Dunno! Oh wait - that’s very interesting- I just dug out my 1983 copy of Silent Flight and it says right there that the section shown on the plans are good for strong conditions but a flat bottomed section can be substituted for lighter air, and yet the plans only show a flat bottomed section. The inconsistency is in the original publication!
Anyhow, back to the Obelix - finished making the templates. After rough cutting the templates on the band saw, I sanded them to the final shape. The convex upper surface contour is easy enough on the disk sander, but the concave curve of the bottom surface templates required the sanding drum.
To clean up any slight waviness I made a semi-flexible sanding stick out of scrap Formica. The idea is to be flexible enough to bend to the concave curve of the airfoil, but stiff enough to bridge the unwanted smaller period waves. I’m eyeballing it here, the occasional unwanted waviness is no more than 0.01” or less. The sandpaper does not extend to the ends of the sanding stick so that the sanding stick can slide along the template without introducing uncontrolled sanding from the ends - if that makes sense…
Anyhow, back to the Obelix - finished making the templates. After rough cutting the templates on the band saw, I sanded them to the final shape. The convex upper surface contour is easy enough on the disk sander, but the concave curve of the bottom surface templates required the sanding drum.
To clean up any slight waviness I made a semi-flexible sanding stick out of scrap Formica. The idea is to be flexible enough to bend to the concave curve of the airfoil, but stiff enough to bridge the unwanted smaller period waves. I’m eyeballing it here, the occasional unwanted waviness is no more than 0.01” or less. The sandpaper does not extend to the ends of the sanding stick so that the sanding stick can slide along the template without introducing uncontrolled sanding from the ends - if that makes sense…
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