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Hi Chad,
Thank you for the offer of donating epoxy, it is extremely generous!! I
will contact you off the list to work out the details. In response to
your numbered comments:
1) I am still working out the details on exactly which epoxy I should
use. I know that the epoxy I used was not ideal, but it was cheep. Your
observation about a wood finish need to flex is probably correct. I do
not know how much a Laminating epoxy will change the stiffness of the
lay up. As the carbon fiber tube has sets over the past few days it has
gotten a bit stiffer. However, I think a second ply will do a lot of
good. (Should become 4X as stiff)
2) I am aware that the strength of CF is in the plane of the fiber. I
was not very surprised that the tube was bit "squishy". Here is the
analysis of stresses on the CF runner I posted today in the "other list"
in response to one of Paul's questions.
According to RAETECH corporation peak pressure during a backfire
is 75-100 psi. The strength of each ply of carbon fiber is 650
lbf/in. If we assume a worst case model of a closed cylinder.
Longitudinal force is Fl=P*R/2 and hoop force is Fh=P*R. So
Fh=100lb/in^2*1in= 100lb/in
Fl=100lb/in^2*1in/2= 50lb/in
So (650lb/in*2ply)/(100lb/in) gives us a safety factor of 13.
3) The inside of the tube is rough. I attribute this to the rough
surface of the pipe insulation foam I used. I am looking at using a wax
forms for the manifold. Wrapping the foam with Saran Wrap is a good
idea. I will try it next time.
4) I wanted to try curing the tube in the oven but my brother was not
too thrilled with the idea.
4) I am aware of peel ply. However, at this stage of the game it seems
like extra work, particularly since I am only doing one lay-up. Further
down the road a will look into using peel ply.
Alex Madsen
-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
Behalf Of Chad Robinson
Sent: Monday, July 05, 2004 9:50 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Carbon Fiber Manifold Tube
Alex Madsen wrote:
> I have finally got around to working on my carbon fiber intake
manifold
> design. I got some 2" carbon fiber sleeve. Not having any bulk epoxy
on
> hand I went to the hardware store and got some EnviroTex Lite Pour-On
> (the only bulk epoxy they had). Pour-On is sold as a heavy duty wood
> finish but it is clear epoxy 2 part epoxy. Unfortunately the stuff
takes
> a long time to dry. I used 2" foam pipe insulation as a form for the
> carbon fiber. The test sample was 1 ply of CF. After it was fully dry
> (it took 2 days) I tried to melt out foam pipe insulation with
acetone.
> However, acetone did not have any effect on the insulation and I was
> forced to cut it out with a big knife. While not unexpected the CF
tube
> was not as rigid as I had hoped. My calculations say that it would
have
> plenty of strength to withstand a backfire but I think a more rigid
> manifold would be ideal. I will try making a sample with 2 layers
> today. A second layer will also greatly reduce the likelihood of pin
> whole leeks (something I have not tested for yet). Attached are some
> pics of the CF tube.
>
> I will be sending out pics of my design work later today.
A few comments from a composite builder, Alex.
1. An additional layer would help stiffness and reduce or eliminate
problems
with pinholes. But there are some other things to consider first. I'm
concerned about the epoxy you're using. There are many types of epoxies
and
they have a wide variety of characteristics. Some actually remain rather
soft
and pliable after they cure, and this is especially true of wood
finishes.
Wood itself is flexible, and moves with changes in temperature,
humidity, etc.
Wood finishes thus need to be flexible as well because a very hard
finish
would quickly crack.
I'd be happy to help pay for a gallon kit of "real" epoxy and have it
shipped
to you for your research, since this benefits everybody. E-mail me off
list
(crj@lucubration.com) with your address if you want this. As an
alternative,
let me know where you got your CF sleeve and I'll get some of the same
stuff,
lay it up with my MGS, and send you a sample so you can compare them and
see
if the difference is worthwhile.
2. The real strength of the CF, given the weave you're using, is along
its
long axis (compression/tension). That's good, but it also doesn't help
your
rigidity test. Additional layers, especially of different weaves, would
help.
I'm guessing that you have weight to burn, since your sample MUST be
several
times lighter than an aluminum or steel tube of the same length. You
could
probably get 2-3 layers in there while still being lighter.
3. An epoxy system designed for wetting out fiber will act very
differently
when you put it on the carbon fiber. It will go on almost like water and
very
quickly soak into the fibers. I'm betting if you examine the inside of
your
tube it's pretty rough. I don't know too much about the intake design
aspect
but I'm guessing you want a relatively smooth inside tube.
If you wrap your foam insulation core with Saran Wrap or box sealing
tape and
get a good "wet" you will get exactly that. It's important to stipple
out air
bubbles with a disposable paint brush while you do the layup. The CF
makes
them harder to see but they'll be there. (This will also help distribute
the
epoxy more evenly and remove pinholes.) Regardless of the method you use
you
probably want your core to be as smooth as possible. An alternative
might be
to give it a light coat of wax.
4. A composite epoxy system is designed to improve its physical
characteristics even more if you "post cure" - throw it in an oven for
several
hours at a certain temperature (varies with system). Your tubes will get
even
harder if you do that. Do you have an oven big enough to fit the tube?
5. You could achieve a much smoother outer surface if you used something
called peel ply on top of the epoxy (and then sanded and buffed the
outer
surface) or Saran Wrap or 4-6 mil plastic sheeting (no sanding required,
will
produce a glass-like finish). I would recommend the former because with
the
Saran Wrap trick you tend to get minor variations in the surface so
you'd be
sanding anyway if you want it to look really good.
Regards,
Chad
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