mounts. I have both the F1A (small size) and F1H carbon fibre tail booms and am looking for some method other than trying to use epoxy and microballoons to mount my platforms. The internal sizes are not compatible with my requirements and the boom diameters. Is there a "split ring" bottom that could be squeezed to fit the boom dia where I want to locate the stab?
A22. The standard moulded tail mounts can be cut and squeezed to fit. The best way is to cot the lower portion off and bind in place with Kevlar thread and fix with cyano and epoxy.
See also Publications - "How-To-Do-It" No's.3, 8, 9 & 11 for help in using these hi-tech material.
Or see the custom kits page for hi-tech wings.
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