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(The Twister...pg.4)
The size of the blade you can make depends on the forming angle.
The maximum size for a blade formed at 15 degrees is six inches
by one inch. As the forming angle is reduced, the blade size
can be increased. See figure 1 below.
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Fig. 1
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In Figure 1 above, the inside, smaller “blade” measures
1” x 6” and the outside, larger “blade”
measures 1 5/8” by 7 inches. Maximum blade side is 2 ½”
x 7 inches.
Next, cut the laminations to the desired shape, but do not glue
them. Place them between the sheets on the 12-degree forming
line, lock them into the clamps, and twist to 90 degrees, then
80, then 70, 60, and 50 degrees. Observe that the deflection
of the vertical reference lines on the top of the sheets at
the face of the clamps is greater than with no laminations between
the sheets, and that it decreases from 90 to 50 degrees. The
degree of deflection will vary with the thickness of the laminations
and the degree of twist.
Find the “flatline” at 90, 80, 70, 60, and 50 degrees
of twist. Note that the flatline is at a lower is at a lower
angle as the twist is reduced. The forming line for any given
degree of twist will be the angle at which the blade centerline
is flat end-to-end, plus two degrees or so to account for the
rebound of the lamination materials.
Release the twisting sheets and the laminations, and place the
laminations with the centerline on the 6-degree forming line,
and twist as before. Note at what degree of twist the centerline
is flat. Repeat for the horizontal midline (the zero-degree
line), and try the pull-down strap.
The degree of twist can be increased incrementally by shimming
under the right side nail-pin.
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Making
a Blade
Tape a wax paper fold to the top side of the bottom twisting
sheet so that it can be folded over the laminations. Make the
fold open to the top. Apply the glue to the laminations and
place them on the wax paper with the top layer facing up, the
root end on the right, and the leading edge facing you, and
with the centerline on or two degrees above the diagonal forming
line appropriate for the planned degree of resultant twist.
(See chart at end). Tape the ends of the laminations to the
wax paper to hold them in place.
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The top sheet is put on, the sheets are locked into the clamps,
and the pin is inserted for the left pivoting clamp. The right
clamp is then rotated to the desired degree and pinned.
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If the laminations slide off each other, remove and use a little
masking tape on the edges at the corners. The location of the
“flat line” is checked with the straightedge and
the laminations are repositioned in the sheets if desired. The
spring-loaded, rubber-tipped v-clamps are then used to clamp
the top and bottom edges of the twisting sheets to make sure
that the edges of the laminations are tightly closed.
Polyurethane glue foams a few minutes after application. The
foaming pressure may push the edges of the laminations apart
if too much glue is used and they are not well-clamped. It hardens
in ten hours. To release the blade, remove the right-end stop-pin,
release the draw hasps, the sheets and the blade. Allow the
blade to rest for an hour after removal before sanding.
Look at your blade blank from tip to root and at the leading
edge profile. The lengthwise axis of the blade should be straight,
as in an axle between two wheels. For a washed-out blade, there
will be a slight concavity of the top lengthwise axis of the
blade, and a consequent lowering of the rate of twist from the
blade mid-point to the tip.
If practicable, measure the end-to-end twist graphically by
holding the root end flat to a flat surface and measuring the
lift at the tip from the trailing edge to the leading edge with
a ruler. Draw it on graph paper, and measure it with a protractor.
Or, use a pitch gauge. Keep track of the laminations used, the
grain orientation, the twist applied, etc., for reference.
The twist (pitch) and twist profile resultant from any given
degree of twist (and degree of pulling down of the blade) will
depend upon the location of the blade centerline in relation
to the forming line, the lamination materials, the planform
and length, convexity, and whether and the extent to which the
blade is pulled down for washout. The consistency of these factors
will determine the consistency of the results. If you really
want to find out if you are creating a good blade profile, after
sanding, cut the finished blade in half along the chord line.
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Below are data on the results of twisting all-balsa and plywood-backed
balsa laminations measuring 1” by 6”.
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(END)
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