Have you calculated how extra epoxy you have due to this ?Anyway, the best surface prep would be a mini rake that cuts square edged channels into the foam . Putting the epoxy joint a little more into shear , rather than tension.
Have you calculated how extra epoxy you have due to this ?Anyway, the best surface prep would be a mini rake that cuts square edged channels into the foam . Putting the epoxy joint a little more into shear , rather than tension.
I was thinking something like ganged micro-gouges. Or, a simple powered version: a palm-sized tool with a bunch of very thin circular blades (Dremel-size) on a common shaft cutting kerfs in the foam at the chosen spacing and depth.Anyway, the best surface prep would be a mini rake that cuts square edged channels into the foam . Putting the epoxy joint a little more into shear , rather than tension.
That's an issue, and the weight is why the folks who are most concerned about weight first fill the voids in the large-cell XPS with an epoxy/micro mix rather than straight epoxy. But, that either adds another bagging session or requires a well-timed application of the layup (after the micro has cured enough so the microballoons won't migrate into the layup, but early enough that the new layup will still bond chemically to the micro layer underneath).Have you calculated how extra epoxy you have due to this ?
+ Stability of results..If we, instead, are making our own deliberate voids in too-smooth foam, at least in theory they could be tailored (depth, frequency/pitch, width) to give the desired "tooth" without more absorption than required. In theory...
I think that's a good point in that the skin to foam bond's shear strength, and flat-wise tension is what should be tested rather than peel. If it is consistantly better than the foam shear and tensile than it's as good as it will ever perform.+ Stability of results..
And you need 1/5 of you foam shear strenght for our sub 200 mph applications.
But that 4/5 is dead weight, and high penalty in sub 200mph region. Where your benchmark is fabric wing..I think that's a good point in that the skin to foam bond's shear strength, and flat-wise tension is what should be tested rather than peel. If it is consistantly better than the foam shear and tensile than it's as good as it will ever perform.
" Stability' over time? That's important, and I'm not sure how to test/measure that. It would be simple enough to apply cyclic shear loading of some type at an accelerated rate to simulate years of use, but we'd still have other issues (thermal cycling, chemical compatibility, etc) as an unknown. There's no doubt that the 40+ years of field experience with Dow billet foam provides a lot of confidence. For consideration:+ Stability of results..
Sorry, could you explain this a bit more? Do we have a rough estimate for the required shear strength of the epoxy/foam bond?And you need 1/5 of you foam shear strenght for our sub 200 mph applications.
Sorry, could you explain this a bit more? Do we have a rough estimate for the required shear strength of the epoxy/foam bond?
Sorry, slowest kid in the class here, holding up the class. "Load from ribs to spar" ? In our stressed skin wing aren't all the loads that go to the spar being passed from the skin (not the ribs)? The ribs are needed to reduce skin deflection and keep them aligned to carry those loads. Or, I misunderstand.For our 1.02 psi load one 0.2" rib spaced each 6" is ok, and it have 8 reserve factor. For load from ribs to spar.
This is the "inflation load' pulling the skins out from the aerodynamic chordline of the airfoil? If so, and if your skins are no foam FG/epoxy only and the ribs are pure foam, then it would seem (to my naive eye) we have primarily a tension load on the rib (if negative external air pressure) with a peeling load to the degree the skin is bulging out and pulling on the first line of contact with the foam rib. If our rib has FG capstrips that fold down over the rib sides, or if our skin/rib joint would be loading the foam rib sides in shear.1/5 was from cri cri example - 1/4" rib each 1.8" . 2/14 of original 100 kg/m3 pvc.
Ok. Not a problem. Solid foam can take much more load, than it need. We have that 1.02 psi on surface. And in my example - pvc foam was able to hold concentrated load - 30 times more - 0.2" thick ribs spaced each 6". But it was pvc foam. Xps should be 3-4 weaker..Sorry, slowest kid in the class here, holding up the class. "Load from ribs to spar" ? In our stressed skin wing aren't all the loads that go to the spar being passed from the skin (not the ribs)? The ribs are needed to reduce skin deflection and keep them aligned to carry those loads. Or, I misunderstand.
As we strive to reduce the amount of foam with thin ribs (which is a good goal for weight savings) it starts to make the skin to foam bond more important.Ok. Not a problem. Solid foam can take much more load, than it need. We have that 1.02 psi on surface. And in my example - pvc foam was able to hold concentrated load - 30 times more - 0.2" thick ribs spaced each 6". But it was pvc foam. Xps should be 3-4 weaker..
But full xps core leaves us with one problem - it weights around 30 kg for two seater ul. Where you wing budget is about 60kg per both wing... Spar is in 10-20kg range... And we have no reserve for controls skins etc..As we strive to reduce the amount of foam with thin ribs (which is a good goal for weight savings) it starts to make the skin to foam bond more important.
AND as Vigilant1 points out it adds a little peel to the bond at the corner of the ribs. In keeping with this thread's title; what test coupon would combine the flatwise tension and peel in the corners from the pressure load, and the shear from the rib bending? I still think the goal is to assure the foam fails rather than the bond. If that is consistently achieved, I think the joint can be considered good and the XPS sheets could be made into effective ribs.
The degree of peel at the rib edges may be influenced significantly by the inter-rib wing skin deflection (more deflection = higher peel angle) and also by the wing skin stiffness as an independent variable (stiffer skin increases the bond area taking the peel load). So, simple, general tests may only be able to provide a trends/indications of the quality of the epoxy/laminate-toam bond. From that, candidates could be selected and detailed tests with actual proposed wing skin/rib section could be tested.In keeping with this thread's title; what test coupon would combine the flatwise tension and peel in the corners from the pressure load, and the shear from the rib bending?
All true. But:But full xps core leaves us with one problem - it weights around 30 kg for two seater ul. Where you wing budget is about 60kg per both wing... Spar is in 10-20kg range... And we have no reserve for controls skins etc..
It is. And as best solution / benchmark - Rigid D cell + fabric covered aft. portion of wing. For any sub 150 mph airplane. And in a wing - foam or build-up, spar have similar mass if done from same material. Or spar + strut combos.. So we need just light way to transfer loads from Cl to spar.. And stay in desirable skin deflection region.- Many other alternatives end up just as heavy. As Billski has sketched out several times, if we have a sandwich skin, ribs, joining tapes, standalone spar, etc all the other stuff can equal the weight of the solid foam block. Building the spar right on the foam block saves a little spar weight compared to a standalone spar.
This way has a lot to be said for it. We still get low drag laminar flow over the "easy" portion of the wing (approx front 30%) and probably don't even need a laminar airfoil to do it. Rigid skin D-cell takes drag loads and covers the place where much hangar rash and bird strikes occur (the LE). Fabric + ribs aft where loads are lower. Very light. Too bad the ailerons, flaps, and the need for something to attach them all to re-complicates the picture.And as best solution / benchmark - Rigid D cell + fabric covered aft. portion of wing. For any sub 150 mph airplane.
Is this the wide-open grid, relatively stiff fiberglass drywall tape, or the silky tape often used for reinforcing basement crack waterproofing? I can see GG working with the former, it would seem too thick for the later.I use fibreglass drywall tape on the rib edge , glued on with gorrila glue and stippled with little tiny holes to increase surface area.
What coverings are you using, and what glue?The coverings get glued on. to the tape.
? "Glass tape on the covering surface"? Is this the glass tape you bonded to the edge surface of the rib, or did you attach more tape directly to the wing covering where it touches the top of the rib?I tested ribs with glass on one side and 1in glass tape on the covering surface . When I ran the weight of failure, I was 10x over 3G flying weights.
In any of your tests, did you take a wider tape (say 3") centered on the rib edge and then folded over to wrap down the rib faces about 1" on each side? That would seem to take all the peeling load off the foam due to the uplift of the wing covering and instead give a more moderate shear loads to a larger portion of the rib face. As a bonus, if a stiff adhesive/matrix (e.g epoxy) is used, you'd also get a stiffening "C" channel around the rib's perimeter.The ribs tested to about 90# on the tail section of the foil naked . Glass tape the edge went to 140#. Glass on the rib face went to 150+#. The fail was the rib collapsing just about 160 or so . 1 in XPS foam .
It is not..Very light. Too bad the ailerons, flaps, and the need for something to attach them all to re-complicates the picture.
Remember in the monolithic foam thread where I mentioned varying skin thickness? If you layup root/tip/Dtube in a heavier, then finish with a single ply of <100gsm over top pf all. 80gsm/10mm XPS backing is enough to require not-accidental force to push through a surface.Very light. Too bad the ailerons, flaps, and the need for something to attach them all to re-complicates the picture.
From a building and owning perspective, I still prefer a rigid skin airplane. Maybe I need to work on that.