Carbon Fiber?

[Pushboy]

I'm curious about some of the carbon fiber items that some have added into the aircraft -- things like floorboards, wing root blank, panel, centre console sides, accessory jack panels behind the shoulder on seat - maybe even the fairings on the trailing edge of the tail and leading edge of the rudder?

I'm considering making some gap seals for the horizontal/elevator and I have kinda gone down a rabbit hole here, and I'm looking for some advice - More specifically on Carbon Fiber and using it in EAB.

I got to reading about galvanic corrosion and how it relates to carbon fibre and dissimilar conductive metals. Is this an issue for the afore mentioned items? And if so -- what are people doing about it?

Any advice would be great. Thanks.

[olsonCCR]

Great question. I'd love to hear solutions others have found as well! I've been playing with making parts out of Carbon Fiber as well. I did all my floor boards, a yaw trim mount for my autopilot, fuel gauge covers etc., and am working on a fuselage door now. I've only had the confidence to replace non-structural items up until the door which can be easily replaced if necessary. That being said, where ever the CF is attached to any metal I separated it with a small white plastic washer, and used hysol to create a barrier for the yaw trim mount and plan to do the same with the baggage door if it turns out (not looking so great at this point!).
Kevin

[Pushboy]

Hey -- I appreciate the comment... I was starting to think I was crazy and the only one who considered this.

It sounds like you have chosen to isolate. The plastic washer is a good idea -- I had considered small vinyl "donut" stickers -- similar to what you use for 3 ring binder paper when you accidentally rip it out. (I got that idea while repairing my build manual when I got over zealous with the vacuum cleaning... oops!)

What type of fasteners (material) have you used? I've read that stainless is pretty good -- with titanium being a even better option. However I've also read about oversizing fastener holes thru the CF plate, then filling them in with epoxy, - and then re-drilling to correct smaller size. This technique is used often in the construction of wood core composite boat construction, as it isolates the wood core from the water.

Also -- you mention quite a few of the parts that I'm also considering constructing -- what type of layups have you done and what method did you use on them?

Thanks again.

Dave.

[olsonCCR]

Hey Dave,
I really like the idea of the paper binder donuts. That's a great tip. In tight places the washers were a pain. I would dab a little hysol on them to get them to stay in place. I'm going to try the donuts next time. For me, simpler the better. With my door hinge for my baggage door, I had to re-place it because I bent it and am now filling in each hole and redrilling. Hopefully everything will be isolated pretty well when it's finished. Only time will tell if I start to get some corrosion.

I'm still a total beginner working with CF. Mostly, everything I have been making has been cut from flat panels I bagged up. One surface is glossy with the peelply side being a dull finish. I was originally using a vacuum pump and commercial vacuum bagging material for my panels and a piece of glass as my base but gravitated to simple household vacuum bags and my shop vac. I have found using my shopvac, the vacuum pressure is way too strong if you aren't careful and it will leave an imprint on the part where the valve is located but the bagging process couldn't be easier for simple flat objects. I tried using foam and plexiglass and CF surfaces as well, so I could get the shape I want but a glass sheet has definitely been the best for the base as it's easier to release. That being said, most of my moulds have been one-off designs because I end up damaging them.
Here's a couple pics.

[Navycposd]

The technique to keep the vacuum adapter from dimpling the panel is to offset the vacuum from the panel.
Make a track with your bagging material and your absorbent blanket from the panel to a box off to one side and draw your vacuum there. If you are worried about drawing too much adhesive into the vacuum or adapter, you can double up the blanket as it leaves the panel (fold a piece of it in half).

Good to see a vacuum storage bag and a shopvac will actually pull enough vacuum to make a reasonable piece. I am going to have to try that for my next project.

[olsonCCR]

Thanks,
A Standard shop-vac definitely has enough suction. Unfortunately when using standard household vacuum bags, the valve is centered and there isn't always enough empty space. I usually take the time to double up the backing material but do get a little lazy once in a while, and use too small of a bag for the part size. I've been pretty lucky and have had space to position my cutouts away from any imperfections without too much waste. I plan to hold off on any more custom parts until after I finish this darn thing.

The technique to keep the vacuum adapter from dimpling the panel is to offset the vacuum from the panel.
Make a track with your bagging material and your absorbent blanket from the panel to a box off to one side and draw your vacuum there. If you are worried about drawing too much adhesive into the vacuum or adapter, you can double up the blanket as it leaves the panel (fold a piece of it in half).

Good to see a vacuum storage bag and a shopvac will actually pull enough vacuum to make a reasonable piece. I am going to have to try that for my next project.

[Pushboy]

Wow - that's pretty cool that a standard vacuum can pull enough to make a decent part. And the household vacuum bags is a great idea.

I made a vacuum pump from an old fridge compressor motor. It's a bit of a Frankenstein machine, as it uses an old vacuum choke from an 80's pickup truck combined with an old relay I had - this turns the compressor motor on and off as a set vacuum pressure. It works pretty well, and the advantage is that if I have small air leaks in the bag set up the compressor will turn off and on but maintain the vacuum pressure. I haven't made a ton of things with it, and its def not strong enough for an infusion set up -- but it works well for plain vacuum bagging operations.

[Brandon Petersen]

So, we've tried literally every composite part on the Kitfox in carbon to see what the advantage was, if any. It's a common misconception that carbon is lighter and better than glass. The reality is that a 6 oz carbon and a 6 oz Fiberglass cloth will yield the same weight in the end. We found the biggest weight savings in the cowl set. 60% lighter than our standard resins and layup. Primary because we are able to to do fewer layers, less cloth and resin, with reinforcements in strategic areas. The floor boards, on the other hand, were exactly the same weight as the varnished wood floor boards. The wing tips with light provisions were 1 ounce less than the standard resins but were very difficult to make and didn't look nice. The tail post and rudder caps were lighter, but more flexible causing distortion when the fabric was shrunk. Adding enough layers of carbon to mitigate this quickly took away any weight savings.

Then there's the seat and instrument panel.... The panel looks cool, and the seat is actually lighter. At the time that we started down this path, Nikk had his accident. As we studied the post accident pictures and looking at how he was affected by the impact.... We immediately decided to forego any composite panel. His face impacted the instrument panel, and because it was aluminum it bent, versus carbon would've shattered and become very sharp, potentially cutting him worse and may have even caused the injury to be fatal.

We are currently testing a seat made from a hybrid material. We are still considering if it's even with going to be worth it, recognizing that the cost is prohibitive for it to be safe. Straight carbon for the seat would be foolish and unsafe in the case of an accident.

At the end of the day, most of the parts we make are better off left as glass. We only offer the carbon in what we consider to be a good value and something we can produce and not have to apologize for.

B


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[VictorV]

At one point I investigated fabricating the cargo compartment floorboards with CF. There's an up-front cost to
getting all the vacuum pump and tools. Then throw in the cost of the CF materials and it was going to end up
costing $1k and it would only save 1/2 lb. There's probably a cheaper way to do it but way too much work
just to save 1/2 lb not too far from the CG. It was obviously not worth the effort.

When I started the project I had all these ideas for cool things I could do to improve the end product. I eventually came
to the realization that KF has obviously spent a significant amount of effort trying different materials etc
and that things are the way they are because it's the best material/design for the job.

A classic example is wheel size. I had thought that the big bush wheels were cool and allowed you to land pretty much
anywhere. Then I discovered that it costs about $20 in rubber every time you land on a paved surface plus
the initial cost. More importantly, though, was that with the SS7 wing and shock monster if I used wheels larger
than 26" the angle of attack sitting on the ground (i.e. landing attitude) was greater than the critical angle of
attack for that wing. This meant I would have to perform wheel landings all the time. Of course the STI is a
different story and is much better suited to gigantic 35" Alaska Bushwheel.

In other words, choose your improvements carefully...... unless you're Mike Patey.

V.

[bbs428]

"In other words, choose your improvements carefully...... unless you're Mike Patey."

Lol. Well said Victor!

In the beginning, I to thought I had better ideas about how to go about building this aircraft. While some worked out well, a few did not and caused issues further on.

RTFM more than once and then once more.