Plywood ribs in humid climate?

[cooley]

Hi all,
this is my first posting to the forum.
I currently fly a supercub but thinking about
building a Kitfox. I looked at builder blogs - the design looks straightforward,
I would like to clarify a few things though..

- I live in very humid and hot part of Texas, (based in KDWH). Entire life is a constant fight with mold, fungi, rust, etc. I know that the ribs are protected with varnish - but I am not sure if this is enough for our climate.
Does anyone fly a Kitfox in Houston area? What is your experience with the ribs, how are they holding?

- it seems like the ribs are attached to the spars on epoxy. Does not it get soften when ambient temp is above 105?

[efwd]

120 degrees in PHX doesn't cause any problems.

[David47]

I have used a heat gun to soften Hysol to adjust a false rib position. I had it set at more than double that temperature in order to soften the adhesive so I don't think your ambient temp will be an issue.

[Dave S]

Paul,

Familiar with your somewhat sticky, humid climate;)

The Kitfox design is good.....the key to preventing problems with the wood parts, which is true for any aircraft with wood parts, is to assure that that there is complete coverage of the wood with the epoxy varnish. That includes spots where a person may have to drill a hole or run a screw/rivet into or through the wood - assuring that such points are sealed completely.

Back when I was doing our kitfox, I had much great advice from a local polyfiber distributor and aircraft restorer and incorporated his ideas, based on 40+ years of experience dealing with corrosion, rot, rust and mold. Unfortunately the person passed a year or so ago and with him a great body of knowledge on these issues.

One deal which he recommended to me was when painting the spars with epoxy primer to overlap the epoxy varnish on the ribs and the epoxy primer on the spars - the epoxy primer, varnish and hysol are completely compatible and the overlap prevents any entrance of moisture and oxygen or mold spores.

Another point is to consider well any place where materials are overlapped such as pieces of metal joined at a lap seam, fabric over aluminum etc. Lap seams are notorious for holding moisture. Personal experience here pulling fabric off aluminum sheets on ancient airplanes to reveal that half or more of the aluminum had returned to bauxite.

You may want to pay more attention to corrosion, rust and rot prevention in a moist climate but consider that plenty of kitfoxes have been operating for many years in warm. moist climates just fine:).

[cooley]

Thanks for the answers, guys!
I am getting serious about building one. Been flying for about 10 years, owning my plane for 4 years. Want to build one now. Need to make a decision on 2-3 years commitment now...

[Jerrytex]

I am right up the street from you in B/CS. As humid as Houston is. No problems with the ribs. But like most have said, complete coverage of varnish should keep them sealed. This time of year is bad with the cold fronts and condensation. There are times I walk in the hangar after a cold front, and it will look like it rained in the hangar. It's crazy. Only humidity problem I have had is the cargo bag molding.

[Guy Buchanan]

The Hysol 9460 epoxy adhesive sold by Kitfox maintains about 75% of its maximum strength at 120F, 50% at 150F. 25% at 180F. (Lap Shear - Link (http://www.scottsales.com/Structural-adhesives/HYSA9460-EN.pdf)) West 105, a typical liquid epoxy shows a Heat Deflection Temperature of about 120F, which is about the maximum temp you want to cycle to and not experience any distortion. (Call it the maximum use temperature.) So I suppose you could see some strength reduction in really hot weather, but as has been said, nobody appears to have ever been hurt by it in a Kitfox.

That said, it gets interesting when you paint your aircraft a nice, dark color. Surface temperatures in sun vary quite a lot by color. (Here (http://www.tom-morrow-land.com/tests/cartemp/)'s a very good, unscientific study.) Which is why Burt Rutan wanted all his aircraft painted white, or mostly white. Room temperature cure epoxies can't realistically take the heat. (Anyone taking their Kitfox cowl off right after a hot flight will notice how soggy it is.) Modern composite aircraft are frequently made from much higher temperature cure epoxies, (typically 250F,) which raises the use temperature accordingly. (Meaning: degree for degree, cure vs use.) Kit aircraft suffer from the problem that though the parts may be high temperature epoxy, the attachments done by the builder are not. However you can easily grossly over-design the attachments so loss of strength is not an issue. (And room temp cure parts, like our Kitfox cowls, can be grossly over-designed as well, with minimal weight increase.)

Fortunately, there are no painted structural composite parts on a Kitfox exposed to the direct sun. (No, a cowl is not structural.) So you can realistically paint it any color you want. (You will find the dark colored cowls will shrink and move over time making them substantially less pretty than when you first painted them. I know that from personal experience.)

Can you tell I was bored this afternoon? Geez what a waste of brain cells!