(Chopstick 2... pg 2)
Rotor
mechanism details:
Turn the main rotor pulley from med-soft 1/8”
balsa, preferably “quarter grain” (also
called C-grain). I mount the blank on the cutting
disc mandrel between 1/32” ply washers, then
turn it true at low drill speed starting with coarse
abrasive paper. The narrow V groove must be sharp
at the bottom for belt grip, and a narrow angled
scalpel blade used flat like a chisel seems to work
as well as anything. Plenty of practise needed for
this job, as I seldom get more than one good pulley
in three attempts! The tail rotor is pre-mounted
with cyano to its shaft and then turned in the chuck.
Mount the main pulley on a short length of dummy
shaft with its triangular ply bearing plates lightly
PVA glued to either side: when the glue begins to
grab, spin the shaft between your fingers and adjust
the running accuracy by eye ---- elastic belts are
tolerant of slight wobbles.
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Main rotor soldered-type hub details, showing
drive spring and Teflon belt guide. |
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The pulley drive spring is no fun to make! This
comprises several turns of thin (0.020”) brass
or steel wire wound on a dummy shaft in the direction
of a screw thread; this can be soldered to the shaft
during final assembly. An alternative to this process
is to wind a steel wire spring on the next smallest
available wire size (0.025”) to the rotor
shaft, then loosen it until the shaft can be twisted
into it; if the turn direction is the same as in
the photo it will grip the shaft securely in the
driven rotation direction. This is not an easy job,
but it does avoid soldering, and also allows painless
shaft replacement.
The complete main shaft bearing and bearing assembly
can be slid into place from one side if the split
tail boom is held in place with just one of its
side cheeks. I prefer to use a drilled channel section
bearing of thin hard aluminum shaped to fit snugly
over the boom. Adding the other side cheek then
unifies and completes the structure.
Any alternative light, minimum-play bearing
you are comfortable with is fine, including a Peck
Polymer 1/32” nylon nose bush, but be aware
that excess soldering heat can run down the shaft
and ruin these! The shaft should be vertical, though
a small tilt to the (pilot’s) right is acceptable.
All wire and tube used is from “K&S metal
centres”, found in good model shops worldwide.
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Fuselage and tail rotor assembly:
Not really much to say about the 1/8”
thick fuselage structure, except that the only really
firm stock required is the motor stick and canopy
laminations. All other wood should be medium to
med/soft with straight grain: if you use nothing
heavy for the tail boom/rotor assembly you can usually
get away with virtually no nose ballast on the finished
model. The canopy outline does not have to look
like a “T Rex!” Choose your personal
favourite.
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Tail rotor assembly: note slightly skewed shaft
mounting for safer belt run. |
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Final assembly and adjustments:
The
upper rotor mast bends have to be made with the
pulley and bearings in place, so take it carefully!!
The pictured rotor-head underside shows where the
bends are located: the non-soldered arrangement
shown is the simplest of the options, and gives
you a bit more leeway in adjustments because of
the movable “wire insulation” spacers.
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underside rotor-shaft wirework
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If you do initially make a total “pig’s
ear” of it a simple disc cut will allow withdrawal
of the shaft, and another attempt. Shaping the little
wire eye to mount the soldered bearing tube is tricky,
so practise on a piece of spare wire first! Static
balance the main rotor resting on its fly-bar wire
before mounting to the shaft, then weight the paddles
for accurate lateral balance when assembled.
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Rotor tracking:
Clamp your geared rubber winder to
the bench, and hold the model’s rotor hook
in your other hand: then wind on a hundred or so
turns and observe of the spinning rotor disc’s
left side edge-on: if the coloured blade tip is
flying higher bend the fly-bar following it downwards
to correct it, and vice versa. A little adjustment
goes a very long way, so don’t rush this task!
The elastic drive belt needs the lightest tension
you can get away with to avoid power losses, and
may benefit from some little Teflon tube and wire
guides to keep it on “the straight and narrow.”
Belt throwing usually starts on the inbound side
of any pulley so keep an analytical eye open for
it! I make my belts from “lycra rigging elastic”
from Sam’s models in the
UK, but two good alternatives are thin "sewing
in” elastic from knitting supply outlets,
and thinnest elastic stringing thread (0.5mm) from
beadwork suppliers. If possible the thread used
should be a textile-covered version for best results.
The joining knot must be very small.
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Craig Limber’s splendid action shot of
his own “Chopstick 1” - not an easy
shot to take: “Chopsticks” are “auto-focus
invisible”!!
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