Structure of a spar - RC Groups
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Jan 20, 2013, 09:34 AM
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Structure of a spar

Hi all,

I'm thinking about an easier way of making a carbon spar. Mi idea is to use an aluminium profile shaped in "U" as a jig, 12mm wide —inside—. Wax it, use PVA if neccesary, and lay inside the wetted carbon tow. Just over the carbon, in the 2/3 parts of the spar —from the root—, the balsa vertical engrain —12mm wide—, and in the tips, Styrofoam Hi-load 60. Let it cure, take out, and make the upper caps in the same jig. Let it cure and take them out. Trim the core of the spar —balsa and foam—, and glue all with epoxy and micro balloons.

Name: Spar.jpg
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Description: Spar structure. Balsa endgrain for the first 2/3 and Hi-load foam for the tips.
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Description: Aluminium profile shaped in "U"

The problem is that usually spars are wider in the root, regarding to stiffness, than in the tips, wich should be lighter. My spar has the same wide in the roots and in the tips but, because the core, it should be stiffer in the roots and lighter in the tips, and much easier to build. The compress load on the foam shouldn't be critical in the tips , but I'm not sure about that. I don't know if it could be neccesary to wrap with glass sock, and my idea is to avoid it. Do you think this could be neccesary?

Last edited by teknos; Jan 20, 2013 at 10:00 AM.
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Jan 20, 2013, 10:58 AM
Registered User
Upvector's Avatar
If you have not seen this information,
you should look into it thoroughly.
The spar has been engineered to really good specifications and
needs, but you must understand the details of the information
to get all of the benefits.

Wish you luck,

my build partner and I hope to have our Supra planes
in the air this season,.. a lot of time, (3 years) and energy have gone into
the construction, but the knowledge of materials and techniques will change how
you build anything in the future. We used an aluminum "L" shape to build our spars
.. it allowed us to taper the spar and save some weight at the tips, where you want
to save weight. It makes the plane respond much better to control input, and also
will show lift and air movement much better.

Hope you can post your efforts.

Jan 20, 2013, 02:57 PM
Registered User
Hi Ray,

I know Drela's technique, but as he sais, «It's prudent to use endgrain balsa in the spar center over the wing saddle». As the wings of my plane —a prototype, not a Supra— will be made in one piece, and with the same wide in root as in the tips, ¿why dont use both types of cores to avoid the excess of weight in the tips?
Last edited by teknos; Jan 20, 2013 at 05:42 PM.
Jan 20, 2013, 06:51 PM
Registered User
Upvector's Avatar
...absolutely... the choice of material is definitely yours to make, and if it fits your design
I'm sure it will work.
For foam in the tip spar, where the size of the spar is smaller, it is critical to make sure the spar caps are held in place firmly, so a lot of care is to be taken to bond the spar caps to the foam. We also wrapped the tip spars, (like the center spar) in fibreglass cloth. We are happy with the combination of foam, vertical grain hard balsa and carbon fibre tow, that produced a very strong spar, with a little bit of ability to flex
during launch.

For us, we had to learn how to take out the epoxy that only added weight, but maintained the strength. That was (with us) a very important part of staying within
the weight limits. We will know this season how well we did,...or didn't do.

Jan 22, 2013, 12:59 PM
Master of the Wind
G Norsworthy's Avatar
Start here if you want to skip all the aero design and go straight to the spar.

I've done two planes this way and it works very well.
Jan 22, 2013, 05:51 PM
Registered User
Neil Stainton's Avatar
Originally Posted by G Norsworthy
Start here if you want to skip all the aero design and go straight to the spar.

I've done two planes this way and it works very well.
Sorry, but just linking to page 6 doesn't work for me, I have my page length set to 100 posts, and there aren't 600 posts in the thread. Can you please give a post number?


Jan 23, 2013, 01:11 AM
dare to thermal
#76 - #90

Jan 23, 2013, 06:16 AM
Registered User
Neil Stainton's Avatar
Thank you Bernd and G Norsworthy.

Very simple and effective spar construction.

Jan 23, 2013, 08:39 PM
Registered User
Is exactly what I was thinking. I'll try this in my next plane —really soon—, but with a straight spar of 15 mm wide —the same in the root as in the tip.

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