Lifting roof XPStream - making improvements

[dave campbell]

incredible ! i agree with your assessment of the hole method. small holes are hard to fill without using injection. i run into similar problems when filling holes in concrete countertops with grout. seems i am forever chasing bubbles, but i have the luxury of rubbing them with my fingers, as i am using cement--not epoxy. come to think of it, the late 90's was a hey-day for epoxy grout and when combined with a small tile (lots of grout lines) i was plagued with bubbles. uggh.

[OP827]

Thanks for your kind words Tony.
Also thanks for sharing the video, very exciting to see a garage build foam sandwich airplane!
Here is what I am thinking about the particulars of both this video airplane and trailer surface. The fuselage is almost round and has 3 layers of FG cloth wrapped around, so such FG skin configuration just by itself w/o a foam is a very strong structure and there are also plywood and other points that will keep the plane FG skin intact.
In my build I am trying to get away with just one layer of 6oz cloth to make it light. The surface area for delamination of my trailer (thus thermal expansion displacement/force buildup) is substantially bigger than on a smaller fuselage like this. Delamination would typically occur on a flat surface area where a thermal expansion pushes FG away from the foam (buckling effect) and when the strength of the foam attachment becomes less than the push, the delamination would happen. Of course it is repairable by drilling a bunch of holes in the FG and inject all with epoxy and that will fix the problem, in fact better than new, as that way the holes will act as some strong anchors for the FG attached insde the foam core. I see how the sample goes, I just would like to have some preventive measures. I do not think I need a lot of attachment to avoid delamination, that is why I trying to come up with some small cuts, like tiny homeycomb. Cheers!
Oleg

[dales133]

That's it!
I'm going to build a plane!
Iive always been fascinated with flight ever since I was a kid and heard of the Wright bros and even more fascinated that v they wernt actualy first to fly the designated distance to be recorded as actual flight.
It was actualy 2 kiwi farmers that beat them to it by a matter of hours due to time lines.... but none likes rewriting historicaly questionable claims because they are usualy ingrained in national folklore.
My grandfather was a aparantly the first person to fly livestock across the cook straight... a lamb...or so he always told me. ...

[OP827]

OK, another test sample result is here, see the picture below. I know, I know, such testing is not representative of real forces that the skin would usually experience, but a destruction is always the ultimate test to see what fails first. So I tried to tear the epoxy FG off.. and what a different thing have just happened.. None of the other samples (hot wire multi-groves, tiny holes etc.) destruction tests had such a result, usually the cloth would come off as one piece, but not here. The tiny cut lines seem to hold the FG cloth so strong that I could not tear it off without tearing the cloth apart as well, wow.. I think it is good news then? The amount of extra epoxy required to fill the tiny cuts is very little and would be same or less than the tiny holes, but the improving effect is much greater. The secret is obviously in its structural geometry that grabs more of a foam surface. I also noticed that after about 20 hours of the sample epoxy curing the laminated surface is significantly stiffer than with plain(no cuts) foam or holes. It is actually takes some effort to do a imprint by finger to put a ding into the surface. I am thinking it could be a better way to go with such laminate... .. What is your opinion?

As far as time is concerned to make the cuts, it will be not much longer than a pricking roller after I shall make the jig to make maybe 10-15 cut lines in one move.



I also wonder if it would have some similar effect with FG cloth and TB2 combination (thinned?).., but it would require much more time (than epoxy) for TB2 to cure or rather dry to a full strength to see the proper result, maybe a week.

[OP827]

This interesting test sample made me wonder what was going on inside the lamination so I cut a small coupon out and quickly dissolved the foam with acetone to see the geometry and quality of structure that formed inside the foam.. The epoxy has impregnated every tiny cut and created a perfect 3D torsion boxes strongly attached to the skn! No wonder it works structurally so good. BTW, I could not really make a dent with my thumb pressure on this coupon surface. What can we say, is it good or what?

I will be using this lamination technique going forward. I wonder how that would affect the lamination of an airplane, it may not require so many layers of FG cloth. Looks like a good idea to me to save on weight while still providing a strong structural skin. Why I did not figure out that earlier...

[KCStudly]

In my work I found that scoring the foam with a razor knife sometimes chunked out pieces. Perhaps I was going deeper, or my blade was not super fresh, but I wonder how you managed to get such perfect scores.

On the airplane usage, I'm thinking that the fact that it tears at the grid stress risers may be a good enough reason not to use the technique in a highly stressed application like a flying machine. Maybe. It does seem like a novel idea, and not to take anything away from your creative experiment, but I would be surprised if the technique hadn't been tried by "experts" before. Is there any precedent out there?

I'll be interested to see how this works out for you and to see how you plan to hold 10 blades and have them be easy to refresh.

[OP827]

In my work I found that scoring the foam with a razor knife sometimes chunked out pieces. Perhaps I was going deeper, or my blade was not super fresh, but I wonder how you managed to get such perfect scores.

On the airplane usage, I'm thinking that the fact that it tears at the grid stress risers may be a good enough reason not to use the technique in a highly stressed application like a flying machine. Maybe. It does seem like a novel idea, and not to take anything away from your creative experiment, but I would be surprised if the technique hadn't been tried by "experts" before. Is there any precedent out there?

I'll be interested to see how this works out for you and to see how you plan to hold 10 blades and have them be easy to refresh.

"Experts" are commonly using special high density structural foam, it is significantly more expensive than XPS foam. There is also pre-scored foam made to comply for a curved surface, like this:


but I did not see this thin cuts technique anywhere yet. I was searching the net for quite a long time trying to see what is being done to the foam like this against potential delamination. And I would not recommend or try it on the plane build without a lot of various testing made first.

EDIT: To make a clean cut I place a fresh blade at a shallow angle towards the foam. If I use the blade on foam only, it lasts quite a long time. I will make a jig to be with replaceable blades, it is just a set of spacers with 2-3 through pins to hold with blade pieces and then the set is tied together with 2-3 screws thru them, with a handle of course.

[OP827]

Here is the jig built to do precise cuts in the foam for reinforcement. I found blades in dollar store that would fit the the idea. The two pins are actually not necessary as after all blades are stacked, the setup can be aligned in the vise (with soft pads) and through bolt is tightened. That's it. I also made a cover out of ply and wood scraps to store the jig safely, safety is first . Cheers.

[noseoil]

Wow! Great idea about the scoring lines & saturation depth. I'm wondering if it's necessary to go in both directions with the cuts? I know the grid effect is stronger, but is it necessary for each panel you have? The spacing of the cuts would have an effect on the stress transfer within the foam, the depth & the direction would also come into play.

I'm impressed with the simplicity of this idea & the result! Congrats.

[dave campbell]

very nice R and D. great jig---great idea !

[OP827]

Thanks Noseoil and Dave. One direction will suffice where only de-lamination is a concern. It seems to hold pretty strong. If I wanted structural component to improve, then more cuts and direction can be added, it is a flexible method for the depth and distance between the cuts. I think I will also use this for the areas of reinforcement under the ply on foam, for example. There seem to be many new possibilities with this. De-lamination was always a weak point of all foam sandwich structures... looks like this can be the ticket!
I made XPS+TB2+FG and Bondo+Polyiso samples just for the heck of it..

[OP827]

Polyester resin FG on polyiso foam has rendered a similar result to epoxy FG on XPS. The tiny cuts work similar to capillary channels in wood to suck the resin inside them. Polyester resin is more economical as we all know.
Prior bondo samples with no cuts would just peel off as one sheet.
This method is now working for both polyester and epoxy.
Next is TB2 sample, waiting a couple more days for it to dry and cure.
Oleg

[tony.latham]

Keep up the tests!

Here's an interesting publication: Moldless Composite Homebuilt Sandwich Aircraft Construction. It's a bit old, published in 1988, but those were the glory-piooneering days of homebuilt aircraft composite construction.

http://www.prototechnical.com/~imcmahon ... MCHSAC.pdf

The only thing I could find on surface prep of the foam was using 36 grit sandpaper. But since all of these layups were on curved surface and a multilayered sandwich, it's different than skinning foam with one layer of fiberglass. But I think anyone skinning a foamie with fiberglass will find something applicable in this publication.

By the way, I flew my homebuilt to Oshkosh, the Experimental Aircraft Association's Mecca, twice. It was a tube and fabricate craft. But of note, all of the many foam composite aircraft I ever saw were white. And boy, were they slick.

Tony

[OP827]

Thanks Tony, I will look into this publication, it just would not load by some reason, will try later. Great to meet a pilot here. I fly hang gliders, I like thermal streams and cross country flying. I did try flying motorized plane and ultralights, yet I find more fun with free flight. Would be cool to build a rigid wing glider that can be light and foot launched with wheel landing gear with this technique.. I just thinking. I built several home built hang gliders back in 1983-90ish. Cheers!
Oleg

[OP827]

Back to the build, to report on today's progress. The frontal assembly is permanently epoxied in place. Does not look as much, but it is a milestone for me, today was the day when trailer assembly has started.