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Rib design and rib stitching

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olgol

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I would like to hear some opinions about alternative rib designs for ultralights with front and rear tubular spars, and how they are related or suitable for attaching the top covering to the ribs. Here are some questions/thoughts that I would like to discuss:

1. Is rib stitching needed on true ultralights? Do you rib stitch your wings? I understand that most of the UL planes do not have rib stitching. In level flight and probably up to a load factor of 2g, it is unlikely that the top covering would lift off the ribs, although I have no way of verifying this. But at higher load factors (a pull-up at 3-4g) I am sure the covering will lift off the ribs. Which again may be no big deal at all.

If the general opinion is "forget rib stitching" on UL planes, then the following questions do not apply. Any rib design will do.

2. If we assume that the top covering must be attached to the ribs either via rib stitching or by gluing it to the ribs, then ribs made from round tubes (typical rib design on UL planes) are not the best option. Gluing is not going to work at all, rib stitching will pull the covering around the tubes and will produce deep ridges along the ribs.

3. If the above (2) is correct, ribs must have a flat flange of some sort. Then rib stitching or gluing the top (and bottom?) covering is easily done.

4. Wooden ribs although technically possible, would not be my choice with aluminum tubular spars.

5. Foam ribs with plywood/wooden caps (affordaplane, SkyPup) are another alternative. Although seemingly low tech, reports claim they have great strength to weight ratio. I am a little concerned about longevity of such ribs and their resistance to accidental bumps and minor ground handling abuse. If the foam is crushed under the rib cap, a rib may have to be replaced (and the wing covering opened).

6. Has anybody seen "standard" aluminum stamped ribs used in UL wings with front/rear tube spars? I know such ribs are used on airplanes with "normal" spar/D-tube wings, like N-3 Pup, etc. This rib design to me seems the most professional, although probably the most difficult to manufacture at home. Does anybody know of manufacturers that can produce/supply custom designed stamped ribs?

Thanks in advance for your opinions and suggestions.
Oleg.
 

mstull

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Oleg,

1. I've always used PK screws on my U/Ls, just because the Stits manual says to. But on this plane I only have bows made of 1/2" aluminum tubing to support the upper wing surfaces, experimenting with using nothing to support the bottom surfaces... no ribs at all, nor stitching (nor PK screws). I can see the underside of the upper wing in flight. It always stays perfectly flat and tight.

I did put some (widely spaced) PK screws to attach the upper surface to the bows. But I seriously doubt they're really necessary on a true, legal U/L. I've never rib stitched... It doesn't sound like fun. I've always used rib caps on wood or styrofoam core ribs so I could use PK screws. The rib caps serve double duty making normal ribs much stiffer. I used thin aircraft plywood or carbon layup for the rib caps.

4. Even without stitching or PK screws, you really don't want the fabric to ever lift off the ribs. It might move around just enough for the fabric to wear against the ribs. As long as your ribs or rib caps have some width, the fabric sealer (PolyBrush) glues the fabric to the ribs fairly well. I use rib caps at least 3/4" wide. The manual does describe a way to glue the fabric to the ribs (in the Under-camber Appendix)... which would be stronger than just the sealer. You have to use lots of PK screws for under-cambered areas. But for tubular ribs, I would use at least a few screws, just to prevent that chafing.

I've also found that putting the finishing tapes on top of the PK screws is a waste of time. I put the screws on top of the finishing tapes, letting the tapes serve double duty as reinforcing tape. The screws go in before painting (see the attached photos). The manual says to cover the screws with tape to make a water tight seal. That might be worth the trouble, if you plan to leave your plane out in the rain, and want it to last for many years out there. But then you'll need reinforcing tapes, that have to be saturated with sealer, that add weight.

2 and 3. Right, you can't rib stitch around round ribs. The fabric would tear. But PK screws work fine. The trick to putting PK screws in the tubular ribs is to find just the right drill size to pilot the holes. Too small, and you'll break the screw heads trying to screw them in. Too large, and some might strip out as you're screwing them in.

I don't use any screws in the front 1/6 of the airfoil. With the sharp curve there, the fabric pulls down hard on its own. From there back, I've gradually spaced the screws farther and farther apart with each wing. I'm up to about 6" spacing.

5. I've never used plain styrofoam ribs because of your same reasons. But if you scrim the styrofoam with very light fiberglass on both sides, it becomes much more structural. You can also use scrimmed balsa wood. I really like the way those ribs come out.

Be very aware that the tip and root ribs have to be extremely strong to withstand the tremendous fabric tension. Just shrinking it tight puts a couple hundred pounds of side load on them. And when you pull Gs, the force multiplies. Don't underestimate that force. Brace your aluminum bows, triangulating against that side load. For styrofoam or balsa or wood core ribs, use much thicker styrofoam with a couple plies of regular layup on both sides, and extra wide, thicker rib caps.

I just remembered I used tiny 3/32" rivets instead of PK screws on my aluminum bows, with little AN960PD-4L washers. That's the middle picture.
 

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steveair2

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Oleg,

Falconar Avia Inc. sells the Hipec Covering system.
They claim no rib stiching required. The fabric sealer glues the fabric to the structure. I plan to use this on my biplane.

Steve
 

PTAirco

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But for tube ribs the Hipec system won't work since there is no glue area to speak of. Looks good for wood ribs though.
 

mstull

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I've always been tempted to try one of the urethane based covering systems (which I think Hipec is). It takes much fewer (about half as many) coats, and can give a much shinier finish than Stits. But that stuff is super toxic. You have to use a remote source respirator. That is fine in a production setting, where buying the equipment would pay off, and where you can dedicate a separate, specially ventilated room for painting.

With Stits, you can use an ordinary respirator with organic filters. Stits sealer also glues the fabric to the ribs. I don't know which covering system's sealer glues better. And I don't know which comes out lighter. Stits (Poly Fiber) is vinyl based, and comes out extremely light. A non-toxic covering system is also available now. I can look that up if you're interested.
 

BBerson

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I use aluminum truss ribs made from .020 bent angle. PK screws hold the fabric into the flat flange just like on my old Aeronca. You could make Aeronca type sheet metal ribs with a simple form. Use .020 6061-T6 for easy forming.
Consider this, the srews/rib stitching holds the ribs in position. The ribs are weak without the fabric attachment. Also rib attachment prevents fabric loss from progressing along the wing if one side came loose.
BB
 

olgol

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Thank you kindly, guys.

Mark, can you post more pictures of your tube ribs and especially their attachment points? How do you form the tubes into the airfoil shape? Do you use a form block of some sort? How do you account for the springback when you bend the tubes? From my limited experience with bending 1/2" tubes over the knee, it is not a very precise process and the knee gets tired really soon. The springback was enormous too. My attempts to make a contour with lower radius (by guessing the required radius) and bend the tubes over it failed miserably. The radius has to be so much smaller than the final shape, it will require a lot of experiments to figure out the form block shape.

If PK screws are used to attach the covering, why not use a narrow strip of aluminum over the rib and screw (or rivet) the covering to the rib with this strip? A reinforcement tape under the aluminum strip should help against wearing through the fabric.


BB, can you describe how you form the rib from an angle? Is this an off-she shelf aluminum angle or do you bend a strip of aluminum first? I think Kolb uses similar ribs?

My preference would be for what Mark showed on the picture. Just upper bows made from tubes (3/8 or 1/2"?). No lower rib parts at all. Ribs from alum angles would have to be made as a truss, with the lower straight part and some cross-members.
 

BBerson

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I make aluminum angle by shearing strips and bending on a sheet metal break. The curve is made with a fluting tool that looks like a hand plier or with a shrinking tool that leaves tooling marks, I am not sure which is better.
Sounds like you prefer to work with tubes.
BB
 

mstull

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You bet, Oleg,

I used 1/2" by .035" wall 6061 T-6 for the rib bows, looking for light weight and stiffness, yet bend-ability without creasing or breaking. 2024 is too stiff. 3/8" is too flimsy. I was hoping to ask you, when you're done, how you bent yours, since that bending process was frustrating for me. I shouldn't complain, since all in all, they were quicker to make than most other types of ribs. Maybe I cared too much about making them with a perfect airfoil. Here's how I did it:

First make a full sized rib template. Make your aluminum frame, with enough straight compression ribs to keep the fabric from pulling the spars together.

Then figure out how you plan to attach the ends of the ribs to the spars. I used tiny aluminum brackets made from flat or angle 2024. I was concerned about how to get a perfect airfoil without having anything protrude up where the sheeting or fabric goes. So all attachments have to be either underneath or in a dipped area. I ended up flattening the front ends of the ribs to get the shape I wanted where the brackets attach.

I'm not suggesting the way I used is best or the only way. In fact, I have mixed feelings about attaching the ends of each rib, trusting just a couple stainless steel pop-rivets. Engine vibration tends to loosen pop-rivets. But so far my biplane hasn't fallen apart with about 160 hours on it. Maybe the attached fabric helps stabilize those attachments. Tip and root ribs will need extra brackets top and bottom, and larger brackets with enough room to stick an extra rivet or two... and brackets that only stick out one way.

Bending an accurate airfoil shape into the ribs is difficult. The aluminum is pretty stiff and wants to keep bending in one place, rather than bend a little over its whole length. So I made a special rib bending jig. I started with a scrap of thick construction plywood about 2' by 3'. It might help for you to look at some tubing benders to get a feel for what it takes. The end of the rib needs to be supported against a block, in a way that helps keep it from collapsing. And the curved block your bending it around also needs to be somewhat concave for support. And it's best to have more than one block pair designed for the tighter and straighter bends. The blocks have to be very closely spaced, so the tubing just fits between them, and very strongly attached to the plywood board. You can screw about a foot long piece of 2 by 4 under it, so it will clamp in a vise. But I just did mine on a table top, letting the thick plywood push against my body.

I start at the front with the sharpest bend, and bend that. Then slide the tubing between the blocks a half inch or whatever to bend the next little part of the rib, carefully trying not to bend it too far. I felt it would weaken it if I had to straighten it out more than a smidgen. And in the tighter bended area, it wants to collapse. Leave an inch or more straight to trim off later on the front, and several inches at the rear for a handle. Anyway, you bend just a little, and then compare it to your template... Bend a little more, and compare again... maybe 50 or 100 times for each rib. The more time you spend, the more accurate you can get them.

After the first few inches are bent, you definitely want to put a mark on the rib that lines up with a mark on the template. This is important both as you're bending, and when you assemble the ribs onto the spars. The marks need to end up lined up so the airfoil is the same on all the ribs. Once you get to the straighter part towards the trailing edge, you can bend it at greater spacing, maybe every inch, or even two near the end.

I found that process frustrating, and moderately hard work. But it only took about 20 to 30 minutes per rib. My ribs came out with a slight to moderate side curve, so they didn't lay perfectly flat on a table. But that doesn't hurt anything aerodynamically. And it's not enough to easily notice on the finished plane. I wish there was a way to make a jig that bends the whole rib to the finished shape in one step. Maybe someone smarter than me can figure that out. But particularly for the straighter end, it springs back quite a bit. So it takes a deft touch to flex it on the jig, just enough to actually bend it, but not too much. It helps if you draw lines on the jig to help find that critical point.

Each rib should be hand fitted onto the wing frame, to trim its ends just right. Cut out the ends of the template to fit the spars, and put it in there right next to the rib. Line up the mark on the rib with the template, and hold the rib in place. Trim the front of the rib, flatten it, and attach the bracket to the rib. Then attach the bracket to the front spar. Then the rear bracket is attached after sighting from behind the rear spar along the top of the rib, so the rib comes out flush vertically with the spar. You don't want the fabric to see a step there. The brackets will normally have to be bent slightly so they lie on a tangent to the spars where they are riveted.

I know this all sounds complicated. And it's kind of hard to explain in text that can't be misunderstood. But if you just try to make them fit, you'll figure out what I mean, or come up with another way on your own. I'll try to find some self descriptive pictures.

As far as your other questions. You'll find it hard to attach a rib cap to a tubular rib that comes out flat enough. It will try to tilt one way or the other at each rivet or other attachment, twisting too, and probably be quite visible when you're done. And you're just adding more steps and weight. It works out fine to just use PK screws or pop rivets to attach the fabric directly to the ribs. On the other hand, the rib cap would add a lot of attachment area for the fabric to "glue" to, possibly making PK screws or rivets unnecessary.

Yes, these bows are quick and easy to make, compared to most other ribs.

Here's some pictures:
 

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steveair2

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Falconar claims the sealer coat forms a fillet between a tube type rib and the fabric.
This attaches the entire rib to the fabric.
There website shows a 200+ man standing on the back side of a test panel.
 

steveair2

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When I built the control surfaces for my Uncles Kolb MkIII, I used a bender to bend the trailing end of the tube ribs. The bends went parallel to the inside edge of the trailing edge tube. The rib and the trailing edge tube were both 3/8" if I remember right.
They were joined by two 1/8" rivets.
This may be a good way for an ultralights main ribs to attach to a main or rear spar.

Steve
Steve
 

mstull

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Stits sealer also creates a fillet like Steve describes, making an attachment width of about 1/4".

Marking, drilling and adding those few PK pop-rivets only takes about an hour for the whole wing/wings/plane. So there's no reason not to add them.

Here's a caption for my last picture above: The wing is upside down in this picture. It shows both a compression rib and a regular bow rib. The fiberglass reinforcement, on the sheeting between the ribs, keeps it from dipping between ribs. The single ply of fiberglass tape is laid up over a triangle of balsa. The 2" by .065" wall 2024-T3 spar is reinforced with a little carbon layup in highly stressed areas.
 

Midniteoyl

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I'm wondering why you didnt add about 1" to the length of the ribs and just flatten it (a vise would've worked) to create the attachment point rather than go through the trouble of making the brackets?
 

mstull

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Jim,

That's a good idea. If you build yours that way, let us know how it comes out.
 

mstull

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Sorry Jim. I guess I evaded your question.

When you collapse the rib bow tubing completely, it no longer has any/much stiffness. That would be okay if you could attach it to the spars right where the tubing first goes flat. But you couldn't bend the crushed tubing sharply without breaking or drastically weakening it. So to attach it that way, you'd probably have to attach the rib bow to the top of the spars. So those ends would probably protrude, making a bump in the fabric, and interfering with the leading edge sheeting. There might be a way to do it, and still have it come out okay. You'd have to experiment with it. For someone who doesn't care at all about looks, it would probably work fine. I wouldn't care much about looks, except I find that others do, and might consider it shoddy workmanship.

I just decided to haul my biplane to Sun-n-Fun after all. I'd be glad to meet any of you who show up. Y'all know what my biplane looks like. Here's a picture of how the PK rivets come out after painting. It's a nice, clean, mechanical look.
 

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needswings

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The view from up their must be spectacular. Wish I could be there to see it in the flesh...
 

Midniteoyl

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Sorry Jim. I guess I evaded your question
Lucky my internet went down, 'cause I was gonna call you on it :)

Thanks for the explanation.. I'm not a UL guy, but I do like the inventive way you guys work and am always trying to learn. I figured the flattening making the end weaker, but also figured that it couldn't be weaker than a blind rivet, could it? I did not take into account the effect it would have on the lead edge sheeting...
 

mstull

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Jim,

I don't claim the way I attached my ribs is the best way or only way. I figured out a way that I felt fairly sure would be reasonably sound, and work out as far as being able to accomplish it. Since I don't work off plans, I am quite flexible about adjusting the design (especially things like attachments) as I go. If I hold two parts together and see a better or easier or lighter way to attach them, I'll go with it. In this case no better way appeared. Even if I did flatten the ends, I'd still have just pop-riveted them to the spars.

Right, you'll see a lot of things on U/Ls that wouldn't be acceptable on "real" planes... like structural parts that are just pop-riveted. I'm sure they'll work lose with enough hours. I considered that when I designed it. I like to make a new wing every winter anyway. So I wasn't too concerned about longevity.

This was my first aluminum wing design. All my previous wing frames were primarily carbon composite, and would last much longer. I like to design a wing that I can build in a couple months. So that compromise gets balanced with all the others. U/L design is all about the compromises... particularly for weight.

I experimented with a totally different rib attachment on my Lexan sheeted wing, that I've never heard of anyone trying. It seemed to work fine, as long as the ribs Tee straight into the spars. It wouldn't work with the more tangent attachment of rib bows.

Attachments are one of the more challenging things in my aluminum structures, since I'm not a welder, and I prefer to use 2024 that's not very weldable.
 

mstull

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Here's a couple pictures of my rib bender, Oleg. The middle block is what the ribs are bent around. The left block works for sharp bends. The right block with the spare rib in it is for straighter bends. The second picture shows how the edges of the blocks are shaped to help keep the tubing from collapsing, and from popping out of the jig. I'm not suggesting this is the only or best way to bend them.
 

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