Hi everybody,
After learning that it's possible to purchase carbon pultruded strips from China in 0.3 mm thickness, I wanted to test if it were possible to form a spar cap out of these strips to match the curvature of an airfoil, so that the maximum possible depth of a spar could be achieved.
It turns out to be possible to vacuum the strips into the required curvature because they have good bending compliance perpendicular to the fiber direction. For this test, I took 0.45 mm unidirectional carbon plate (which isn't an exact match to the Chinese supplier, but it's what I had available to me cheaply from a hobby store) and cut them into 30mm wide strips (same as the Chinese supplier).
I started by laying down 1/16 in balsa wood on the table, and putting packing tape over it as a release film. The amount of curvature should be controllable by the thickness of whatever spacers you put down here. (Disregard the foam blocks to either side)
Several strips are glued together with epoxy, laid down across the spacers, and vacuumed down to the table.
The final result is a curved carbon pultrusion, as you can see from the gap between it and the straightedge. The idea is to use this as part of the core of a sandwich panel, butted up next to the foam in the D tube. In this thread you can see how the outer surface looks with the cap installed. There's a kink at the joint between foam and carbon (I tried to see if I could cantilever the carbon out beyond the shear web during the layup, so the clamping forces made that kink), but the foam and carbon curvatures match pretty well.
After learning that it's possible to purchase carbon pultruded strips from China in 0.3 mm thickness, I wanted to test if it were possible to form a spar cap out of these strips to match the curvature of an airfoil, so that the maximum possible depth of a spar could be achieved.
It turns out to be possible to vacuum the strips into the required curvature because they have good bending compliance perpendicular to the fiber direction. For this test, I took 0.45 mm unidirectional carbon plate (which isn't an exact match to the Chinese supplier, but it's what I had available to me cheaply from a hobby store) and cut them into 30mm wide strips (same as the Chinese supplier).
I started by laying down 1/16 in balsa wood on the table, and putting packing tape over it as a release film. The amount of curvature should be controllable by the thickness of whatever spacers you put down here. (Disregard the foam blocks to either side)
Several strips are glued together with epoxy, laid down across the spacers, and vacuumed down to the table.
The final result is a curved carbon pultrusion, as you can see from the gap between it and the straightedge. The idea is to use this as part of the core of a sandwich panel, butted up next to the foam in the D tube. In this thread you can see how the outer surface looks with the cap installed. There's a kink at the joint between foam and carbon (I tried to see if I could cantilever the carbon out beyond the shear web during the layup, so the clamping forces made that kink), but the foam and carbon curvatures match pretty well.
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