Wing Bottom Covering Question

[jiott]

I am in the process of covering the wing on my SS7 and I see that the bottom side is slightly concave. I also noticed in Tom Waid's build Log that he had the fabric pop loose from a rib at a concave place while shrinking the fabric. So I am assuming that the covering process, at least for the bottom surface, should follow Appendix C in the Polyfiber book where it gives the process for covering an undercambered wing. The Kitfox build manual only says to precoat the ribs with Polytak cement. How did you guys treat this?

Jim

[enyaw]

Prior to covering the wings, we pre-coated the ribs w/poly-tak. Since the bottom of the wing is concave, in order to glue the fabric to the ribs after doing the initial shrinking, we employed soft dive weights (https://www.facebook.com/photo.php?fbid=397737606269&set=a.394594281269.174307.132606786269&type=3) as clamps.

[Tom Waid]

The separation occurred only on the ribs that are not stitched due to the tank being in the way. It happened when I was applying the first brushed coat of Poly-Brush. Apparently it soaked through and loosened the Poly-Tak. If I had to do it over I would put an extra coat of Poly-Brush over the ribs before shrinking. Perhaps that would prevent the soak-through.

My Fix! (http://tropicaltuba.com/Kitfox%20Project/Kitfox6.htm#6 June 2011: A fix!)

[t j]

Mine is a model 4 but the same wing I think. I did not butt the tank up to the #3 rib so I could lace it. Also, record the measurements where the spacers are between the bottom of the tank and the bottom partial rib. You can then use a curved needle to rib "Stitch" it. it you want the show quality, plan your rid lacing spacing before putting in the spacers and place them so the stiching will line up with the lacing on the rest of the wing.

[HighWing]

t j was spot on in my opinion. The fabric can be laced under the tank. This is where the previously mentioned home made lacing needles are very handy. They can be bent to specific dimensions needed to clear the spacers. I found that rather than lace directly under the rib to the other side, I would have to go forward a hole on the same side, then back down the hole to the other side - pulling the cord through. It takes a bit of time and results in the day per wing time frame I experienced while lacing.
Lowell

[jiott]

I also left a gap between the tank and rib #3 so I can lace it. Should I still follow the Appendix C instructions and glue the fabric to the ribs (using 1:1 Polytak and MEK), only shrink to 250, then rib lace, then shrink to 350, then apply poly brush? Thanks for the comments.

[jiott]

By the way, for others who may be installing the tanks on the SS7, the Kitfox manual says to fill the gap between the tank edge and rib#3 with Hysol. This prevents rib lacing #3 rib. I left the gap open and used small blocks of wood between the side of the tank and the side of #3 rib, exactly like it tells you to do on rib #1. John McB approved this method because thats the way they do their SLSA.

[czkree]

When I had done this on my s-7 I mixed about 75% tak - 25% mek. The thicker mix was used on all the cap strips. Then wicked in with a mixture of about 25% tak -75% mek. Maybe not by the poly-fiber book, but I have had no problems.

[Dave Holl]

Gents
I left a small gap between the tank and no2 rib, filled the gap with hysol and then drilled small holes in the hysol to rib stich.I also put in the small wood strengthning pieces between the tank and no 2 rib similar to between the tank and no 1 rib.
I also rib stiched the ribs on the underside of the tank.
I followed the Polyfibre manual for ribstiching undercambered wings. Ie rib stiched before final heat tightning.

[jrevens]

Just some comments & a question about what I've done with the Oratex & wings so far...
As some others have done, I filled the gaps between the lips of the tanks & the #1 & #3 ribs with Hysol & flox. I then drilled holes for lacing the #3 rib. I've found that the Oratex glue is very strong, & I've had no problems with it holding in the underside concave areas, even with a full shrinking before any lacing. I'm thinking about possibly using reinforcing tape & staples on the middle underside of the tank, probably with a little Hysol as Highwing suggested. Any comments?
Does most everyone run a piece of 2" finish tape over the centerline of the rear spar on the top only? Also, would that tape be installed first before the rib tapes?
Thanks in advance for any replies or comments!

[jiott]

John, I don't know anything about Oratex, but on my Polyfiber wings I did use reinforcing tape and rib stiched the middle underside tank rib. I felt much better having this slightly concave area rib stitched. I spaced that rib from the tank using small wood blocks located between stitches, leaving a small gap between rib and tank for the stitching. I used a small, very flexible wire for a needle and used a separate piece of thread for each stitch. Took a little bit of finese but was not difficult.

Per the book, I ran a 2" finish tape on the top centerline of the rear spar, before putting on the rib tapes. The fabric is so close to the spar on the top that any motion of the fabric during flight would cause it to contact the spar, which is why you need a reinforcing tape in that area.

[rosslr]

Hi John,

sounds like you are making great progress on the covering. In regard to your questions: Does most everyone run a piece of 2" finish tape over the centerline of the rear spar on the top only? Also, would that tape be installed first before the rib tapes?

We only ran it on the top. We put it on last - after the rib tapes.

cheers

[jrevens]

Thanks, Jim and Ross!
I'll try to post some more pictures of the Oratex job sometime soon... it's going good for me, but it sure goes slow when you only work on something about 1 hour every 2 or 3 weeks! :o It definitely takes longer for the application than Polyfiber before painting, but it's real nice to look at it when it's done and realize that you've got a finished, airworthy part. I'm very pleased with the way it's looking. The stuff is so tough... it's even hard to poke a needle through it!

[jiott]

The Oratex sounds really great except I still have one major concern about it. That is its protection from UV damage to the underlying fabric. I know they say it has built in protection, but the fact that it is somewhat transluscent makes me wonder how good it is. You can get sunscreen for your skin that is very translucent, but mountain climbers who really want total UV protection use zinc oxide which is not transparent.

I have built and flown RC model planes for years and used mostly Monocote covering. It is supposed to be UV resistant, but after about 5 years or so my heavily used models would begin to get weak enough that you could poke your finger thru the covering. I guess I'm a show-me type that will have to wait around for 5-10 years to see how the Oratex airplanes weather the sunlight.

[jrevens]

I understand your concern, Jim. Just a couple of points I'd like to make. First of all, Oratex has been on aircraft for over 10 years now in Europe. It would be nice if we had some longer local experience, but really long term experience isn't available yet. However, the science of these types of materials is is not exactly black magic... it's not like the old days when Ceconite, for instance, was first used on aircraft. The fact that the coated material is slightly translucent does not mean it is more vulnerable to UV rays. It's like comparing apples to oranges. Speaking of that, we can wear sun glasses that are very translucent and block 100% of the UV. You cannot compare Oratex, in many ways, to Polyfiber, or certainly not to Monocote. The Germans who invented this stuff say it is UV resistant (due to the coating), fuel proof, much stronger than Polyfiber & with a much higher temperature rating (482 deg. F.). I wonder if the Polyfiber system is 100% UV proof? I'm not trying to change your mind or sell you Oratex, but I'm just trying to explain some of the reasons I chose to use it. I guess I'm a little bit of a pioneer... so be it.

Anyway, that being said, I hung a sample of Oratex on the back of my house, with a direct Southern exposure, several months ago. We get a lot of sunshine in Colorado - blue, clear skies over 300 days a year on average. We are over a mile above sea level and the sun's rays are less filtered by atmosphere than they are in most of the rest of the country. Our temperature also varies very widely, from occasionally over 100 deg. to 20 deg. below zero or less. This will be a good test, and even after only a year's time the exposure should be equivalent to many years of average use for a hangared airplane.

To the moderators - perhaps this should be transferred to one of the Oratex-specific threads?

[jiott]

Good points John. Your test piece hanging in the sun is a great idea.

[jtpitkin06]

I predrilled rib #3 where it butts against the tank. I took care to put the spacers on the tank bottom rib caps where they would not interfere with the lacing pattern.

I glued everything with polytack and shrunk to 350. Like Lowell it took a bit of creativity on the bottom of the tank working curved needles up and round and down and though. sometimes retracing the cord to get the needle through the proper hole. I saw it as a challenge. In the end it is all glued and stitched tightly to the under camber including under the tank.

I did have a few spots that tried to pop off when shrinking the fabric, but once the stitching was installed it pulled things down where I could use some fresh Polytack and reglue the fabric and press it down with a Century covering iron (Hobby shop item).

John