# Covering ultralight wing with undercamber

### Help Support HomeBuiltAirplanes.com:

#### pylon500

##### Well-Known Member
Would point out that just 'large head pop rivets' is not really suitable as the heads will chew through the fabric, the large plastic washers are a slightly better idea, but fiddly and expensive when you start buying two or three hundred of them!
My usual (before I found the Foxbat method) way is to lay the fabric glued around the edges, give a light shrink till flat, dope a pre-ironed, one inch wide pinked strip along the rib, then run a length of half inch wide fibreglass reinforced packing tape along the rib.
Using a scriber, locate the riveting holes and prise the fibres of the tap and the fabric aside and insert large head rivet.
Finish with a two inch pinked fabric strip and final shrink before dope layers.
I've never tried stitching...

#### Protech Racing

##### Well-Known Member
Cover the rear section spar to spar, cover the nose section from spar to spar, rivit the seam top and bottom. Nice and clean .

#### Mohawk750

##### Well-Known Member
I'm pretty sure the strips that are on your ribs are not structural but added to provide a wider surface for the tape. If they are structural it's the poorest application of structural reinforcement I've ever seen. Flimsy aluminum strip held on with 8 rivets over a 24" span, what possible structural bennefit could it bring? tension? torsion? compression? buckling? No, the rib is already designed to manage those forces.

Now, rivets on approx. 3" spacing for fabric attachment, or holding a strip to support the the tape for fabric attachment is right out of the AC43-13 recommendations for low speed fabric covered airplanes.

Again, get the straight answer from Ron at Thunderbird and don't take my word for it.

BTW, your project has peeked my interest in the Sorrell designs so last night I got out my SNS-2 Guppy plans to review and then sent an info request off to Thunderbird last night. I received a nice reply from Ron. He answered all my questions and he's going to add me to the mailing list. Seems like a good Guy.

You are lucky that there is someone out there that is still supporting the design. Make good use of him.

#### Built2Fly

##### Well-Known Member
Again, very interesting discussion here. Let me add one more dimension that I haven't mentioned. This might add another potential solution to the problem.

I drew an illustration of my wing below. My wings are a bit short, 10ft long. (It is a bi-plane, I have 4 wings, so each can be a bit short). There are 6 bays for 7 ribs. Leaving 1ft for the bow, each of the bay would be 1.5ft wide.

My fabric is about 62" wide, so a bit over 5ft. I plan to lay the fabric across instead of along the wing span. This layout is according to Poly-Fiber manual, recommended by Ray Stits. It makes sense as there will be only one seam across the airflow.

Now the math part comes in. I can cover 3 bays on one section of fabric, but I won't have enough for the bow on the outer section. So whatever I do, I have to do 3 sections anyway. A better configuration would be to do 2 bays in each section (still 3 sections) and have enough for the outer section to cover the bow.

Why does this matter? It matters because for each section, the fabric will wrap around the butt end of the ribs in additional to the cap strip. This creates additional sheering force that holds the fabric down on every other ribs. That might buy me enough leverage to skip the lacing or riveting?

@Mohawk750, I am still waiting for Ron's reply now, but I do not expect a definite answer. Even when you have all the designs, it is hard to tell if removing one piece would matter if you were not the designer who went through the calculation.

@Mohawk750, @BBerson, I can not decide if that strip is structural or not. On one hand, it seems to not hold much load in any directions. On the other hand, it has a formed edge for strength, and it looks like it is there for some reason. My thinking is to err on the safe side and not to remove it (though I am really tempted for the 2lb weight saving).

@pylon500, @Mohawk750 is right that the rivet method is a method referenced in the FAA Aviation Maintenance Technician Handbook. The Poly-Fiber manual also referenced it, but does say that the metal washer is preferred over plastic washer, as plastic deteriorates sooner. But your fiber glass method is a very interesting idea.

Last edited:

#### TFF

##### Well-Known Member
Many Tailwind builders use the Maule method for the tail, of gluing on each section, which would be ribs here. Technically it is under repair in the manual where you find the method. Each cell is glued and minimum overlap is observed. First bay is glued to the ribs. The next bay is an extension. One side glued to the rib while the other is the overlap to the previous fabric; done all the way down. It’s a lot of glue though.

#### Mohawk750

##### Well-Known Member
Now the math part comes in. I can cover 3 bays on one section of fabric, but I won't have enough for the bow on the outer section. So whatever I do, I have to do 3 sections anyway. A better configuration would be to do 2 bays in each section (still 3 sections) and have enough for the outer section to cover the bow.
There is nothing wrong with span-wise seams on the leading and trailing edges. Your wing and fabric dimensions will work for chord-wise covering but it will be more effort, will take longer and use more glue and therefore be heavier. If the only reason to cover chord-wise is to avoid mechanical attachment then I do not reccommend it.

Do not depend on glue only attachment to ribs, use rivets or rib lacing. Spend the time to do it correctly the first time. You will not be happy if you have to go back in a year and add rib stitching, tapes, paint etc. which will look ugly and add more weight.

My2c

#### Built2Fly

##### Well-Known Member
OK. So here is the response from Ron of Thunderbird.
1. The cement of Stewart system is strong enough to hold the fabric just fine without rib stitching. Most people do that without a problem.
2. The cap strips are not structural. They do provide a wider area for attachment. Also, without them the stamped ribs might have sharp edges that needs to be cleaned to avoid cutting fabric.
I think that makes sense. He definitely answered the questions we are struggling about. Now I need to think about what I need to do given all these information. So basically two options:
1. Leave the cap strips on. Do nothing special, just glue the fabric. The cement will hold it. Easiest, quickest.
2. Remove the cap strips, clean the rib edges. Glue the fabric and use rivets to hold it down. More work, but 2lb weight saving.
Now I got something to think about over the weekend.

#### Mohawk750

##### Well-Known Member
I'm glad Ron got back to you and what he said makes sense. If gluing only you will want the wider surface.

Or you could take the belt and suspenders approach, remove the rivets between the spar and trailing edge, glue and rivet the fabric on using the same holes for added insurance!

#### Tiger Tim

##### Well-Known Member
Leave the cap strips on. Do nothing special, just glue the fabric. The cement will hold it. Easiest, quickest.
Do that one. No point in going off-script your first time out when you have the advice of the current manufacturer.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member

There is a lot of misunderstanding about the way a glued fabric to rib bond works. After a certain minimum width the width of the bond makes no difference in the strength or durability of the attachment.
The only critical area is the edge that is being peeled.

Once the fabric starts to pull away from the rib the glue also starts to deform or stretch. This transfers the load to the next infinitesimally small line of glue that then shares the work load. When the load is removed everything returns to the original configuration. Only if the load exceeds a critical point does the joint then start to fail by peeling away. I can't find the drawing I was looking for, and don't have the ambition right now to draw one myself, but think of a couple of sheets of whatever attached by a field of little glass posts glued on. Try to wedge the part from one side. Do the same with the little glass posts replaced by little rubber posts with the same ultimate load strength.

Go one step further and imagine a small length of the edge load of the glued rib being replace by the edge load of a single rivet, screw or even the traditional rib stitch. There is a reason we put a line of reinforcing under the stitching.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
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 purpose of 1-1/4" strips is because the rib flange can't be formed that wide width. The rib flange is likely 3/8" wide and with a 1/8" hole so only 1/4" wide for structure. These wings have no rear spar so that rib is fully cantilevered out from the D-cell. There is a concentrated load at the d-cell. Some use wide gussets at that joint. The designer apparently chose to use a full strip instead of an abrupt edge gusset. But it must have a gusset of some distance, in my opinion.
To change a design requires a sand bag test of a sample rib cantilevered from the D-cell to ensure it does not buckle.

#### cdlwingnut

##### Well-Known Member
HBA Supporter
Log Member
The plans for my ultra piet say i don't have to rib stitch if i didn't put the under camber in, but I did so rib stitching isn't an option. I would personally do it anyway.

#### Built2Fly

##### Well-Known Member
A lot of good ideas. I am really struggle to understand this design here. Here is a closed-in picture. D-cell and the leading edge of the wing is downward direction. The trailing edge of the wing is the upward direction. This picture shows how the half rib is connected to the spar.

The spar is a simple aluminum U channel. The half rib is connected to the spar by only 4 rivets (as pointed by the red arrows). The cap strip is only connected to the spar by one rivet. The blue lines outlined the cap strip which is mostly obstructed by the rib in the picture.

Please ignore the other wing that is behind.

What puzzled me is that this cap strip apparently did not hold much load. The rib flange is about 3/4 inch wide. So the cap strip is not making a huge difference in glue areas (and not to mention the glue joint strength argument by @Hot Wings). So I really don't quite see a reason why they are here in the first place.

On the other hand, this is designed by Sorrell, so I have to assume that they know their stuff. There might be a reason. But I can't seem to see any legit reason for it.

@Tiger Tim's suggestion makes sense. This is my first build, so stay on script is probably the safest route. But that aside, I really like to understand what those cap strips are there for, and why Sorrell put them there.

#### Protech Racing

##### Well-Known Member
Cap looks structural to me . It puts that rivet into double shear.

#### TFF

##### Well-Known Member
You don’t stop at the spar cap of the D, you have to wrap it all the way the leading edge to the prescribed overlap. Usually 2 to 3 inches. The planes that leave the D uncovered and only have the open areas covered have a locking channel that the fabric locks into. Earlier Corsair outer wings are like that. I believe early Nieuport 28s only covered the open areas and one almost killed a Eddie Rickenbacker; the Americans were very happy to move to the SPAD for that one reason.

#### Built2Fly

##### Well-Known Member
These wings have no rear spar so that rib is fully cantilevered out from the D-cell. There is a concentrated load at the d-cell. Some use wide gussets at that joint.
There is actually a rear spar. You can see it in the picture of Post #18 (where I numbered all the ribs). The rear spar is also supported at the root by being bolted on the fuselage. I do see the gusset argument. I guess that force with this wing is taken care of by the drag bracing tube inside the wing. There is only one rivet holding the cap strip to the spar here.

Cap looks structural to me . It puts that rivet into double shear.
Interesting point. But I don't see a force that yanks the rib backward. So that double shear makes no difference here.

You don’t stop at the spar cap of the D, you have to wrap it all the way the leading edge to the prescribed overlap. Usually 2 to 3 inches.
I think you mean the fabric covering. I agree with you and I planned to wrap it all the way around, completely covering the entire wing, including the D cells.

#### BBerson

##### Light Plane Philosopher
HBA Supporter
No, that diagonal brace is not a rear spar. The ribs are not attached to that diagonal brace. They are cantileverEd from the d-cell. Those capstrips are flanged on both sides to make them rigid and make the rib like an I-beam instead of just a C channel. I-Beam is stronger than a C channel.
I suspect the capstrip continues inside the d-cell, but I can't see it. That would join the front rib to the aft rib.

Last edited:

#### pwood66889

##### Well-Known Member
A little late to the conversation, but have been involved in 1 1/2 cover jobs on Ercoupes. Ont top only, and one complete with IRAN on wing structure.
The 43013 foil has a "cusp" under the "D" cell in front, so it has a problem similar to this thread.
The coupe uses "PK" screws to hold down the cloth, on both the top and bottom of the wing. Only a couple are used in front to keep the concave shape.
There are washers under the PK #4 or #6 screws; covered with pinked tape of course.
When the Ercoupe wings were "metalized," aluminum skin was put on instead of the cloth. That started in the 1950's, when the original "Grade A" cotton was still in use but wearing out. The "Stitts Process" (TM) solved a lot of problems, especially with the "Razor back" covering. Now fabric wings lasted longer = 17 years in my case!
The trade-off was heavier wings that not have to be recovered every so many years. Both planes I had custody of had fabric wings.

#### Tiger Tim

##### Well-Known Member
I really like to understand what those cap strips are there for, and why Sorrell put them there.
A couple ideas on the reason come to mind:

1) Wide cap strips could be to provide sufficient gluing area to skip rib stitching. It’s been a while but IIRC stitch spacing on any other airplane is governed by something like speed or wing loading and a UL has neither so some may be assessed as good to go with glue alone provided there’s enough of it.

2) Could be structural. A wide strip like that would go a long way towards preventing the ribs from bowing to one side or the other, especially when the fabric is tightened.

3) Could be cosmetic. Formed aluminum ribs are sometimes fluted along their edges and the cap strip would bridge along those flutes to provide a smooth surface. For a UL it probably wouldn’t matter if the fabric was just bridging dips along the rib’s edge but Sorrell’s designs show a certain pride of making things nice and I for one appreciate that sort of thing.

#### Built2Fly

##### Well-Known Member
@BBerson, sorry I was not very clear in my description. Here is a picture showed what I am referring to (main spar, rear spar, and drag tube). And also the attaching points to the fuselage.

I had a shot inside the D cell. It seems that the cap strip ends at the main spar.

The I-beam explanation makes some sense. Also confirmed by @Tiger Tim's comment.

Apparently all of us here have our thoughts and opinions. It may be a mystery about what Sorrells were thinking when they designed this. I do like many aspects of their design. Going through this is fun and educational.