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rotax618

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The spar webs don’t have to be continuous, many designs use vertical ply only butt jointed With gaps between the sheets. As long as the spar caps are stable there is no problem, unless you are relying on the web to increase the cross sectional area of the caps.
 

Victor Bravo

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Yes, I understand your spar caps are larger, but with your shear web design the structural bits holding them at the right distance apart have gaps, right where they attach to the caps. My only point is that this MAY have an effect on the amount of shear and bending load the structure can take, compared to a "solid" shear web.
 

rotax618

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A spar has to be properly designed and proof tested, try to think of a spar as a truss, top and bottom chord and the webs substituting for the diagonals.
 

mcrae0104

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The interruptions in the webs might be OK. I'm glad it'll be tested before flight. Because of the thickness of the ribs, I would be concerned about the bending moment at the rib locations. The moment of inertia in these sections will be quite a bit lower. Unfortunately Bruhn doesn't present a method for analyzing this situation.
 

rtfm

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You’ll want to give that one a really good think before committing to glue.
Hi Tiger Tim,
There are ribs which need to be bonded together (the end-cap ribs (1.5mm ply), for example, are bonded on the outside of the 9mm ribs on either end of the wing section). So while that was curing today, I turned again to the laptop and (as you cautioned might be the case) I discovered a serious oversight. Check out the screenshot below:
1620372321050.png
I hadn't figured in the necessary angle for the strut attachment, which now requires the final rib pairs to be more widely spaced than first envisaged. This also means that the shear webs for the inner part of the wing section have to be reduced in length (reduced and not lengthened, thank goodness...)

I think I'll leave the bonding to cure for another day while I consider this some more. I REALLY don't want to re-build this... A bottle of Chardonnay wouldn't go amiss while I ponder...

Duncan
 

rtfm

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Oops, I made a typo in post #400, where I said:
According to Greenberg's plans for the HM293, his main spar has two15 mm x 15mm strips of Spruce (225mm^2 cross section), sided by two sheets of 1.5mm ply. My wing (considerably deeper than the airfoil he recommends) has two strips of 19mm x 19mm Hoop Pine (361mm^2 cross section) which sandwich a single 3mm sheet of ply.
That should have read "722mm^2 cross section" So quite a lot more beefy. Sorry.
 
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mcrae0104

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Hi McRae
What do you mean by "bending moment at the ribs"?
I meant the bending moment that is resisted by the spar at each location where a rib passes through the spar.
14030323-69B9-403C-A628-0351EE7DD68E.jpeg
The question I was getting at is, will the caps, acting as four individual 19x19 beams, be sufficient to carry the bending moment? I’ll look forward to the testing.

Another thing to consider is web stiffeners. With a conventional full-length web, the ribs act as stiffeners. In his design there are none. You may find that some stiffeners are needed on either side of the rib openings, especially near the root and strut attach (where the shear is greatest).
 
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rtfm

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I meant the bending moment that is resisted by the spar at each location where a rib passes through the spar.
The question I was getting at is, will the caps, acting as four individual 19x19 beams, be sufficient to carry the bending moment? I’ll look forward to the testing.

Another thing to consider is web stiffeners. With a conventional full-length web, the ribs act as stiffeners. In his design there are none. You may find that some stiffeners are needed on either side of the rib openings, especially near the root and strut attach (where the shear is greatest).
Hi,
I'm just a simple sheep-watcher, and while I try my best at this technical stuff, I'm mostly out of my depth. Could you draw me some pictures?

Duncan
 

mcrae0104

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Maybe think of this as a truss with the discontinuous webs acting as discrete diagonals and verticals?
The only trouble is that a truss’s members must be axially loaded only (except for the misnamed vierendeel “truss”, which relies on its members to resist bending—takes us right back where we started).

Hi,
I'm just a simple sheep-watcher, and while I try my best at this technical stuff, I'm mostly out of my depth. Could you draw me some pictures?

Duncan
Yes—busy day ahead, but I’ll be back. Are you asking about the stiffeners in particular, or bending without a web generally?
 

BBerson

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It appears the web is continuous provided the plywood rib to web glue butt joint is adequate in vertical shear.
If desired, the rib could have a slot cut after assembly and a plywood web doubler inserted.
An oscillating saw can cut a slot flush with the web.
 

Hot Wings

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takes us right back where we started
Maybe not? Using your example of the vierendeel maybe consider the sections with the shear webs to be just very wide verticals and the rib discontinuity to be very small openings? He then has 4 stubby spar cap sections at each rib that are themselves small cantilever beams.
 

TFF

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This is illustrating what the simple task a top and bottom capstrip solves on a rib for a wing. Keeps the rib together so a continuous spar can be used. A slot to slide the spar through. I always figured without a continuous web, the rib would need to be designed to be the local web.

Model airplane wings are built sometimes with only webs between ribs, but most models are overbuilt for handling and survival. Very few test the limits of materials. Those models are very fragile that do.

Most of this is about design choices and how to make it work. I think it’s more of curiosity on seeing how you make it work, not that it’s good or bad per say. Most just go with the simplest spar and would rather have more complex ribs than the other way around, even if it seems more involved labor is created. Most of it is not thought in those terms.
 

Victor Bravo

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It appears the web is continuous provided the plywood rib to web glue butt joint is adequate in vertical shear.
If desired, the rib could have a slot cut after assembly and a plywood web doubler inserted.
An oscillating saw can cut a slot flush with the web.
I believe he has thick foam ribs that are also interrupting the shear webs?. So the (traditional model airplane) structural joint between the solid rib and the shear webs is there in some of his rib locations, but replaced by a half inch thick slab of foam in other places. This foam will not transmit that much shear from one web section to another (IMHO).
 

rtfm

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Hi Victor Bravo,
The foam does not act as a shear web in any way. The 3mm shear webs are bonded to the upper and lower spar caps and do ALL the work. I'm busy having my morning cuppa, but will soon repair to the workshop and will thread the ribs on the jig. I'll take some more photos.

Duncan
 

BBerson

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Webs normally have vertical stiffeners (vertical wood sticks glued between the ribs). The vertical stiffener spacing determines the shear buckling allowable. The foam might work if bonded to the web?
I don't think I would use separate spar webs, the spar loads are more critical than the ribs.
 

mcrae0104

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I'm just a simple sheep-watcher, and while I try my best at this technical stuff, I'm mostly out of my depth. Could you draw me some pictures?
I started out drawing a page or so of diagrams, but I realized we'd need to start all the way from statics up through beam theory. I think it would clog up your thread (and there are already many books that present the matter more capably than I can).

Instead, visualize that you sawed your spar in two, right through one of the places where there is no shear web. The "missing" half of the wing is producing lift, which would have to be transmitted into the remaining half. The path where these forces (both shear and bending) can be transmitted is limited to the four caps (four little cantilevered beams, as @Hot Wings mentioned). These four itty-bitty beams are a lot weaker than they would be if they were tied together into one big, tall, strong section. They're a weak link, if you will.

Moment of inertia is a measure of the bending resistance of a beam based on its geometry (independent of the material used, which also has an influence). The concept of moment of inertia works like this: the deeper the beam, and the more of its mass is concentrated at the top and the bottom (the extreme fiber, as the engineers call it), the stronger it will be. Even though you have four caps spanning the gap, collectively they do not have nearly the same moment-resisting ability that the "whole" beam does. Therefore the caps would need to be exceedingly oversized to work in the "weak link" portions of the spar.

One might object that oftentimes beams have holes in the shear webs and they work just fine. That is true, but the size of the permissible hole is limited, and the location of the hole is restricted. For example, look at engineered wood I-beams commonly used in light construction. They don't want a hole near a support, where the shear is greatest, because the hole degrades the shear capacity too much. Also, this kind of hole does not extend the full depth of the beam (after all, they're holes, not slots), which would interfere with the beam's ability to pass forces between the web and the caps (shear flow). If the ribs were very thin--i.e. plywood--I think your scheme might have a better chance of succeeding, but because the foam is relatively thick, the discontinuity in the shear web gives me pause.

My two cents would be that if you don't go through the engineering side of it, it's best to mimic a proven design with similar loads and then test it. Or, complete your wing as-is, and test thoroughly--which I understand is your plan. I'm looking forward to your continued progress. This is a cool project.
 

mcrae0104

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Webs normally have vertical stiffeners (vertical wood sticks glued between the ribs). The vertical stiffener spacing determines the shear buckling allowable. The foam might work if bonded to the web?
I don't think I would use separate spar webs, the spar loads are more critical than the ribs.
Yeah, probably, if there is a fiberglass scrim over the side of the rib, providing a proper bonding surface for the edge of the shear web, as well as a way to distribute this force into the rib. Testing might reveal that some "corner blocks" would be needed to help. By the time one goes through all of the engineering (or iterative destructive testing), the time savings of a one-peice rib might not seem so valuable.
 

rtfm

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Hi,
Could someone tell me where I've gone wrong here? The answer is ridiculous.
1620440973801.png
Besides, this assumes a cantilever wing (I think).
 
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