Realistic Strengths of Homebuilt Carbon Fiber?

Hi all


Was purusing the Marske flying wing glider website.....pretty cool tooo....look
ma, no horizontal stabilizers or computer controls....

http://www.continuo.com/marske/carbon/carbon.html

Anyway, he also sells extruded carbon fiber rod....the stuff is amazing! 14
times the tensile strength per pound of 6061 AL and 4 times the stiffness per
pound!

Now, that sure makes you want to incorporate the stuff into the more critical
areas of a composite structure (like the spar cap etc etc...)...

The problem is.....your gonna have to incoportate it into your hand laid up
epoxy/fiber construction....and I'd always wonder about how much I could expect
the strength of my homemade composite material to be....

So, if I was designing using this stuff....I'd probably just use "normal"
numbers for homebuilt level composite strengths and throw in some of this stuff
for a fudge factor/piece of mind when it came to ultimate strength...

Which brings me to my question.....if you are doing homemade carbon fiber
layups...and you are reasonably careful in its construction....what kind of
real world numbers can you expect for strength and stiffness per pound over
aluminum?

2x? 3x? 4 x?.....

take care

Blll

Earlier, (BllFs6) wrote:

...Which brings me to my question.....if you
are doing homemade carbon fiber
layups...and you are reasonably
careful in its construction....what
kind of real world numbers can you
expect for strength and stiffness
per pound over aluminum?

Jim's comparisons between aluminum and carbon for strength and
stiffness per pound are the only ones I know of, although there must
be others. Jim's a great guy to talk with on this topic; he seems to
have lots and lots of practical experience with a wide variety of
materials and techniques.

Actually, now that I think on it, there are some carbon properties
results in the Test and Research section of the NTSB report on the
Minden Nimbus 4 crash (though not comparisons with aluminum):

http://www.ntsb.gov/publictn/2002/AAB0206.pdf

See pages 35 and 36. For reference, I believe that one MPa
(megaPascal) equals about 145 pounds per square inch. Note that those
properties are achieved by people with lots of experience and lots of
high-tech tooling, and who take their jobs pretty seriously.

Here's a couple of pictures of my Graphlite-strip based wing spar:

http://www.hpaircraft.com/hp-24/update_4_dec_02.htm

http://www.hpaircraft.com/hp-24/update_21_March_04.htm

As you can see in the photos, I also used Graphlite strips as
stiffeners in my wing plugs. But only to give them enough stiffness to
make them easy to handle without creasing their fiberglass skins.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24/

(Bob Kuykendall) wrote:

Here's a couple of pictures of my Graphlite-strip based wing spar:

http://www.hpaircraft.com/hp-24/update_4_dec_02.htm

http://www.hpaircraft.com/hp-24/update_21_March_04.htm


Bob,

I can't quite figure out the structure of your spar shear web from the
information on your website. Could you enlighten me please?

David O -- http://www.AirplaneZone.com

Earlier, David O wrote

I can't quite figure out the structure of your spar shear web from the
information on your website. Could you enlighten me please?

Basically, the spar is female-molded in two C-sections, with an
exterior wrap of bias cloth and then the carbon strips and the foam
shear web stiffener. The bias cloth is doubled on the back of the C.
After they're cured, the two C-sections get bonded back to back.

Here's a cross-section diagram of one of the spar C-sections:

http://www.hpaircraft.com/hp-24/typical_x-section.GIF

That cross-section is pretty much as shown on this Marske page:

http://www.continuo.com/marske/carbon/carbon.htm

Scroll to the figure titled "typical I-beam spar construction."

Also, note that in the photos on the Web site, the spar stub still
isn't finished with mating pin provisions or the final shear wraps.

There are some pictures of the plug we used to make the spar molds on
this Update page:

http://www.hpaircraft.com/hp-24/update_3_june_02.htm

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com

(Bob Kuykendall) wrote:

Earlier, David O wrote

I can't quite figure out the structure of your spar shear web from the
information on your website. Could you enlighten me please?

Basically, the spar is female-molded in two C-sections, with an
exterior wrap of bias cloth and then the carbon strips and the foam
shear web stiffener. The bias cloth is doubled on the back of the C.
After they're cured, the two C-sections get bonded back to back.

Here's a cross-section diagram of one of the spar C-sections:

http://www.hpaircraft.com/hp-24/typical_x-section.GIF

That cross-section is pretty much as shown on this Marske page:

http://www.continuo.com/marske/carbon/carbon.htm

Scroll to the figure titled "typical I-beam spar construction."

Also, note that in the photos on the Web site, the spar stub still
isn't finished with mating pin provisions or the final shear wraps.

There are some pictures of the plug we used to make the spar molds on
this Update page:

http://www.hpaircraft.com/hp-24/update_3_june_02.htm

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com



Thanks for the detailed reply. What had me puzzled is the surface
denoted by the arrow in the following picture doesn't appear to be
glassed.

http://www.airplanezone.com/NewsgroupPix/Temp.JPG

I'll be out of town for a couple of days. I'll pull the picture when
I return. Thanks.

David O -- http://www.AirplaneZone.com

Earlier, David O wrote:

Thanks for the detailed reply. What
had me puzzled is the surface denoted
by the arrow in the following picture
doesn't appear to be glassed.

http://www.airplanezone.com/NewsgroupPix/Temp.JPG

Ah, thanks, I see now. The denoted area _is_ laminated with
fiberglass. The surface texture that the arrow points to is just dust
on the surface. The nearby streaks are where the dust got rubbed off.

Those spars have been waiting around for quite a while, now - it's
been about a year and a half since the parts popped out of the molds
in early September 2002. And I've not been known to do much sweeping
or dusting around the shop...

Thanks again, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24