Carbon or S-glass?

[Jon Matcho]

And now... for a thought I can't seem to get out of my head the past few days. I want a light plane. Not the lightest possible, but lighter than usual. I am doing some vacuum bagging, and taking good care to have clean layups. But... I have been wondering if there's any particular part, or set of parts, or system, that I could <gasp> replace an equal number of plies of glass with carbon fiber or a better glass fabric (S glass?).

 

Stray thoughts for carbon parts:

• The entire canard
• Could have done the entire fuselage in carbon, but I'm done except bottom and outside
• Heat duct and seatback brace?
• 1 ply carbon outside fuselage + 1 ply glass
• Entire canopy
• Cowlings
• Strake skins
• Winglets
• Rudders and/or elevators only?

I absolutely will be keeping my main spar and wings glass, but would consider a "better glass" there as well.

 

Go ahead, shoot me, or set me straight. I'm ready.

[TMann]

I'm doing all control surfaces, wings, center section spar, canopy frame and cowling.

 

No for canard, winglets, fuselage and especially anyplace that will have a copper foil antenna.

 

Be aware of the differences in CF and how it affects antennas, aluminum parts as well as how it breaks up in the event of a collision.

 

It also wets out differently and if you use it, V-Bag it.

[Neverquit]

Wow! This topic is as old as the hills and the digging of coal is just as controversial. Mark Z will make you go to the archives and do some learnin' for sure. I can't afford the stuff for savings of a few ounces but if money is no object, what's the method to you madness? I've also learned that CF is very sensitive to orientation for strength. As T-Mann says, bag it.

[Jon Matcho]

...especially anyplace that will have a copper foil antenna

Doh! I'll claim a momentary loss of memory for that one.

 

Be aware of the differences in CF and how it affects antennas, aluminum parts...

Right... how does one go about using it for something such as the instrument panel (just for looks)? Won't the aluminum touch it at some point? Just have to seal the edges of the instrument holes? What about the elevator/aileron torque tubes?

 

...as well as how it breaks up in the event of a collision.

Right... a shardy mess. Glass is sounding better.

 

Mark Z will make you go to the archives and do some learnin' for sure.

Hopefully I can get this post to scroll out of sight before Marc gets here. I know... I think one of my own first posts somewhere was, "Hey, anyone ever think of using carbon fiber?!" :o

 

I can't afford the stuff for savings of a few ounces...

Me neither, but I was looking for several pounds of savings.

 

...what's the method to you madness?

Saving weight. It's like buying more square footage for your house. Hard to do after a certain point.

 

I've also learned that CF is very sensitive to orientation for strength.

No more so than fiberglass I believe.

 

There may not be an obvious answer for carbon fiber. What about S2 glass for the entire canard? Lighter and stronger?

[mak790]

Jon Have you spoken to Chris Esselstyn? He built his Cozy IV out of carbon (I mean cowls, sparcaps,and etc). I think it's the best way for you if you want to use carbon. Chris did it and it works. I believe you heard about him but just in case.

 

http://www.cozybuilders.org/Oshkosh_Presentations/2006_Esselstyn/index.htm

 

Mak

[chasingmars]

How much do you want to spend per pound of weight savings. I always try to figure out how much a given savings will cost in time and money before I decide to try it. Weight not in the plane is worth something, and that value will be different for everyone, but really, that value is not infinate.

 

Let me suggest a little thought experiment. Carbon vs plans 7725.

 

Plans 7725 is listed at 300x300 lb/in strength, 8.8 oz fabric weight.

Carbon 282 is the same 300x300 according to ACS, 5.8 oz fabric weight

 

38" 7725 is $5.90/yd at ACS today, 39" 282 is $45.50/yd.

 

density for glass is about 2.6, for carbon about 1.75, and for MGS about 1.2.

 

With these starting assumptions for a 50/50 v/v fibre:resin ratio

(Some math happens )

 

1 sq yard of 7725 E-glass laminate weighs 12.9 oz (68% glass w/w)

1 sq yard of 282 E-glass laminate weighs 9.8 oz (59% carbon w/w)

The resin used is nearly the same, by the way, so no real savings there.

 

Savings is 3.1 oz at a cost of 39.60, or 205 $/lb. This assumes that you can substitute layers 1:1, which you can't by the way, and that you have absolutely no waste, so cost goes up from there for wherever you need to adjust the laminate schedule. Assuming that about 1/2-2/3 of the cloth you cut remains on the plane (wastage in cut plan, trimming, etc) and you're looking at a cost of 300-400 $/lb saved.

 

i.e. if you can't afford it for a savings of a few ounces, you can't afford it to save a few pounds.

 

For me, I cost weight savings at $100/lb and 10 hours work/lbs.

 

for what it's worth (or not worth),

 

EDIT : THE ABOVE SHOULDN'T BE TAKEN TO MEAN 282 CAN BE SUBSTITUTED FOR BID WILY-NILY. It's just an example. In reality, you do *need* to account for the differnt properties of carbon compared to glass (in part, that carbon's not well suited for shear loads).

[chasingmars]

(Regarding sensitivity to fibre orientation

No more so than fiberglass I believe.

Unfortunately Jon, here you believe wrong. I've read some academic papers on this - carbon is substantially more sensitive to perfectly straight fibre runs when compression loaded than glass, especially when it comes to impact toughness, but also generally. Essentially, carbon is weak in shear so when the fibres aren't well aligned, you get shear loading introduced that causes premature compression failure, as I recall. I'll see if I can't dig up the reference for a more accurate recollection.

[Jon Matcho]

Woah... glad I got that thought out of my head. It's gone now. Thanks for all the feedback. Weight is a big thing for me, but so is cost. Cost wins here.

 

I'm sure there will be plenty of small opportunities to save weight along the way with just good technique, as well as not introducing a lot of weight-inducing mods. I was just looking for something "obvious" to make me want to burn the $ on carbon fiber.

 

S2 glass looks good, but is twice as expensive as regular glass. It's stronger (in the general sense), but not lighter.

 

Rutan cloths shall continue. Maybe I'll just do the cowlings in carbon.

[rviglierchio]

Nemesis NXT (Neoteric eXperimental Technology) is a two place, retractable gear, single engine, kit airplane of all molded carbon fiber construction allowing a very sleek aerodynamic profile.

 

 

I parked next to Relentless last Sunday at Salinas and learned a lot about the hottest looking plane in the sky or on the ground. Carbon works fine, just has to be done correctly. One has crashed and the pilot walked away and rebuilt it.

 

By the way he is going to E-Bay it soon starting at $400,000. 500 HP wasn't enough so they are building a new one starting with 700 or so.....

[TMann]

As stated in this thread and others, if your objective is saving weight then stick to the plans and use proper technique. If on the other hand your objective is structural then CF might be justified.

 

I'm using 716 in my wings because I am building closer to the Berkut plans than the Long-EZ plans.

 

The old argument has surfaced again about the alighnment of the fibers. I think a lot of this comes from the difficulty of working with the spar cap tape and has been tranferred to the CF tape. If you have ever worked with the West 703 CF tape you would be dropping that argument in a flash. Not even close to the same experience.

 

The work around for aluminum to CF contact is to use some sacrificial fiberglass to form an insulator. Bolt holes can be drill slightly oversized, floxed and then redrilled to their proper size.

 

As far as replacing BID with 282 .... Not for me. Way too much expense (except for maybe a yard for that snarky instrument panel.)

[Hennie Engelbrecht]

All

 

I came accross avery interesting article on the Gurit website titled "Guide to Composites:

http://www.gurit.com/downloads.asp?section=000100010037&itemTitle=Brochure+Downloads

 

Many comparisons are made between various materials and associated characteristics. Loads of good information!

Hennie