Carbon fiber laminated wing spars

[N74DV]

What would it take to have approved a set of new wing spars that have been laminated with a layer of carbon fiber cloth on each side of the spars?

I can only think of one negative issue with this which is it may make inspection of the wood more difficult or impossible??

[VictorBravo]

Re: Carbon fiber laminated wing spars

BAD idea...

Carbon cloth will do very little, it is a bidirectional material not suited for spar cap loads.

Unidirectional carbon would do some good in terms of breaking strength, but the spar design would need to consider the carbon from the start, not an add-on.

The BEST choice for a carbon spar is the "new" solid pulltruded carbon rods, sold by AC Spruce as "Graphlite".

This material could make a spar that is 2X stronger and half as heavy as the Spruce. But you would be designing the spar from a clean sheet of paper. The rib attachments, drag wire fittings, compression struts, etc. would all have to be different.

You might be able to make some strength improvement by laminating the carbon rods to the top and bottom of a wooden spar, but you would then have to wrap the spar in fiberglass cloth to hold these rods in position. If done properly with an engineer looking at the hidden problems only engineers can see (calling Mr. Jarrett...), you could potentially make a huge increase in the breaking strength of the spar, but chances are it would add weight if you started with the existing spar.

However, the engineer would likely tell you that the wing would likely stall before you actually broke the spar on a properly built stock wood spar wing anyway!

[astjp2]

Re: Carbon fiber laminated wing spars

I was going to do a replacement carbon fiber spar as my masters thesis project but once I got looking into it, it is not practical unless all of the ribs are carbon or glass also. Carbon is highly corrosive to the aluminum ribs. If the 2 are joined together, within 1 year the ribs would be gone. The carbon is 6 times stronger and about 100 times stiffer than the aluminum spars in like a cub for the weight. They do not flex which changes the loads on the airframe and strut attach points. The concept is good but the problems outweigh the gains. Tim

[VictorBravo]

Re: Carbon fiber laminated wing spars

That's probably right on the money.

It would be very tempting to build a replacement wing, Rutan style foam cores and glass skins, with the carbon rods as the spar caps. It would be pretty unlikely that you would save any weight that way.

[N74DV]

Re: Carbon fiber laminated wing spars

If the 2 are joined together, within 1 year the ribs would be gone.

hmmm..... several of my RC helicopters and camera mounts are carbon and aluminum and the two materials contact each other almost everywhere. Had them for years and have never noticed any corrosion.

[Hank Jarrett]

Re: Carbon fiber laminated wing spars

The stiffness of the carbon is the problem. A common failure in spars is crushing in the extreme fiber of the compression side. Just laminating some carbon on the surface of the wood would probably cause the load to build up in the carbon (since the carbon doesn't yield anywhere as much as the wood) and the wood won't pick up it's part of the load. Instead a high sheer builds up between the carbon and the wood and the carbon just buckles up on the compression side as the spar flexes around the tension side. It would look kind of like the top spar cap was being blown off.
You really need to design the spar from the ground up as a composite spar.
As to corrosion from contact with the composite, all you need to do is electrically insulate the metal from the composite and keep any electrolytes away (paint her and keep her dry, kind of good advice anyway).
Hank