Finally, I found some information, for my questionning, from the master itself Chris Heinz. My only question remaining is the thickness of the material of the floor stiffeners for CH640.

CH in is book "Flying on your own wings", page 176 discuss on Stiffeners : "The graph on p. 162 shows that the effective width (of the flanges of "Z" or "L" stiffeners) is b' = 10 t ( " t " is the thickness of the material use for stiffeners). Because (b') this will be very small (to small for riveting) for our thin stiffeners, b'/t = 20 or 30 times (of " t ") are better and still not too heavy (CH said that any dimension over 10t only add weight - not resistance). 

Note : (parenthesis comments are from me).

Hypothesis : we want to use 0.025 alum material    -  If 
t    is thickness of material
b'  is the flange

Then : 

for t = 0.025
t = 0.025 x 10 = b'
b' = 0.25 (or 1\4 on an inch) - this is the minimun effective resistance of a bend stiffeners
t = 0.025 x 20 = b' of .500 inch or half inch for the flange
t = 0.025 x 30 = b' of .750 inch or 3/4 inch for the flange

and Then 
for t = 0.032
t = 0.032 x 20 = b' of .640 inch or  5/8 for the flange
t = 0.032 x 30 = b' of .960 inch or 15/16 inch for the flange

Has I understand it, this exclude the radius bend material dimension itself. This explain why the plan have always, most ot the time, dimension from 18mm to 20 mm for the flanges.

Then for my   " Z " stiffeners - in 0.025 or 0.032 :

First side at the base Z shoud be = +- 20 mm
Second side (vertical bar) required 2 times 20 mm = +- 40 mm (one flange for connecting the 2 radius)
Third top side +- 20 mm

Radius :

First base radius = 3 mm radius (for 90e angle)
Top base radius = 6 mm radius (for 135e angle)

Tell me if I am wrong or miss something

Christian Tremblay
CH640 from plan 0059

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