(The Twister...pg.3)
Testing the “Twister”
Put the left pivoting clamp in the vertical position and insert
a 10 d nail through the hole in the backplate through the frame
upright. Now rotate the right pivoting clamp counter- clockwise
90 degrees to expose the hole at the 90-degree line. (If twisting
with your bare hands is difficult, use a ¼” drill
bit to drill a hole down into the top of the right clamp backplate
for a helper tool.) Insert a 10 d nail from the back side of
the upright through the hole in the backplate as before. Observe
that the reference lines on the top twisting sheet at the face
of the clamps are no longer parallel to the clamps.
Hold a six-inch straightedge rule to the top side of the sheets
along the horizontal midline (the zero-degree forming line).
Observe the bowing along that line. Now hold the straightedge
to the surface along the 12-degree diagonal forming line. Pivot
the straightedge in an “x” motion to find the angle
at which you have a flat line. As the twist of the sheets is
reduced, the flat line will be at a smaller angle to the horizontal
line, and as the twist is increased, the flat line will be at
a higher angle. When forming a blade, to achieve a flat centerline
from tip to root without using the pull-down strap, the centerline
of the blade will placed on the diagonal angle appropriate for
the degree of twist (plus two degree or so depending on the
recovery characteristics of the laminations used). For example,
the 12-degree line is the approximate line for 70 degrees of
twist.
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Sheets shown twisted to 90 degrees and straightedge held
to top side of sheets along horizontal midline (zero-degree
forming line).
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Sheets twisted to 90 degrees and straightedge held to top
side of sheets at about 10 degrees to the horizontal midline.
Note flat surface.
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Turn the “Twister” on its side. Hold the straightedge
to the bottom side of the sheets, along the horizontal centerline.
Observe that there is a large gap at the middle between the
straightedge and the surface of the sheet. A blade formed with
its centerline along the horizontal midline at 90 degrees of
twist will be strongly bowed down, as if formed on a cylinder.
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Pivot the straightedge to the 12-degree line. Observe that the
line is flat.
Now pull down on the sheets until the top side of the sheets
looks flat. Hold the straightedge to the bottom side along the
midline. Observe that there is still a small gap in the middle.
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Now pull down on the strap until the top side of the sheets
is slightly concave. Hold the straightedge again to the bottom
side. With the straightedge touching the sheets at the center
and held level, there are gaps at the ends of the straightedge
between it and the sheet surface. Pivot the straightedge to
the 12-degree line. Observe the large gap in the center. This
means that a blade formed with its centerline on the horizontal
midline with this amount of tension of the pull-down strap will
be washed out, because the result of pulling down to this degree
is a straightening of the blade leading edge and a decrease
in the twist from the blade mid-point to the tip.
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A blade with mild washout can also be formed without the strap
by placing the centerline at a higher angle than nominal for
the given degree of twist.
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