HiTech F1A spar construction

[Anonymous]

Pete McQuade asked the following:

At Lost Hills two years ago, after the Finals had wrapped up, you told me how M&K build their extra-stiff spars. I’d like to experiment with them a bit now. I think I remember the general method you described, but I’d like to make sure I have the details right. Is there any writeup on the method, similar to the excellent Digest series you wrote on building composite wings back in the 90s. (I’ve lived by those for the last 10 years!) If there isn’t any such writeup, would you mind putting your thoughts into an e-mail, to get me started on the right foot?

I directed Pete to Rene Linmberger who has kept up on this better than I. See next post for Rene’s reply.

JIM

[Anonymous]

Pete, here is a little overview of the current spar technologies i am aware of:

1. all in one molded d-box and spar
this is my favorite. Attila in hungary has this technique down to professional level. he makes d-boxes for all the top fliers including all of victors. basically there is a ground (yes, ground, not machined) 3 part alu mold, a core of the spar made from very hard rubber and a core for the d-box made from very soft rubber. unidirectional, HM carbon prepreg is layerd on top and bottom of the hard rubber spar core and 45deg carbon prepreg is wound around the d-box rubber core and the spar core. the whole bundle is then put in the 3 part mold and hot-cured. the resulting d-box & spar is unbelievably stiff, both lateral as well as torsional. this method is pretty expensive due to the tolling cost but leads to the ultimate results.

2. spar made from DPP carbon
Maarten Van Dijk in holland, makes amazing pultrudes HM carbon strips in various sizes. these pre-cured parts are extremely stiff, extremely straight and have a very low resin content. several people build spars from this. the general technique for assembly is like such:

(directly from Allard)
Spar caps of HM carbon pultruded strips, assembled in 1.1 x 4 mm three strips side by side and the cut with a diamond circular saw to a double taper. Core is a sandwich end-grain medium balsa/80 grams carbon+epoxy at 45 degree bias/ medium end-grain balsa. Wrapping where the joiners are

this can lead to very stiff spars too but is generally heavier than the molded prepreg approach due to the extra adhesive and core material. Matt Gwain’s Cstsales now sells DPP products in the US btw!

3. spar made from carbon roving (tow)
some people make spars from raw carbon tow/roving. you need some tooling for this, but it can be made from home depot alu stock. basically you create a “U” shaped bottom mold, put the carbon in, put the filler material in, then the top carbon, then close the mold. the problem of this technique over the others is that the carbon orientation isn’t optimal and resin content will be much higher. there is a good writeup on this here (in german, sorry, but there are pictures too):

http://www.iag.uni-stuttgart.de/people/thorsten.lutz/thermiksense/t_jeder.pdf

i am sure others can contribute other methods but these are, in order of stiffness, the methods i am aware of. if the tooling required for method #1 is not an option for you, #2 can be a cheaper option.

[Anonymous]

I suggest you consider adding this to your NFFS discussion thread
Nick
Carbon Fibre Wing Construction
by Martin Gregorie

http://www.gregorie.org/freeflight/

Lots of good stuff, The spar construction is the CF caps with balsa web and kevlar thread wrapped. Still a good spar but not as stiff as the one piece molded ones.

[Anonymous]

You can also find a load of good stuff in past Free Flight Forums; available from Martin Dilly and the NFFS I think.

If you live in places where you don’t hit anything hard then the UD d-box construction is fine but if you fly where you can hit, for example, free range haggis then you need a d-box material with more impact resistance such as carbon or hybrid (carbon-kevlar) fabrics. Whilst this wont give you the same stiffness and will be slightly heavier it is worth the price for better damage resistance. Any of the methods described above are still applicable.

CHE

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