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Covering ultralight wing with undercamber

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Built2Fly

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I was looking at my wing the other day and noticed that it has some undercamber.

The original construction for Hiperlite use a pre-sewn envelope for wings. That fabric is held down to the rib caps by some double sided sticky tape.

The Poly-Fiber manual stated that the cement is good for sheer force but not good for pull force. So it uses rib lacing, and alternatively uses pop rivet or screws to hold it down.

Adding to the consideration, Hiperlite is an ultralight. So would the 3M contact cement be strong enough to hold it down? Or do I need to add rivets? What is your suggestion? (I am using the "poorman's Stewart System" with 3M 30-NF.)

This is the underside of my wing. The wing is up-side-down on saw horses.
20210117_095646.jpg
 

Mohawk750

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I hate the idea of adding more rivet heads to the ribs but riveting the fabric is fast compared to rib stitching though some folks really enjoy that part. My Taylorcraft used martin wire in the original application and I replaced it with pop rivets right back in the same holes the wire clipped into. I was able to do a whole wing in a few hours with a hand puller.

It looks like your ribs already have a strip of aluminum riveted to the caps presumably to give a larger surface for the tape application. If you are not using the tape could you drill off those strips and reuse the holes for fabric rivets? That would be cleaner and lighter and less work than rib stitching.
 

Built2Fly

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It looks like that the front and the back ends of the rib cap strip go underneath the other structures. So I would not feel comfortable drill them out.

@Mohawk750, it is good to know that Taylorcraft is stitched / riveted.

@Bill-Higdon, I didn't know that Avid has undercamber. I will search to find if there are any wing covering video for Avid/Kitfox. That would be a good reference.

20210119_211229.jpg
 

Victor Bravo

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Put a thin coat of fabric cement (Poly-Tak) onto the bottom of the rib cap strips. As you install the fabric, apply a little bit of MEK through the fabric and stick it down onto the under-cambereed ribs and let the MEK soften the glue layer, tacking the fabric down evenly. After all of the rib bottoms are done, go back with the fabric iron on low, and make sure the fabric is glued down to the ribs.

Then finish attaching the fabric to the leading and trailing edges and tips and roots. using your Stewart Systems or similar.

Then shrink the fabric on the bottom of the wing just until snug, nowhere near full tension. Maybe 225 degrees. Then rib stitch it using the method in any of the fabric books for stitching. Now your fabric is stuck and attached to the undercamber.

THEN you can go back and shrink to full tension, and put the coatings, tapes, silver/charcoal, and paint on.
 

pylon500

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Taree Airport Australia
After years of doing covering jobs with Polytac, large head rivets and double layered dopes and glue on wide wooden capstrips, I got to recover an Aeroprakt Foxbat (as we call them here in Oz), and basically, they use a polyurethane adhesive to glue the fabric down, and I am very impressed with the way it works.
The ribs have been formed with the caps around three quarters of an inch in width, and I have total faith in the ability of the fabric to 'stay put'. They even have a forward facing edge of fabric that is simply glued in place, and I'm totally happy with it.
Have a look down the bottom of this page;
'Foxbat covering'
At the paint and covering .pdf's, all is explained, and with pictures!
 

TFF

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It depends on how much camber. The Polyfiber manual says for undercambered has you glue on each rib, then stitch, and then shrink. If it’s a small amount, I think they say you can do the initial shrink like normal, then stitch, then final shrink. My wings were pretty flat, but when I covered my 1/4 scale Sopwith Camel, I glued each rib and stitched it all like a real plane because of the undercamber. I have had model covering pull off the ribs before. Just depends.
 

Built2Fly

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Thanks for all the ideas, everyone.

I found some video on YouTube on a construction of Kitfox SS7. The builder did lace the fabric onto the ribs. I haven't found any other information on Avid.

What I am trying to decide is whether I can get away with just the cement. This is because (1) it is an ultralight with low wing loading, and (2) the undercamber I have looked fairly mild.

@pylon500, it looks like that your Foxbat has a flat bottom. Or was there any undercamber? Thanks.
 

BBerson

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The original construction for Hiperlite use a pre-sewn envelope for wings. That fabric is held down to the rib caps by some double sided tape.
From the photos, it looks to me like the fabric was held with a strip of metal and pop rivets shown in the photos.
 

Mohawk750

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Built2Fly, Thunderbird Aviation is still selling kits for the SNS-8 hyperlight and they list Stewart Systems as the covering supplied in the kit. They offer tech support to their customers, they even offer support for the SNS -2 Guppy for which pirate plans are available all over the internet.

Why don't you give them a call and get the information right from the source?

 

pylon500

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@pylon500, it looks like that your Foxbat has a flat bottom. Or was there any undercamber? Thanks.
No, the Foxbat doesn't have any under-camber, but I was cautious of the glue only concept, so I made a test frame and covered it as per instruction.
Shrunk the fabric the next day, and the following day, applied WAY more force than the fabric would ever endure from the back with no sign of peeling.

testframe1.png

testframe2.png
 

Built2Fly

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Why don't you give them a call and get the information right from the source?
Indeed, I was talking to Ron (the owner of Thunderbird) the other day on some pieces I am missing. I didn't ask about the covering. I thought I knew how the fabric was officially done, since I have the official build manual. But it does not hurt to ask him if anyone has done it without the double-sided tape they supply.

From the photos, it looks to me like the fabric was held with a strip of metal and pop rivets shown in the photos.
The original fabric should be over the strip and the rivets. There is residue of the tape on top of the strip, and that's what holds the fabric down. The metal strip and the rivets holding the strip are both under the tape, so they are not what's holding the fabric.

No, the Foxbat doesn't have any under-camber, but I was cautious of the glue only concept, so I made a test frame and covered it as per instruction.
Shrunk the fabric the next day, and the following day, applied WAY more force than the fabric would ever endure from the back with no sign of peeling.
Thanks for that pictures and the data point.
 
Last edited:

Mohawk750

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Indeed, I was talking to Ron (the owner of Thunderbird) the other day on some pieces I am missing. I didn't ask about the covering. I thought I knew how the fabric was officially done, since I have the official build manual. But it does not hurt to ask him if anyone has done it without the double-sided tape they supply.
It was my understanding that the doubble sided tape was used with the early presewen wing covering system that was pulled over the wing panels and tensioned with lacing at the root. They are now supplying Stewart Systems with the kits and I would be surprised if the tape method is used with this system. I would definitely ask Ron what changes if any were made to accommodate the Stewart system.
 

Mohawk750

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Built2Fly; If this was my project I would be drilling off those wide strips on the rib caps, carefully. I would replace the first and last rivets in the structure at the leading edge and trailing edge with the same type/strength rivet that was removed. I would then follow the covering manual for rivet attachment of fabric using all 1/8"aluminum shank 3/8" large head pop rivets or fabric attachment and finish as per the manual.

This is my opinion, and it's worth exactly what you paid for it! But please, don't do this if you are not comfortable with the idea and please contact the current design holder for their input or other options. If you do decide to go this route I can give you some tips on how to get a nice finish around the rivets.
 

Turd Ferguson

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The Avid Flyer & it's many off spring also have an undercambered wing how do they attach their fabric?
Glue.

Ray Stits said absolutely do not rely on glue. Many airplanes have done it for years and it has apparently been successful. I would go with what Ray Stits said and use some kind of mechanical fastener, probably lacing.
 

BBerson

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The metal strip and the rivets holding the strip are both under the tape, so they are not what's holding the fabric.
What is the purpose of the metal strips?
I would use aluminum pop rivets and washers for the fabric stitching. Same as a Citabria.
 

Protech Racing

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I’m going to cover my sliglty undercambered wing in front and rear sections. Seamed at the spar, this reduces the tension on the undercambeted area and gives a rivet option to help retain the low point seam.
 

Built2Fly

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Very interesting discussions. I did send an email to Ron (owner of Thunderbird) this morning asking about how it is done with the blanket method on Hiperlite this days. I am still waiting for his replies.

What are the purpose of those metal strips (or rib caps)? It seems that their obvious purpose is to provide a wider area for the fabric to attach to. But are they structural? Or providing a part of the strength?

20210116_210510 (1).jpg

This is one of the wings. For rib #1, it is structural. For the other six (#2 ~ #7) I am not so sure. It does not look so, but I don't have the data for the original design. But I am a little hesitate to remove them.

There are 12 strips per wing (#2 ~ #7), and I have 4 wings (biplane). They are about 26" x 1-1/4" x 1/64". So that's about 2 lbs of weight total. Would that weight saving worth the risk and the effort I am taking on this?

The safer alternative is to drill some rivet holes on those strips, cover and glue them, and then with two options later on (a) leaving it with the just the glue, and (b) adding pop rivets at a later time to hold it down when it appears to separate.

Thoughts? Suggestions? Particularly if I am overlooking any risks on removing the rib cap strips?
 

BBerson

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The rib capstrips are obviously structural and should not be removed. They serve to keep the thin ribs from collapsing. A typical Cessna has skin to keep the ribs from collapsing but you don't have skin.
 

Doran Jaffas

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Jun 25, 2019
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315
I was looking at my wing the other day and noticed that it has some undercamber.

The original construction for Hiperlite use a pre-sewn envelope for wings. That fabric is held down to the rib caps by some double sided sticky tape.

The Poly-Fiber manual stated that the cement is good for sheer force but not good for pull force. So it uses rib lacing, and alternatively uses pop rivet or screws to hold it down.

Adding to the consideration, Hiperlite is an ultralight. So would the 3M contact cement be strong enough to hold it down? Or do I need to add rivets? What is your suggestion? (I am using the "poorman's Stewart System" with 3M 30-NF.)

This is the underside of my wing. The wing is up-side-down on saw horses.
View attachment 106540
Depending on wing loading ( actual ) an appropriate adhesive and or rib stitching.
 
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