Translucent linen and rib tapes

[Romani]

At the risk of stating the obvious, I would like to comment on this optical effect wich might be of help to modellers.

Linen fabric as used in Great War airplanes was very thin, like a tablecloth, certainly much thinner than the canvas of jeans, per example. Once you grasp how thin actually is, you realize how fitting is the description of the airplanes of the age as "Kites with an engine"

Being so thin, the fabric is not opaque, it lets light shine through the gaps between the fibers, just like a thin sheet of paper does. For a detailed description of the phenomenon of translucency, go check an optics handbook.

It's also possible that the shrinking effect of dope on the fabric may make it more translucent, due to the gaps between fibers being made bigger due to the fabric being stretched. For a coarse demonstration of this, take a knitted wool sweater and stretch it, you will see points of light appearing.

This means that in an early war aircraft with a clear doped linen finish, the linen under strong sun light will be translucent , and the wood ribs and spars and longeros will be seen as dark shadows underneath. So the rib tapes areas would be dark lines.

But on an airplane with CDL undersides, and opaque paint on the upper surfaces, the reverse happens. The rib tapes appear lighter than the neighboring areas of CDL. Why?

If the uppersurfaces are painted, no light goes through the top wing, so the inside of the wing is dark, lit only by the light that goes through the linen on the undersurface. In this case, the background color is dark, wich makes the translucent linen fabric look darker than it actually is. But the rib tapes, being applied on top of the fabric, have it as background color, so they appear lighter.

A practical demonstration. Under strong light put a sheet of white paper on top of a black object. And then put another sheet on paper on top. Unless you have picked very opaque paper, you will be able to make the dark shape under the first sheet, but if you put two sheets the top one will look white.

I hope I have explained myself well enough and that this is of help to modellers