Replies to This Discussion

Permalink Reply by Jon Humberd on January 16, 2018 at 8:22pm

I am still struggling a bit with the decision of Carbon Fiber or Fiberglass. If I assume 1 yard of fabric/layer and go with 10 layers of Fiberglass, the total would be ~$90. I have heard that carbon prices have come down some recently. If I go Carbon Fiber and cut the number of layers back to 6-7, the total cost would be about twice that of the Fiberglass. I'm ok with spending the extra $100 for Carbon Fiber if it will be less work/layers, lighter, stronger, and possibly better than Fiberglass. Thoughts?

Decisions, decisions...

Also, I have a couple of other questions.

About how much epoxy should I expect to use on this project? I am guessing that I need to buy the 1.2 gallon kit of the West Systems 105 Resin and the 206 Hardener?

Should I even bother with peel-ply on the exterior? If I use peel-ply, would it still have to be sanded (roughed-up) before I fill and sand the exterior for smoothing using epoxy and micro-balloons?

Keep the comments coming. They are all a great source for keeping me up at night studying, researching, and thinking about my approach to the next step of this cowl!

• ▶ Reply

Permalink Reply by Bob Hartunian on January 16, 2018 at 10:20pm

I think I've said enough about material selection. For a cowl with low loads where stiffness is not critical and neither is weight, I would always advise using glass. If you had a part where stiffness is very important, then you could make part from carbon. Both carbon and glass have relatively high tensile strengths, Glass laminate will end up with a stiffness of 3M psi and carbon closer to 9M psi. Carbon is more expensive and more difficult to layup into radii as it wants to bridge because it's stiffer. It is also more difficult to cut/drill than glass. But you seem to want to push for carbon so I'll stop pushing glass. You could make first part with glass to develop technique and a second part with carbon when learning is over.

About 2-3 qts should do it for resin volume.

Some peel plies use a release that transfers to surface of laminate and should be sanded off to assure  adherance of filler material. I don't see any huge advantage of peel ply on this part.

• ▶ Reply

Permalink Reply by Jon Humberd on January 16, 2018 at 11:07pm

I am still working out the last smoothing details. Tonight I taped the joint at the fuselage and applied another coat of my special "hardshell" mix on the entire cowl. Hopefully sanding tomorrow night will get it all nearly close enough for paint. (Although I expect to have at least one other application and sanding before I feel it is worthy)

Bob, thank you for all your knowledge. I did the hands-on with the EAA composite class, but they did not cover anything about carbon and I have never even touched carbon before. I did not know that it was that much more difficult to lay up because of the stiffness. I am thinking that I will do exactly as you suggested. I will try my first cowl as a glass layup, then if the male plug is still suitable for a second layup (and if I feel I can improve my technique) I might try a carbon cowl. I think with a second cowl, I could build a plywood firewall, mount the extra cowl to it, and make a male or female mold with spray foam later on if there was ever a need.

I'm sure I will have more questions in the next few days and I am also sure I am going to owe you a steak dinner if we ever meet up after this is all over with.

• ▶ Reply

Permalink Reply by Henri Luyckx on January 17, 2018 at 2:53am

If any doubt about carbon vs fiberglass take a look at this link:

https://www.youtube.com/watch?v=j7bQYEt_qtU

• ▶ Reply

Permalink Reply by Ken Ryan on January 17, 2018 at 11:28am

It's interesting how little weight is saved using carbon fiber. I think it is a pretty common myth that carbon fiber is MUCH lighter than fiberglass, when in reality it is only a little lighter. In a different airplane I looked into using carbon fiber for floorboards (vs aluminum) and there was virtually no weight savings.

• ▶ Reply

Permalink Reply by Jon Humberd on January 17, 2018 at 3:13pm

Wow. Not one good thing has been said about Carbon Fiber here. Apparently I almost fell into the pitfall with so many other people that think Carbon Fiber must be the latest, greatest composite to build things with. I guess it is just pretty to look at...

Thank you to everyone here for throwing me a life jacket before I drowned in the sea of misbelief.

Maybe I will make two Fiberglass cowls off this male mold.

(Special thanks to Bob for not giving up on me)

• ▶ Reply

Permalink Reply by Bob Hartunian on January 17, 2018 at 9:56pm

Substituting carbon for aluminum without redesigning the part is called "black aluminum" and seldom makes for a proper weight savings or cost reduction. Composite structure requires designers to use the intrinsic properties of the material and to incorporate manufacturing methods to maximize the advantages. I spent 40 yrs working in advanced composites to develop the technologies that could build the 787. One major advantage is in the reduction of parts count on a commercial composite (carbon) transport compared to a metal plane because composite structure can be fabricated in large sections with skins and stiffeners cured in one piece. This eliminates all the separate detail parts that must be individually built with a paper trail and stored as inventory and finally joined with high secondary labor with rivets. When you can cure a 100' wing skin with integral stiffeners, you save approx 20% in weight and reduce the parts count by 90% and all that saves big bucks. Plus composites can be tailored for stiffness and strength putting fibers only where needed.

I have a Pulsar glass plane and am finishing up the Zenith 701. The Pulsar fuselage has 2 pieces joined longitudinally and the Zenith has dozens of parts requiring drilling and riveting and jigging. Pulsar went together in 1/3 the time for the Zenith. Another consideration is the elimination of corrosion problems and fatigue issues prevalent with aluminum.

So to end this lecture, carbon makes for a very efficient structural material when properly designed and manufactured.

• ▶ Reply

Permalink Reply by Jon Humberd on January 21, 2018 at 12:02am

I hope to put this project in the paint booth/clean room tomorrow, where I will paint the plug and prepare for the Fiberglass layup.

I also hope to order supplies in the next couple of days and I have more questions.

What are your favorite hand-layup tools? Cutters, brushes, rollers, etc, etc. What are the must-have items I need?

What Items could I also have on hand to make things easier?

How do I need to lay this thing up? Where should I try to have the seams? Any tips for draping into all the intakes, around the nose, etc?

Any other tips for me before I make a total mess?

• ▶ Reply

Permalink Reply by Bob Hartunian on January 21, 2018 at 12:19pm

A pair of metal snips will cut glass easily.

Take a paint brush and cut off half of the bristle length to make a stipple brush used for poking layup to remove entrapped air.

Always use latex  gloves to keep resin off skin. Some people do get sensitized to epoxies and develop a rash.

Have sufficient cups to premeasure resin components but don't mix until required.

Put a light coating of resin on mold and then place the precut dry glass on the surface to tack it down. Once in place, brush on resin to saturate glass.

You can put a few plies on mold and let them cure if you find the bottom plies are sagging down. If mold can be positioned off plane such that prop is vertical, might keep plies from sagging. If you do let plies cure, sand lightly and wipe with acetone before adding new plies.

Glass plies do not have to be continuous in hard places. You can make smaller strips and overlap with adjacent plies in areas like the inlet recesses. Overlaps about 1" or more.

Wear safety glasses to prevent resin in eyes.

The ambient temperature should be between 60-75F for the layup. Don't layup if it's very humid as that impedes resin cure.

Might be a good idea to practice a small layup on some wood shape before doing the cowl.

If you find plies popping up after resin application, wait for ~1/2 hr and then press down plies as resin advances in cure and plies become tacky. The enemy of a good laminate is trapped air which makes for delaminations.

If you find resin is curing too slowly, you can encourage faster reactions with light heat from a heat gun.

Bob H

• ▶ Reply

Permalink Reply by John Austin on January 21, 2018 at 12:24pm

I found an Olfa Rotary Cutter worked great to cut woven glass without distortion!

John

• ▶ Reply

Permalink Reply by Jon Humberd on January 21, 2018 at 7:45pm

Painted the mold with some left-over auto paint today and almost ready to try a layup. I wish the mold was easily removable, but it is formed tightly around the engine and will have to be destroyed to remove.

A few more questions before I place an order for materials:

I have regular squeegees, is there any need for a notched squeegee?

Is a roller necessary or would it make any part of my project easier?

What size brushes would be recommended for this size project and about how many should I have on hand?

60-75 degrees is no problem, should it be even warmer once completed and waiting for the cure time?

Is there any benefit to wrapping a layer of glass completely around the plug? Would it help with sagging, or should it just be medium sized pieces with overlaps? Where on the cowl would you try to place the biggest/longest overlaps?

I have left at least a 1" gap from exhaust pipes, is there anything else I need to incorporate here?

What else am I missing? What are some other common questions that I might not have thought of yet?

Sorry for all the newbie questions. The replies here are an incredible resource though, thanks!

• ▶ Reply

Permalink Reply by Bob Hartunian on January 21, 2018 at 8:19pm

-I don't see need for notched squeegee.
-A roller and pan might make application easier but a brush and cup of resin
will work. You could precut glass in sections and saturate with resin on
table and then transfer to mold but that might distort glass depending on
size of pieces.
-Brushes about 2" wide should work. I use cheap Harbor Freight brushes.
-Temps around 75F are good.
-Take some glass and test drape to see where it will conform to mold. You
would like to have largest pieces possible but not to get wrinkles or
puckers. There is nothing wrong with having 3-4 longitudinal joints around
the periphery of mold overlapped by 1" or more.

-Where are you located? I'm in San Berdo mtns in S. Calif at Big Bear Lake
area.
Bob H

• ▶ Reply

• ‹ Previous
• 1
• 2
• 3
• Next ›
• Page