CNC Routing One Piece Wing Ribs

[BravoCharleyWindsor]

The idea of using a CNC router to cut wing ribs as one piece from sheets of ply has crossed my mind. Using a CNC router could apply to other components as well, but replacing multi-component glued ribs with a single 2-d component would be a more "radical" use of a CNC router than simply replacing a jigsaw with a CNC router to cut an already flat 2-d component (e.g. the current leading edge profile components).

Please understand I'm not advocating for or against this idea. I don't know enough about these matters to advocate for or against it!

I'm only tring to learn what others may think of it or might have experienced!

But in thnking about it I have developed several pros and cons (which may or may not be correct) to start the conversation. So far they are...

Pros...
Fast (after programming/dimensional verification is complete)
Simple (after programming/dimensional verification is complete)
Uniformity of end results
Accuracy to design, especailly if router can be programmed from a CAD file
Eliminate gluing of component and gussets.
Eliminate rib assembly jig
Incorporate gussets and components into uniform thickness routed profile
Potentially stronger?
Potentially Lighter?
Potetially sanding free cuts if bit sharpness, feedrate, and RPM are properly set

Cons...
Need to have a CNC router or access to one.
Potential burning/roughness of cut edge if bit sharpness, feedrate, and RPM not properly set
Potential to replicate error if not set up correctly
programming
challenge of setting up to cut as many ribs as possible from a sheet to minimize waste
more need to verify dimensions after ribs made, possibly with errors found after the production
Waste (cutout portions may not be suited to subsequent use)
Expense (related to waste)
Need to inspect cut edges for exposed voids (scrap rib if any found, but less likely with good quality ply material)

What are your thoughts on the idea?

I do have one question so far. Would it be detrimental to have all the plys in the same parallel planes of the rib, mutually perpindicular to the covering material and the spar?
Or perhaps beneficial?

Other considerations?

[Bitshifter]

I read that Todd Mason was building a CB1 with laser cut ribs, I don't know how it worked out though. I think I also read about someone getting them water jet cut. I don't think that there are any flying Hatz's with one piece ribs though. If you have a local supplier of plywood it would help since it is very expensive to ship and it will use a lot.
I might be a little concerned that smooth cut edge of the plywood rib may not hold glue like the rougher edge of the spruce ribs. In most places on the plane the plywood is glued to a single side, in your case all the ribs will be glued along the ply side. I have no idea if this would matter at all. Most people seem to build all the ribs first and that is what I did. This method takes about 40 days with one jig, there is no reason why you can't start building the first wing after the tenth rib.
I suspect that one piece ribs would be a good replacement for plans built ribs and I suspect that the weight would be slightly heaver since plywood is heaver than spruce.
It would defiantly be easier to stain or protect the plywood ribs since you wouldn't have all the areas inside the doublers to coat.
If the money is not a problem it would be faster but I am not sure that it would be a noticeable savings of time in the end.
I was surprised as to how close all the plans built ribs were.

If you use 5 ply I don't that the grain would matter but that is purely a guess on my part.

[ha622]

The weight of a plywood rib will definitely be higher than the standard truss type rib. By how much? I don't know. I know that Lorin Wilkinson used routed ribs for his "Snooty Hatz". I believe that this airplane is one of the heavier ones around, not only because of the ribs but also because of many other modifications, which didn't necessarily help to keep the empty weight down.

In Tony Bingelis' book "The Sportplane Builder" there is a comparison of the cost for different rib constructions. The truss rib came out at $1.53 per rib and the solid 1/4" mahogany plywood rib at $2.34. Nowadays, the prices are certainly higher but the ratio probably remains similar.

If you plan to go with routed plywood ribs, I would recommend a high quality 5 ply plywood and orientate the grain longitudinally. This will make the ribs slightly stiffer in the transversal direction and stronger in bending. Generous lightening holes will be needed to keep the weight penalty low. You will also have to think about clearances for the drag/antidrag wires.

Building truss type ribs is not as bad as it seems. I actually enjoyed this part of the building very much.

[painless]

My concerns are based on economics and weight. As has been stated, I think we can assume your cnc'd ribs will turn out heavier than a truss-built rib. How much is the question, but when you consider the number of ribs to be built (36?? I lost count ) the weight adds up.

Waste/cost will far outpace that of a truss-built rib. Quality 5 ply plywood is not cheap, nor is the setup and running of a CNC machine, unless you own one for other endeavors. Factor in the percentage of the ply that will end up on the shop floor as well.

One suggestion for the use of a CNC or laser cutter is to make the plywood leading and trailing edge pieces. There I can see a definite advantage. I live very close to a model airplane kit manufacturer called Balsa USA. Just after I made all those wing parts, for all 4 wing panels and center section I might add, they went out and got a laser cutter!

Dad gum it!

[BravoCharleyWindsor]

Thanks everyone for the great responses! To really evaluate the cost and weight differences would require dimensions and component properties not available to me at this point. Load bearing analysis of the two methods would be even more of an involved techinical evaluation that I'm not at the point of tackling now.

[BravoCharleyWindsor]

Here is the best photo that I've seen so far to depict what I have asked about. Note that grain of ply is even longintudinal per previous posts! Does this stir any additional thoughts or comments?

http://www.warnerair.com/builders/spcwlkr8.jpg

More pics here...http://www.warnerair.com/builders/max.html, and here...http://www.warnerair.com/builder.html.

[DavisD1W]

I know this is an old post, but I just found it and thought I'd add to the record.
The benefit to a traditional stick built rib is that for any given member of the rib, you have the grain going in the most beneficial direction. In the case of a plywood rib, you pretty much always have Half of the layers with the grain going in the wrong direction. Which means that to make up for that, your lightening holes need to be smaller, and thus your rib heavier.
I'm a Mechanical Engineer by trade, and while designing stuff is cool, it must be followed up with a lot of testing (especially when your life depends on it). So once you add all of the testing, there goes your time savings. I think it is best for people to stick to the plans for structural components. There is something to be said for putting your life in a proven design. Every time I catch myself thinking about changing something in the wing, I picture myself flying at 5000 ft. and looking down at that wing (or up for that matter, it's a biplane) and wondering how confident I would feel with my design change at that moment.
So anyway, it's a cool discussion, but I'd stick to the plans.

[stearmoth]

We can only agree to the Warren girder truss spruce built-up ribs! It is has the much higher stress/weight ratio and hence the weight of the rib ist the lowest possible! Probably the only advantage is the saveing of labor time, which should not be a problem, when you have choosen such a high building time, but very, very satisfying Hatz project!
During our build it always has been one of our main aims to save weight where ever possible! Building strong but lightweight ribs is just an easy part of this.
Although we have a great weight penalty with our Bendix wheels, we managed by careful building and parts weight saving to achieve an empty weight of
1054 pounds. That's very close to the 2. Hatz Classic prototype !
Beleave us, the Hatz is a perfect flyer if the weight is close to that weight. If it would be much lighter, its behaviour in strong winds would be more like an ultralight and if its much heavier, there are higer landing speeds and stresses an there is less aeobatic security!
So, we only can recommand building those ribs ar per plans!
One hint however: The false ribs could be made much lighter by using a selfmade plywood from balsa abd thin birch plywood. cutting those from 1/4 birch is definitively not a good idea....


Happy building!

Hans & Sam, H.C.#78, Switzerland

[mmarien]

The CNC Routing caught my attention. I decided to use plexiglas for the rib jig and after the initial cost of having someone cut the jig, I bought a CNC router for the small parts for the jig. The first thing I noticed is the paper plot I placed under the plexiglas doesn't fit. It's slightly longer between the spars. The plexiglas measures as close as I can measure between the spars. I suspect the paper stretched.

I don't have a lot of wood working skills so I also used it to cut the nose and tail parts from plywood. For now, the CNC router is my most used tool. Is that cheating? Some pictures:

Plexiglass jig

20160328_165416 Small.jpg (117.01 KiB) Viewed 8640 times

Cutting rib nose pieces

20160331_145334 Small.jpg (142.25 KiB) Viewed 8640 times

pieces glued and clamped

20160401_143835 Small.jpg (122.08 KiB) Viewed 8640 times

cleaned up nose pieces

20160401_145958 Small.jpg (115.48 KiB) Viewed 8640 times

rib tail pieces right out of router

20160401_150008 Small.jpg (111.89 KiB) Viewed 8640 times

[painless]

Cheater!!

Actually, that's really awesome. Very nice!