Depron and Zepron
The story below is a bit out of date. Depron is now available in
North America from several vendors, including
July 2002: Many European modelers build parkflyers and slowflyer out of a wonderful material called Depron. Depron is sold in some European countries as an insulation material. It is very inexpensive and comes in large sheets (80x120cm) and two thicknesses: 3mm and 6mm.
Many American modelers have been looking for a similar miracle material available on the west side of the pond.
One possible substitute (if you don't need large sizes) is those styrofoam cafeteria trays.
A better alternative is a foam that I called Zepron, which is available from Air Dynamics. Gabe Baltaian from Air Dynamics was kind enough to send me some Zepron samples so I could try my Depron constructions techniques with it.
Zepron comes in 2mm and 3mm thickness. Unlike Depron, Zepron does not seem to be strongly anisotropic, i.e. there is no predominant orientation of the fibers that causes a greater stiffness in one direction. Zepron is less stiff than Depron, but is quite a bit easier to put into shape.
The density of Zepron is a bit higher than Depron as shown
in the table below [if you don't like metric units, welcome
to the 21st century :-) ]:
Zepron can be flexed with a much smaller radius of curvature than Depron. This property makes it useful for building wings of fuses by folding or rolling a sheet of Zepron. In addition, preliminary experiments seem to reveal that heating Zepron with a heat gun seems to increase its stiffness.
Some French modelers say that shaping Depron can be done by plunging it in boiling water. I haven't tried that myself, but I suspect this would work with Zepron as well.
Zepron can be glued with epoxy, odorless cyanoacrylate (the normal CA melts polystyrene), and various other non-solvent based glues and cements. Many impatient builders will dispense with glue altogether and use double-sided tape or Blenderm tape. Blenderm is a really cool adhesive tape made by 3M for the health care industry. It can be found in some pharmacy or ordered online. If you feel like buying the stuff in boxes of 16 rolls, you can order from medical supply stores (like MVAP Medical Supplies), but if you want more reasonable quantities, you can order from Air Dynamics.
Zepron has an advantage over Depron: it can be bent with a radius of curvature without breaking (if you are careful). This allows to build small wings by folding a single sheet over itself. You can also do that with Depron, but it's a little trickier than with Zepron.
Here is how to build a high aspect ratio slowflyer wing. No ribs, no spars, no slow glue, no balsa.