Most of you have probably already heard by now, our supply of hobby grade Depron is now pretty much gone. RC Foam made this announcement recently on their website for those of us in North America. It has generated a lot of discussion on many forums, you can browse through this thread on the RC Powers forum.
It appears that RC Foam is still selling some floor insulation grade Depron, but with slight curvature and I am also assuming without the smooth finish of hobby grade Depron. So other than what we already have, it appears we will be building with alternative foams from now on. You can see as you read through this thread, we have been building parts of our planes with XPS/MPF foam for quite awhile now, but with this new situation, we will be eventually be building them entirely of foams other than Depron.
Since XPS foam is identical to MPF grade B (made by the same company Adams I believe) and other than a smoother finish on both sides, MPF grade A is the same stiffness as XPS and MPF B, just more expensive, I will refer to the foam I used as MPF. For those in the US, MPF B is cheapest and shipping is cheap depending on where you are, for me on the west coast of Canada, shipping from MPF is more expensive than a box of the Grade A, so I will be sticking with using XPS from RC Foam. They are a very good company, have always been very good to me and carry a wide range of high quality carbon fibre products which I will probably be needing more of to reinforce the more flexible MPF type foam.
Well, now that those announcements are out of the way, I wanted to share with you some thoughts after building my first score and fold plane entirely of MPF (other than paperless Dollar Tree Foam for the KFs). I recently built a downsized/modified RC Powers Su27 V5 as you can see below, very simple "paint job" done with magic marker as this is a testbed plane to see the long term properties and durability of MPF foam. I now have 50+ flights on this plane, so wanted to give you my feedback on the build and how the MPF has performed thus far.
In the wing, I used RC Powers recommended wing reinforcement plan, one spar ahead and one spar behind the prop slot. I decided to use 4 mm carbon rod for the main wing spar behind the prop slot, I had a piece that was already cut to a good length and I wanted the extra strength. The spars across the front of the prop slot and across the horizontal stabilizers are 3mm carbon tube. I used bamboo skewers in the elevons and vertical stabilizers. I might have been able to get away without using the bamboo in the elevons, but they are much bigger than what I am used to, so I didn't want to take the chance. I think the tape hinges give some stiffness to the elevons, but I will continue to experiment.
So when comparing how much extra weight the reinforcements added to the plane with one of my Mig-35B builds, the increase is about 0.6 oz/17 gr. This is not too bad actually, I expected it to be much heavier. I think with a better layout of carbon fibre in the wing, like using a "Pi" pattern like we use in our NAMC planes I might have saved a little weight, I will experiment more with our future NAMC prototypes.
So how does it perform? Well, without a doubt, just when handling the plane it feels softer and more flexible, that is just something I will have to get used to. As I might have mentioned in other posts in this thread, I have read that MPF is only 80% the stiffness of Depron, but just in the feel of the foam is it definitely softer also. I flew these two planes yesterday, the Mig-35B made of Depron (other than the KFs and fuselage). When handling the Mig-35B, it felt like it was rock solid stiff compared to the Su27 made entirely of MPF.
Overall, though, I think MPF is much better suited for use in score and fold style planes than in profile planes, just comparing my RCP Mig-29 V1 built entirely of MPF and this Su27. The overall feel of the plane and having more structure to the fuselage adds a more solid feel to the plane in general. I also purposely left some of the foam "unpainted" to see if over time and exposure to the sun it changes consistency like Depron sometimes does. Not that the Depron gets weak, but you can feel over long term use and exposure to the sun, the foam does take on a bit of a crystalline consistency, so we will see if MPF does the same.
You can probably see it somewhat in the picture above, the elevons and vertical stabilizers on the RCP Su27 are big. Using some measurements from the real Su27, they are about 15% larger than scale. I think even in our NAMC planes that reinforcement would still be needed in the vertical stabilizers, just not as much and hopefully none or very little will be needed in the smaller elevons.
As we continue with new prototypes, I will be building them entirely of MPF in order to save my precious supply of Depron (8 full sheets and numerous scraps left :/) for my "show" planes, so I will be able to figure out the best reinforcement plans for them allowing for maximum strength with minimum weight gain. My partners Dave and Stephan will also have input as we all like to use our own techniques for reinforcement and we will be experimenting and passing along our findings. Stephan and Dave will also continue to design our planes to keep them light but strong without weighing them down heavily to keep them flying well using the softer, more flexible MPF style foam.
As Stephan already has, we will continue to provide plans that are specifically laid out for the 24" x 48" size of MPF. Stephan was the first designer to do this for park jets and we will continue to provide this service to our customers to make their building experience better.
For those of us who have long built with hobby grade Depron, it is a bit of a sad turn of events, but one we will adapt to and overcome to continue enjoying this great hobby of scratch building and flying foam park jets :)