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If you take a piece of 3mm foam (say) and bend it – it does so easily.  Now bond ordinary paper to each side, and suddenly the foam becomes extremely stiff.  Why is this?  It is because as one tries to bend the foam/paper sandwich, the paper on one side is being stretched, while of paper on the other side is being compressed.  And the force required to stretch paper is quite large.  And since the foam core will resist any buckling effect, the composite test piece becomes very stiff indeed.  Increase the thickness of the foam, and the stiffness increases exponentially.

Wings operate in exactly the same way.  A wing spar consists of two strips of material (called spar caps) separated by what is called a shear web.  This is usually (in a wooden wing) plywood.    And this web need not be massive.  The venerable Cub, for example, kept the two spar caps apart with 1.5mm plywood (1/16″).  Under load, the top spar cap is stretched, while the bottom spar cap is compressed.  If there were nothing tying the top and bottom together, the load would cause the wing to collapse.  But the shear web ties top and bottom together, and absorbs the differential forces.  At some point, the shear web’s ability to resolve the opposing forces will be exceeded and the wing will collapse.

So when it comes to designing a wing, most folks use spruce for the spar caps, and plywood for the shear web.  Plywood has a shear strength of about 2Mpa – and this has proved quite sufficient.  But it is also quite heavy (680kg/m^3).  Paulownia, on the other hand has a shear strength of 5.3Mpa, and weighs about a third that of plywood (260kg/m^3).  It turns out that Paulownia is by far the better material for shear webs.

As for the spar caps themselves, while spruce has been the wood of choice in the past, it is more than twice as heavy as Paulownia (550kg vs 260kg per cubic metre) but has almost identical shear strength.  And so it turns out that Paulownia is again far preferable to spruce for the spar caps.

In conclusion, the Tiny Cedar Flea wings will be built from Paulownia spar caps, and Paulownia shear webs.  A no-brainer, really.