Plywood vs Stck Ribs and When Are They Intercangeable?

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rbrochey

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I notice that in many designs there is an option for using ribs made with plywood or the good old stick rib types... I opt for anything more efficient to build and with my router table have made many forms very quickly, guitar molds and other things... so my question is, is it always acceptable to make plywood with lightening holes instead of those labor intensive stick ribs if they are the same in every other way? Or is it only sometimes that you can go either route? A case in point, the HM 295 ribs... can you safely and efficiently use plywood wing ribs? I know the rudder ribs are 6 mm X 12 mm spruce sticks covered with a ply web... (I know the web and a singular ply rib is not the same thing) why not a similar wing ribs? Is it weight? What? And if you can do either how do you calculate ply thickness, curious minds want to know... thanks

This

fleawing1.jpg


Or this (if no why not?)

fleawing2.jpeg
 

nerobro

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Plywood ribs are less efficient for weight. They're typically faster to build. That is the tradeoff. If you care about weight, and you have time, you do one. If you have the weight to spare, and prefer saving time, you do the other.
 

BJC

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Years ago, a popular aftermarket wing kit for Pitts used plywood ribs. The wing was heavier, and the ribs broke.


BJC
 

cluttonfred

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Actually, it's not quite that simple as there are in-between types that muddy the waters. At one end of the spectrum are built-up stick ribs with gussets on both sides like a Pietenpol: light, efficient, but time-consuming to make, though personally I find that kind of woodwork quite relaxing. At the other end of the spectrum are simple plywood ribs from a single sheet like an Evans Volksplane, easy to bang out quickly using a template and a router or even bandsaw in stacks, but heavier and less efficient.

PIETENPOL

ribs1.jpg

EVANS (stabilator rib, but wing ribs similar)

28.jpg

Mignet actually used a hybrid method, sometimes stick and gusset and sometimes using a thin plywood web without sticks, even combining the two methods on the same rib as you can see in the first photo in rbrochey's post no. 1 above. The second example above may use Croses-style ribs, which are grooved capstrips with plywood webs and no sticks except those glued in afterwards to attach the ribs to the spars as well as balsa reinforcements at the nose cap. With a couple of aluminum strips screwed to a thick plywood template, you can one rib drying in the jig on each side. See http://pouguide.org/nervures-croses

MIGNET

mignet ribs.jpg

CROSES

croses ribs.jpg

If you take the time to cut lightening holes, the weight penalty vs. stick ribs is minimal and it seems like it would be great way to do the ribs for a wooden kitplane. Provide the jig, maybe in a plastic that glue doesn't stick to very well, CNC cut the webs ready to use complete with holes, and pre-groove the capstrips in bulk. The builder just cuts the capstrips to length with a little hobby miter saw and starts gluing. I bet you could save a little weight or add more glue area by using corner blocking instead of square sticks to attach the ribs to the spars.
 

Little Scrapper

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The Rose Parrakeet has a unique rib design, I have plans and it is a very interesting airplane. Ribs are seen in this video.
[video=youtube_share;UMNGDwsu4_w]https://youtu.be/UMNGDwsu4_w[/video]

Traditional stick building is cheap, quick and fool proof. Just "follow the plans" is always good advice on wing ribs, stick building ribs is really fast. I built my Baby Ace ribs at a rate of one per night with some tunes and a beer.

Stick building ribs is dead simple and fast, especially if you use staples......which I don't because I like nails. Still, with nails it's really quick.
 

rbrochey

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My favorite type that I've seen are ply with capstrips and lightening holes... that would be easy in my shop... I do enjoy the woodworking zen too but I do so much of it that I like to save some time when I can. I've grown to appreciate high quality plywood, in boats and guitars too. Thanks
 

cluttonfred

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OK, here's a question for those with more engineering knowledge than I have, actually the opposite of what rbrochey was asking as I am looking for an alternative to plain plywood ribs.

The VP-2 ribs are routed directly from 1/4" Douglas Fir marine plywood, 58" long and about 9" tall at the thickest point with lightening holes up to 6" in diameter. If I wanted to save weight by using the Croses I-beam ribs (thin plywood web in a groove in the cap strips), what size spruce cap strip and what thickness aircraft mahogany or birch plywood web would I need to approximate the strength of the original 1/4" fir plywood?

45.jpg
 

don january

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OK, here's a question for those with more engineering knowledge than I have, actually the opposite of what rbrochey was asking as I am looking for an alternative to plain plywood ribs.

The VP-2 ribs are routed directly from 1/4" Douglas Fir marine plywood, 58" long and about 9" tall at the thickest point with lightening holes up to 6" in diameter. If I wanted to save weight by using the Croses I-beam ribs (thin plywood web in a groove in the cap strips), what size spruce cap strip and what thickness aircraft mahogany or birch plywood web would I need to approximate the strength of the original 1/4" fir plywood?

View attachment 69430
Ok I'll give it a guess. 5/16"x 5/8" caps with 5mm ply and lightening holes 5" on down
 

cluttonfred

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Ok I'll give it a guess. 5/16"x 5/8" caps with 5mm ply and lightening holes 5" on down
Thanks, is that based on calculations, a similar design, or “that looks about right?” TLAR I can do myself. :p

It also occurs to me...in a two-spar wing like the VP-2’s, where is the rib’s most stressed point? My guess would be the shear loads at the point where the rib attaches to the forward spar as if the wing were balanced on the CP, but I don’t really know.
 

don january

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Thanks, is that based on calculations, a similar design, or “that looks about right?” TLAR I can do myself. :p

It also occurs to me...in a two-spar wing like the VP-2’s, where is the rib’s most stressed point? My guess would be the shear loads at the point where the rib attaches to the forward spar as if the wing were balanced on the CP, but I don’t really know.
No calculations just a "looks about right" from past builds. Also on the highest stress point on the ribs I would say were the most weight of the aircraft is balanced on in level flight. But in flight that area becomes the whole of the bottom of the wing. Rather good questions Matthew.:)
 

mcrae0104

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what size spruce cap strip and what thickness aircraft mahogany or birch plywood web would I need to approximate the strength of the original 1/4" fir plywood?
It will almost certainly be lighter than the 1/4" ply for the same strength. You might need to check stiffness. Here's an analogy:

  • The 1/4" plywood rib is like a simple rectangular beam (say like a 2x12 floor joist).
  • The beam you propose is like an engineered I-joist. More efficient use of materials, not so hard to manufacture.
  • A stick-built rib is like a truss. Pound for pound, nothing is stiffer or stronger.

Here's an approach that could very well get you in trouble if you trusted your life with it, but at least it would get you in the ballpark for a weight estimate. What's the spacing on the ribs? Compare the spacing and stick size of a wing with similar wing loading. 1/4" sticks and 12" apart? OK, build your ribs the same way, but leave out the diagonals and slap on 2mm ply. You might still keep the verticals as stiffeners to prevent buckling. But really run the numbers or test it!
 

cluttonfred

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My question about which part of the rib is most stressed is related to testing. What are the critical modes to test?
 
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