I have been using Fli-Power Value XPS foam for the fuselages of several of my score and fold park jets over the past year and have found it to be a great product for that application and in the process have saved myself a couple sheets of Depron and a little bit of cash :). You can read more details on my first impressions and use of this foam in this previous article I wrote in March of 2015. For brevity from this point on, I'll just call it XPS foam.
I decided for fun to dust off a very old set of RC Powers Mig-29 V1 plans a week or so ago and build another but with several mods. I wasn't looking to move motors around or shorten the plane and other things like I did with my recently completed, highly modified RCP F18 V3, but just tinker with it a bit to see what would happen. If you are interested in the mods I played with, please go to our old blog starting with this article. Anyway, since the Mig-29 V1 is a small, profile type plane, I figured it would be a perfect platform to try out a complete build (other than the KF airfoils where I used paperless dollar store foam) with XPS foam.
Here are a few pictures of the final build still unpainted.
As I was handling the pieces and started assembly, the foam just felt very flexible to me. I think Stephan and I determined that this XPS foam is about equivalent to B Grade Model Plane Foam that has become very popular among many park jet builders. I have read testimonies that it is 80% as rigid as Depron, I'm not sure I agree with that figure, it might be a little generous, but so be it.
So I decided I was going to need some extra reinforcement or my plane would be flopping around quite a bit. Fortunately, I had a 20" piece of 4mm carbon rod that is my favorite for wing spars, but in keeping this a "budget" build, I went with bamboo BBQ skewers for the rest of the reinforcement. These cost me about $.01 each and over spans of about 11" or less are an ideal, inexpensive replacement for 3mm carbon tube, especially when secured with epoxy as they seem to get a little stiffer as they absorb some of the epoxy.
Sorry the picture below is a bit blurry, but hopefully you can see I put two 11" long pieces of bamboo along the nose section of the wing plate, one 4 and 1/4" piece in each elevon, one 6" piece in each vertical stabilizer and one 9 and 1/2" piece along the front of the elevon hinge line. I made horizontal stabs out of the front part of the elevon that is normally moving on the stock build. So this ended up using up 5 bamboo skewers in total.
In airframe construction alone, I saved about $5, a full sheet of Depron to build this plane costs me about $9 and this plane used less than a full sheet of XPS (24" x 48") which costs me about $4.50 a sheet. The bamboo skewers used added $.05 to the cost, I would have used the 4mm carbon rod for the wing regardless if I had used Depron.
So bottom line for a fun, simple build like this, I'm pleased with how the XPS worked out, even with the extra work of reinforcement. I think I still prefer Depron for its natural rigidity for larger score and fold builds for the wing plate, control surfaces and internal structure and because I don't need to spend as much time and materials reinforcing it. I still like the XPS for fuselage and canopy use as it shapes and sands nicer than Depron. Like many things I have learned in this hobby over the last three years, sometimes you just have to apply the right material, tool or component to the task to suit your building and flying needs. This has always been the beauty of scratch building park jets to me, the freedom of expression is vast and there is rarely a "class answer", you just have to experiment and find what works best for you.