Foam cores are not strong, in and of themselves. Cutting
the airfoils in foam blocks disrupts the three dimensional stability of the foam
and sometimes causes the foam to warp slightly. This is not a
problem. The foam cores and shucks will fit back together and be as
perfectly straight as they were when they were cut, when they are placed on a
flat table and weighted down, like when you apply the sheeting. What is
important is that when you skin the wings that it is done on a flat surface with
the shuck marked TOP as the top. The weight or Vacuum bag will hold everything
flat. When we cut wing we have about 70lbs (32 kg) on the block of foam. Remember
that the foam has very little strength and almost no stiffness. The balsa skins
are strong and stiff, which always dominates and carries the load. Once the
skins are bonded in a flat position the wing will always be flat. Think about it
from a fiber perspective. Fiberglass cloth is limp and has no structure. Epoxy
DOES NOT react with fiberglass yet when added together you get a stiff
structural part. The reason is that the epoxy holds the fiber "in
place". The combination gives you the structural part. The same is true for
the wing. The components are all limp and non structural until added together.
Have you ever worried that since the fiberglass cloth is limp that your fuse !
will warp? The wing core has one purpose: to give the skins something to
glue to to "hold them in place". If it is done flat it will always be
flat, just like if a fuse is molded straight it will always be straight (unless heated to soften the epoxy), with a
former or equivalent to hold the shape "in place".