Sequoia Aircraft Corporation F.8L Falco, Construction Manual

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F.8L Falco Construction Manual 10–8 Revision 4, March 1, 2002
the wood. The rib capstrips must be soaked overnight in water, then put into a bending jig until
dry. It is not necessary to bend the wood precisely as there is still plenty of “flex” left in wood.
The birch plywood is bent in a similar fashion. The amount of curvature that can be introduced
into a flat piece of plywood depends on numerous variables, a few of which are moisture content,
direction of grain, thickness and number of plies, species and quality of veneer, and the technique
applied in producing the bend. Figure 8 shows the approximate relationship between thickness
and breaking radius of plywood. The minimum radius beyond which it is impossible to bend a
piece of plywood without fracture decreases when any of the following conditions prevail:
1. Thickness is decreased.
2. Face plies are laid more nearly parallel to the axis of the bend.
3. Moisture content of the plywood is raised.
4. Temperature of the bending form is raised.
5. Quality of the veneer is increased (particulary by minimizing cross grain).
6. Technique of bending is improved.
It should be noted that no constant ratio of radius to thickness can be set for all thicknesses. In
Figure 8, “hot soaked” means that the plywood is thoroughly soaked in hot or boiling water until
the plywood sinks, after which it is bent over a mandrell heated to approximately 300°F; “10%
moisture content” means plywood of this moisture content bent over a cold mandrel.
Said another way, thin plywood is easier to bend than thick plywood. It is easier to bend plywood
with the face grain parallel to the axis of the bend. It is easier to bend plywood if it is wet, and it
is much easier to bend plywood if it has been heated—plywood (and all other woods) become very
plastic if heated to around 212°F. Note from Figure 8 that 2mm (.08”) plywood has a breaking
radius of about 1.5” if bent across the grain at a normal moisture content of 10%, while the
breaking radius decreased to about 1.0” if bent with the face grain parallel to the axis of the bend.
If heated, the breaking radius is reduced to about .03” if bent parallel to the face grain and heated.