I’ve been playing around with putting 2 part polyurethane foam inside composite skins which have been vac bagged and cured in the molds, then closing the mould so the foam expands and fills the skins to give a foam cored wing.
I’ve been doing reports about it on one of our local threads, and was urged to post the results in this composites fabrication. I’ve been reluctant to do so because it’s just backyard, rough and ready, trial and error stuff and I know there are a lot of very experienced guys who read this forum. And I haven’t researched the forum to see what others have done already. But I’ve had some success which has prompted me to start this thread.
I’m basically a slope nut, and when I got my first “moldie”, an Aeromod Minij and later a Miraj, it was obvious that the foam cores conferred huge durability compared to hollow molded planes. But other than Aeromod, who don’t even have a dealer in Australia now, there are no other manufacturers who make foam cored planes. One or 2 others tried and gave up soon. So before long I got some lessons in vac bagging and made foam cored wings that way. Most of those wings are still flying, either with me or my mates, and have outlived numerous hollow moldies.
Just over a year ago I designed a simple plane I called the Scratcho, which has a 64” span, HN1038 foil (i.e. thin!) fairly high aspect ratio wing. I shaped a fuse and made a mould of that and vac bagged glassed tails from flat balsa. Basically the Scratcho flies fabulously – fast and aerobatic and locks beautifully into a DS circuit. The vac bagged one did 185mph on the first day’s DSing, but more than that was just all round great fun and totally free of bad habits. So that enthused me to make a plug and from that, a wing mould. I had never made, owned or used a wing mould before so it was a great learning curve. The result isn’t perfect, but good.
My total goal was to make molded wings with foam cores. In that quest I talked a lot with Timbuck, who was on the same wavelength. At the time he had discovered Ampreg F230 foaming epoxy, and he was very excited about it. One Sunday we had a crew of blokes turn up here at 8am. We cut the glass, mixed resin with fast hardener and laid it up, added spar caps etc and vac bagged it with heat for a few hours. Then we unwrapped it, added a few bits of blue foam in servo spots etc, then mixed 600mL of Ampreg and poured it into the skins, smeared it around, closed the mould and ramped up the heat. At 8pm the same day, there was a bunch of guys leaping around for joy as we broke the mould open and pulled out an incredibly strong wing, filled solid with Ampreg epoxy foam. We called it the “Timbuckerized Wing”.
This was just fantastic. We made another, improved one soon and I owe a lot of my DS learning to the durability of the Ampreg F230 filled wing. But the stuff is quite dense, so wings filled with it are at least 1.3kg, and need reasonable air to perform well. So I began to work on ways to get the benefits of the F230 without the weight, by putting pre-cut blue foam cores into the wing skins before pouring in Ampreg. Several prototypes later I’d made some great slope wings, but they weren’t ideal … I’d found that the Ampreg doesn’t like to expand when used in a thin gap. It wouldn’t completely fill and bond between the foam core and the skin. It seems to need a critical thickness or mass to activate the expansion.
I then cut half –foil foam cores … i.e each core had just the top or bottom half of the airfoil cut out of a thin foam block. They were vac bagged into the molded cured skins, (with some epoxy painted into the skins). Once the foam had bonded to the skins I hot wire cut the excess foam off, flush with the mould flanges so there were 2 fully foam filled halves, ready to mate together. I cut slots for a socked spar and socked aileron stiffeners, and also cut the foam away from the edges of the entire circumference of the wing, to allow some space to pour a thick bead of Ampreg F230 all around. Then the mould was closed. The end result is, to me, perfect. Acceptably light (540gm wing out of mould) and extremely strong. The Ampreg around the edges makes the wing so tough that it’s no problem to crash land the plane into the bushes where there’s no LZ. It’s so good, I wish all slope planes were so tough. But it was quite a lot of work.
Recently, Two-Stroke in Slovenia reported using 2-part polyurethane foam inside a new plank wing. I had ditched the idea of PU foams for filling. Timbuck had played with them and made some Warhead 30 wings by putting foam into the skins and allowing both sides to expand … then he cut the excess off with a knife or saw and bonded them together. That sounded hard to me; I wanted to have the foam expand inside the closed mould, but Tim said the working time was impossibly fast. I asked Two Stroke how he did it … he had done it in 2 steps, part filling with PU the first time, then trimming and putting PU in again and quickly closing the moulds. So this got me interested again.
About a month ago, I had obtained some PU foam called Erapol GP330 which is a commonly available, cheap insulating/ floatation foam. I tested it and was amazed at the expansion. But the stuff goes off fast. You have about 10 seconds mixing time, then about 30 seconds to spread it as it begins to froth but once it starts to gel, you can’t touch it or the matrix will collapse at slight touch. I was worried about the short working time but found that all the other foams were even faster!
I worked out the volumes of my fin and stab moulds, and vac bagged light glass/ carbon skins into them. When they were cured, one at a time I mixed the PU (1/12th of the volume) and smeared it into the skins (both sides) and closed and clamped the moulds. I really didn’t expect it to work, but when I opened them up the next day I was honestly stunned by how good the parts were. Much, much lighter than I’d made before, and very stiff. Even after cutting the control surface hingelines, they were good and strong. This motivated me to proceed with trying it on a whole wing!!
A few days ago I laid up light glass/ carbon skins in my Scratcho wing moulds and vac bagged them down. When the skins were cured I used fine sand leveled in the moulds to calculate the total volume. Yesterday I got a helper and mixed the foam and spread it as a long thick band along each half. We were very prepared and worked fast, but this stuff goes off fast. We had spent too long mixing, and when I started trying to spread and even up the liquid, it was already starting to collapse when touched. So we closed the mold. A few hours later I opened it up, and was very disappointed to find that it was a failure. The foam had not expanded as far at the LE and TE from the central band. So there was a big void behind the LE and TE joins.
This was puzzling, because small amounts of foam such as the leftovers in the mixing cups expand hugely. I had been advised that the foam didn’t like to expand far from a pool of liquid. But I didn’t realize that the problem would be this bad! It seemed that when you start with a lot of liquid in one spot, it starts to expand but the surface tension at the outer edges increases, resisting expansion from the centre and it gradually stops moving, not reaching the potential expansion. This results in a lot of foam of high density, and gaps at the edges. Thinking about this I wondered if spreading the foam liquid more evenly, and coating the entire skins, would allow it to expand more fully and fill the wing better.
I was kicking myself for wasting good materials making nice carbon skins for a failed experiment, then had an idea … and with very little effort I found that it’s not hard to peel the skins back off the rigid foam! So I was able to salvage the skins in perfect shape, scrape almost all the PU foam off them, and try again. To get the skins to sit back into the moulds, I misted 3M 77 glue spray over the skins and mould, which sounds bad but is important because you have to make sure the expanding foams can’t bleed back into a gap between the mould and the outer skin surface. With the 77 spray, they stuck down nicely.
At lunchtime today we did “Take Two” and this time we did things a bit differently:
1) We timed the mixing to only 10 seconds, then quickly poured into both skins.
2) As soon as it was poured, 3 sets of hands set about spreading the mixed GP330 across the whole skin area from LE to TE, very quickly. Even so, we were still working on the spreading when the foam began to collapse to the touch.
3) We used 200mL total liquid this time (1/10th volume) instead of 160mL (1/12th).
I opened it up this evening, and it’s a 97% success! You would find it hard to credit the difference in the result compared to yesterday’s experiment. Basically it's perfect except for 2 quite small sections of the leading edge that have a bit of a cavity behind them. But they're pretty small deficits. I can tap all along the LE and it's solid. The TE is bonded all along, and feels really solid and rigid too, apart from one small spot that I can bend. The wing is definitely stiff both in bend and torsion. Weight is 460gm, which is 40gm more than yesterday's because we used 40mL more GP330!
Sorry for the long rave! In summary so far, I think the method does have potential if you're prepared to set everything up ready and have a few guys to mix and spread it VERY quickly. I'm going to continue with trying it for different things. I reckon you could vac bag a glass and Innegra skin overnight, and next day get 4 guys together and spend 10 minutes doing the foam mix pour and spread at sprint speed, and 3 hours later you can pull out a brilliant very durable cheap wing.
Here’s a short video I took tonight showing the results and explaining a bit about it.
I’m Interested in anyone’s similar experiences!