|
|
|
A New Tool and Method For
FORMING LAMINATED-BLADE PROPELLERS
Oct 7, 2006
|
|
|
|
|
OVERVIEW:
This article describes a device and method I developed for forming
laminated blades for props for rubber-powered outdoor free-flight
model airplanes. The idea behind the device was a “moldable
mold” that would enable the forming of blades of varying
convexity (undercamber), degree of twist and conformation. While
twisting away on the problem, and some sheets of wood veneer,
I hit upon the idea of the “Twister.” I am confident
that this jig and laminating method make a valuable contribution
to the art of forming propeller blades.
The “Twister” is essentially a wooden frame with
pivoting clamps that hold convex polycarbonate sheet formers.
It includes adjustable stops to control the amount of twist,
and a “pull-down strap” to control straightness
of the prop blade. Prop blade laminations are first cut to shape
and glued together. Then they are placed between the polycarbonate
sheet formers (called the “twisting sheets”) which
form and compress the laminations as they are twisted end-to-end
by the pivoting clamps. The clamps hold the sheet ends together
and limit bowing of the sheets while allowing them to shorten
when twisted. The pull-down strap is used optionally to form
larger square-tipped and paddle-type blades and blades with
washout by pulling down the twisting sheets. This straightens
the blade leading edge and reduces the twist toward the tip.
|
|
|
|
The “Twister” is built from materials available
at hardware stores, and can be made at home with ordinary shop
tools. The plan for this article shows the jig with a capacity
for blades up to seven inches long and two and one-half inches
wide. The pivoting clamps will accommodate twisting sheets with
a formed radius of three inches or larger. Clamps and twisting
sheets of a different radius can in any case be substituted.
The “how-to” part of this article gives a method
for forming twisting sheets of any radius as well as instructions
for using the “Twister.”
|
|
|
|
|
The laminating method is a three-layer bias-grain lamination
consisting of one layer of 1/32-inch thick sheet balsa (or 1/16”
for larger blades) as the top layer over two layers of 1/64-inch
thick aircraft plywood. The grain of the laminations is oriented
on a 40- to 45-degree angle in relation to the lengthwise axis
of the blade, such that the grain slants inward and downward
from tip to root when looking at the balsa-layer side of the
blade (which side will face away from the fuselage for a tractor
prop), with the leading edge at the bottom. Orienting the grain
of the laminations this way allows for easy twisting, resists
untwisting and coning under load, and allows for backward flexure
to help avoid breakage. It also avoids buckling of the blade
at the twist transition point if the blade is pulled down for
washout. I use a pattern and a ballpoint pen to draw the blade
shape on the balsa and the plywood, and cut the wood with scissors.
The laminations are glued with polyurethane glue (Gorilla brand
or Elmer’s). This glue is very strong, lightweight, and
sands down easily and smoothly. The plywood allows for a very
thin trailing section without causing reflex of the trailing
edge.
|
|
|
|
|
|
A thin blade (thickness-to-chord ratio under .10) performs as
if its pitch is lower than nominal, and the convexity (undercamber)
compensates for the relatively thin section and adds rigidity.
The radius selected for the twisting sheets depends on the blade
planform, length, twist (pitch) and planned application of the
prop. Where thickness-to-chord ratio is .10 or more, I use twisting
sheets with a radius of about nine inches. This produces a nearly
flat bottom, for reduced drag. For thinner-section blades, depending
again on the planform, etc., I use formers with a radius of
3 ½ inches or 5 ½ inches. The whole idea behind
the jig is to facilitate experimentation.
The “Twister” and this laminating method can
produce a thin, strong blade of any degree of twist and convexity
that is axially straight (flat) from root to tip. It can produce
blades with wash-out or wash-in (hypotwist). Using the instructions
and the plan which follow, you can make your own “Twister,”
and then make blades and props quite possibly unlike any you
have made before. Like me, soon you won’t just be flying
planes, you’ll be “flying props.” If you
aren’t up to making one, but want one, contact me on
the web.
|
|
|
|
|
|
MAKING AND USING THE “TWISTER”
|
|
|
| |
| MATERIALS
Wood
Frame: 1” x 6” pine or similar, kiln-dry,
smooth, knot-free, straight.
Backplates: 1” x 4” pine, or similar, k-d,
smooth, etc.
Pivoting clamps: 1 ½” thick tight-grain,
kiln-dried, smooth, knot-free hardwood (mahogany, oak,
other) or good-quality dense softwood such as southern
yellow pine or douglas fir. ( ¾”–thick
wood can be laminated and substituted.)
Polycarbonate (trade-name: Lexan)
Two (2) pieces 2 1/2” x 11” x .093”
thick.
Hardware
Two ¼” x 2” carriage bolts, washers
and nuts.
Two draw hasps (draw clamps) (National N 208512V35 -
2 ¾”; or Ace 5300231 - 2 ½”).
Two 1 ½” x 2” to 2 ½”
tight-pin desk hinges (National N 211-870).
One 4” x 5/8” steel “mending plate”
(National N-114-405), or similar.
Six ½”steel washers.
Flat-head (bugle-head) wood deck screws (deck screws),
#6 x 1 5/8”.
Pan-head wood screws, 5/8” to ¾”.
Two 10 d. (10 penny) nails.
One Velcro 18” heavy-duty cinch strap. (Sold two
to a package.)
Sheet-metal vent pipe, 6” to 10” diameter
x 14” long, (cut down from two-foot standard length),
or flat metal sheet, 20-guage or thinner, 7” x
14”. (Used for forming polycarbonate twisting
sheets.)
Wood glue (Gorilla or Elmer’s polyurethane) (for
frame and laminations).
Six to eight spring-loaded v-clamps (Craftsman # 65942)
(For clamping twisting sheets)
|
|
|
|
|
|
|
|
|
|
| |
|
|
|
| |
|
|
|
|
FABRICATION
AND ASSEMBLY
(See Construction Plan)
Book-end Frame
Frame Base Plate: (One) 1”x 6” x 12 ½”.
Frame Upright Ends (“Frame Uprights”): (Two) 1”x
6 x 10 ”.
Mark the centerline top to bottom on one side of each frame
upright. Mark for drilling on the centerline 3 ¾”
from one end, which will be the top. On one upright, which will
be the right side upright, use a carpenter’s square to
draw a line at 90 degrees to the centerline at the 3 ¾”
mark across the face of the upright. Next, from this centerpoint,
using a protractor, draw lines at five (5) ten-degree intervals
below the 90-degree line (at 80, 70, 60, 50, and 40 degrees)
in the lower right quadrant of the upright, as shown in the
photo below and on the plan.
|
|
|
| Drill
through the 3 ¾” centerpoint on each upright
with ¼” drill bit. Butt-joint the frame uprights
to the ends of the base plate. The upright with the degree
lines goes on the right, with the lines facing in. Make
sure that the uprights are at 90 degrees to the base plate,
and that the holes in the uprights line up. Apply wood
glue and fasten with 1 5/8” wood screws. Using a
9/64 drill bit, drill a hole about 2 ½” deep
into the top of each upright toward the front corner for
keeper-holes for the 10 d nail stop-pins. |
|
|
|
|
|
|
|
|
| |
|
|
Pivoting Clamps
Each complete pivoting clamp consists of the hinged clamp attached
to the backplate. Each clamp has mating halves. The backplates
are made first, then the clamps are made and attached to the
backplates.
|
|
|
|
|
|
Note in the photo above that the clamps can be wider than the
backplates. “r 5 ½” denotes the radius of
the slot arc.
Backplates for Pivoting Clamps
(Two):
3 ½” wide x 5 1/2” long by ¾”
thick. (Use
standard “ 1’ x 4’”, usually 3 1/3”
actual width. Exact width unimportant.)
Mark the centerline top to bottom on the face of each backplate.
Extend the line across each end to the back side. Mark one backplate,
which will be the left backplate, for drilling on the centerline
at ½” and at 3 ½” from one end, which
will be the top end. Drill through the ½” point
with 9/64” bit (for a 10 d nail). At the 3 ½”
point, use a 5/8” spade blade to drill a 3/16” deep
hole (for the head of the carriage bolt), then drill through
with a ¼” drill bit. Install the carriage bolt,
and pull it into the hole with a washer and nut. Check to see
that the bolt head is below flush. Set aside. On the second
backplate, which will be the right backplate, mark for drilling
on the centerline 3 ¼” from one end, which will
be the top, and drill with the spade blade and the ¼”
bit as before. Install the carriage bolt and set aside.
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
|
