Carving the back of the blades

End View at R=30 in

1 ill i

S S in

1/8 MI

6 ill

All that's left to finish the job is to carve the airfoil profile on the back side of the blades.

As a rule the thickest part of the airfoil is always 1/3 of the way back from the leading edge. (in other words, the blade is 3 inches wide at the tip, so the fattest part of the airfoil will be one inch back from the leading edge, at R=30 inches the blade is 6 inches wide so the fattest part of the airfoil will be 2 inches back from the leading edge). Another general rule is that the airfoil is about 1/8 as thick as it is wide. This ratio changes gradually as you approach the root so that at near the root it's about 1/6 as thick as it is wide. This is not terribly critical so long as you're close, but you never want to be less than 1/8 as thick as you are wide. At the tip the blade is 3 inches wide, so the thickest part of the airfoil should be about 3/8 inch thick. Anything between about 3/8 inch and 1/2 inch will be OK at the tip, it should not be more or less than that though.

working the back of the blade with a drawknife

A drawknife is a good tool for roughing out the back side of the blade. It also moves along very quickly with a hand plane or a power planer.

carving the airfoil with a power planer

So turn the blade over so you're looking at the back. At R=12 inches measure back 1/3 of the way from the leading edge to the trailing edge (the blade is 7.5 inches wide at this point so measure back 2.5 inches from the leading edge) and mark it. Do the same thing at the tip (it's 3 inches wide so measure back 1 inch). Draw a straight line between the marks. This line marks the thickest part of the airfoil and it should never be disturbed, it's the one place on the back of the blade where you'll do no carving. Between this line and the leading edge you need to carve a nice rounded surface as shown in the 'end views' in the image. Between the line and the trailing edge it could be slightly rounded but almost a flat surface. As with all other steps, it's best to do one operation to each blade so they come out the same. I find that even a change in mood can affect your final work, so it's good work along all three blades at the same time. Once you have this roughed out then finish it with sand paper. The leading edge of the blade should be rounded . There should be no sharp surface between the front of the blade and the back of the airfoil. As the airfoil approaches the root of the blade (around R=12 inches) is should just taper out to the original profile of the board. This area involves some concave surfaces which are impossible to do with a plane. Best is to use a draw knife or a spoke shave. The trailing edge should be brought down fairly thin (about 1/16 inch) and fairly sharp. Ideally it should be very sharp, but you don't want to make it so thin that it's very fragile. Now all you have left is to assemble the blades. We'll discuss that in the next section.

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