Sitka Spruce preferred, other wood species allowed
The manual specifies a number of acceptable wood species. Douglas Fir seems to be most popular. This species is quite a bit stronger than spruce, but unfortunately it's also a lot heavier. Using Douglas Fir, you end up with a substantially stronger ship, but it will also be somewhat heavier than it needs to be, which will ultimatley translate into a slightly reduced available
payload. Since an IBIS contains approximately 80 qubic dm wood, the weight difference between using Sitka Spruce and Douglas Fir would only be about 6,5 Kg / 14.3 lbs.
Is all of this important? You decide: it depends on the mission profile. After all, 7 Kg extra payload can be turned into almost an hour extra endurance if you put it in the fuel tank, so it might well be important to some...
Aircraft Spruce (Sitka spruce) is perhaps the nicest wood species for building IBIS. Aircraft spruce (Sitka spruce) has the highest strength to weight ratio, meaning that you can build light and still end up with a plane that meets the design specifications. Unfortunately, spruce is rather expensive. Aircraft spruce is more expensive still, as it underwent a lot of scrutiny before being allowed on the market as such. For my project, I've decided to go for Douglas Fir.
The skin surfaces of wing, canard and fuselage are covered with birch plywood. In case of the wing and canard surfaces, these are vacuumbagged to the underlying foam and wood structure with a polyurethane glue, resulting in a strong structure with a finish that can be as smooth as those of glass ships. The birch plywood skin on the canard wing is 0.8 mm strong, for the wing skin 1.0 mm is used.
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