May 2007 47
TTow Syssttem
Bllade
Scrap balsa on the front of the hanger stabilizes the glider from
the downwash of the helicopter blades.
The hanger uses a small amount of balsa, string, wire, and aluminum
tubing. Not shown is the Silly Putty for the timing mechanism.
by Bill Mixon
A fun project converts an electric
helicopter into a tow model
THIS SIMPLE, lightweight, low-cost release mechanism is
used to carry items to altitude to drop with the Blade CX
helicopter. The heart of the release unit has its roots in FF
Glider technology; it uses a Silly Putty timer.
FF Glider fliers have used the Silly Putty timer for many
years to activate a DT device on the model to avoid its being
lost in a thermal. We will use the Silly Putty timer in a similar
manner, but instead of operating a control surface it will
activate a release pin.
Various objects can be towed aloft with the Blade CX, such
as small gliders, parachutes, mock bombs, etc. Once you learn
to fly the great little helicopter, this is a great way to expand
the fun you can have with it.
Most of the construction materials used in the release unit
can be found at the local hobby shop. The steel wire and
aluminum tubing will be in the store’s K&S metal racks. You
can find the Silly Putty, rubber bands, and thread in a
department store. Let’s get started.
Release-Unit Construction: Cut a suitable 17-inch length of
1/8 square hard balsa to build the unit’s boom. This piece of
balsa needs to have a good grain structure and be as straight
and stiff as possible. If a good piece of 1/8 balsa is unavailable,
you can substitute 3/16 square balsa or other material such as
hardwood rod or carbon-fiber tube.
Cut two pieces of 1/8 balsa to 3/8-inch long and another to
1/4-inch long. Glue these three blocks in place on the 17-inch
boom, as shown, with medium cyanoacrylate.
Sand the outside of a piece of 1/16-inch-diameter aluminum
tube, to aid in glue adhesion. Use an X-Acto knife to carefully
roll the tubing under the blade on a flat surface and cut it to
four lengths: one at 1/8 inch, one at 1/4 inch, and two at 3/8 inch.
Use the tip of the X-Acto blade to skive a chamfer on the
inside of each end of the 1/4-inch-long tube. This helps to
ensure that the pin slides smoothly into this section of tube; all
others can be left alone.
Bend a coil or loop at one end of a 5-inch-plus section of
.025-diameter music wire, and snip off any excess wire at the
loop. Cut the wire to a finished length of 27/8 inches long. Use
fine sandpaper to go over the music wire approximately 1 inch
back from the straight end.
Slide one of the 3/8-inch lengths of aluminum tube over the
wire all the way to the loop. Slide the 1/8-inch-long piece on the
wire and position it 1 inch from the end of the wire. Lightly
crimp this tube in place and soak with thin cyanoacrylate.
After the adhesive has cured, scrape any excess glue from
the wire so it doesn’t bind inside the other tubes. Position the
two remaining tubes on the wire with the 1/4-inch length at the
front. Glue the three tubes to the blocks on the boom, taking
care that the tubes are in alignment.
For extra strength wrap aramid (Kevlar) thread around the
tube/boom and soak it with cyanoacrylate. Make sure no glue
gets inside the tubes! When finished, the wire release pin must
slide freely in the tubes or it will not work properly.
Cut a 11/2-inch length of the 1/32-inch-diameter brass rod.
Using the 3/16-inch-diameter aluminum tube as a fixture, bend
the ends around the tube and twist together with pliers. Bend
the twisted length down along the side of the tube and remove.
CX
05sig2.QXD 3/23/07 1:30 PM Page 47
The loose rubber
band is looped
around the
crossmembers to
keep it from
dangling when
released.
Test-glide the simple glider before using
the Blade CX helicopter to tow it to
altitude.
When the Silly Putty timer is released, tension on the mounting pin
pulls it back. The aluminum stop crimped to the wire captures it
within the rear and middle guide tubes.
Medium cyanoacrylate holds it all together. Scuff the aluminum
tubing for the best adhesion. Use a braided string on the
mounting pin.
The tension on the mounting pin is adjustable. When the timer is released, you don’t
want it snapping open.
The 1/8-inch tube is packed with Silly Putty, which binds the rotator pin, slowing its
rotation when the rubber band is connected.
48 MODEL AVIATION
Photos by Nathan Mixon and the author
Materials Needed for Tow
System Construction
• Needle-nose pliers
• Sandpaper
• X-Acto knife
• Razor saw (optional)
• Thin cyanoacrylate
• Medium cyanoacrylate
• Foam-safe glue for glide
Tow System Materials
• 1/32-inch solid-brass rod (K&S
stock number 160)
• 1/16-inch aluminum tubing
(K&S stock number 100)
• 1/8-inch aluminum tubing (K&S
stock number 102)
• 3/16-inch aluminum tubing
(K&S stock number 104)
• .025 music wire (K&S stock
number 500)
• T-pin
• Straight pin
• 1/8-3/16 square hard balsa, 36
inches long
• Office rubber bands (3.5-inch
loop, #19)
• Kevlar or other thread for
reinforcement binding
• Fishing line
• Silly Putty
• 2mm Depron (for the glider)
05sig2.QXD 3/26/07 9:07 AM Page 48
The hanger is designed to hold its cargo far enough out of the rotor’s wash that the
performance of the helicopter isn’t compromised.
The Silly Putty timer is typically set for a 15-second delay. Practice by dropping with
parachutes or Ping-Pong balls over a clear target.
The Blade CX helicopter is primarily a
zero-wind machine and is best to use in
an indoor gym or similar open space.
Never stop flying the helicopter to watch
the gliding payload. Anything released
should be allowed to fall in a safe area.
The finished hanger for the Blade CX weighs only
approximately 3 grams and doesn’t affect the helicopter’s CG.
The hanger is mounted to the landing gear with rubber bands.
Be sure the release string remains free to move.
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05sig2.QXD 3/23/07 1:35 PM Page 49
50 MODEL AVIATION
Tie a release line to the front portion
of the loop in the release pin. A braidedtype
line works best here because it will
slide around the guide loop better than
some other types of lines. I used
Spiderwire Original Braid fishing line on
this unit.
Run the other end of the line up
through the guide loop and back toward
the timer. Tie a loop in the line so that it
is approximately 51/2 inches from the
center of the timer.
Loop another #19 band through the
loop in the line. Because of variations
between brands of rubber bands, you
may have to adjust the line length to suit
you.
Test the Operation: Pull the rubber
band aft and loop it over the pin in the
timer. There should be just enough
tension on the bands to slowly rotate the
timer, and the release pin should be fully
engaged. When the timer pin rotates far
enough that the band slides off, the
release pin should pop to the released
position.
Set the time for release by rotating
the pin accordingly. The rate at which
the timer rotates can be adjusted by
increasing or decreasing the band tension
and by the amount of Silly Putty in the
tube.
A typical time from takeoff to drop
altitude with the Blade CX is roughly 15
seconds. It’s best to experiment with the
timer now and get familiar with its
operation.
Starting at the front, mount the unit to
the bottom of the helicopter’s landing
skids, as shown, using rubber bands.
Make sure the release line is free to slide
and won’t be interfered with by the
bands or any part of the helicopter.
Loop the rear band over the rear
frame crosspiece before mounting the
unit to the helicopter. This is to keep the
release line from being shot out in front
of the helicopter when released. The
spacing of the frame’s crosspieces
should set the boom pointed slightly
upward to clear the ground better.
The finished release unit weighs a
mere 31/2 grams, so in most cases
rebalancing the helicopter will be
unnecessary. However, it does need to be
checked. If necessary, the battery can be
shifted or a small amount of weight can
be added to the aft portion of the
fuselage body.
To tow an item, remove the rubber
band at the front from its pin so you can
slide the release pin freely. Place the
object you want to drop into position,
and slide the release pin forward.
Pull the release line/band back and
loop the band over the body of the timer.
Slice off the excess rod, leaving
approximately 1/4 inch of twisted length.
Glue this in place as shown. This will
serve as the guide loop for the release
line.
Cut two pieces of balsa to 23/4 inches
long and glue them in place on the top of
the boom, ensuring that they are
perpendicular to the boom and parallel
with each other. Wrap both joints with
thread and soak with thin cyanoacrylate
for reinforcement.
Time for the Timer: Cut a section of
3/16-inch-diameter tube to 3/4 inch long.
Plug one end by gluing a short piece of
wood on the inside or by crimping the
very end with a pair of pliers. Either
method works well.
Cut a 7/8-inch length of 1/8-inchdiameter
tube and clean any burrs from
the ends. Set this tube inside the 3/16-
inch-diameter tube you just made and
mark the 1/8-inch-diameter tube
approximately 1/16 inch out from the
outside edge of the 3/16-inch-diameter
body tube.
Remove and push a T-pin through the
center of the tube at the mark. The pin
will go through without crushing the
tube, although it does require a good bit
of force. If necessary, predrill the tube to
make it easier.
Cut the excess T-pin from each side.
You need only 1/8-3/16 inch of pin to
extend from each side of the tube.
Roll a small amount of Silly Putty to
a size that will fit inside the 3/16-inchdiameter
tube, and pack the tube
approximately half full with the Putty.
Push the 1/8-inch-diameter tube/pin
assembly slowly down into the Puttyfilled
tube until it bottoms out.
Glue a balsa gusset to one side and
flush with the rear of the main boom you
constructed earlier. Adhere the timer to
the back of the gusset/boom with
medium cyanoacrylate, with the timer
body flush with the edge of the boom.
Rubber Bands and Release Line: Hook
a #19 rubber band with a 3.5-inch loop
onto the loop in the release pin. With the
pin in the released position—pulled all
the way back—mark the wood boom
approximately 3.5 inches back from the
loop in the wire. At the mark, push a
straight pin up through the boom at an
angle, snip off the excess on the top side,
and soak the area with thin
cyanoacrylate.
With this band in place over the pin
there should be virtually no tension
pulling back on the release pin. Too
much tension here can damage the
mechanism upon release because of the
pin slamming aft with excessive force.
Reconnect the front band to its pin.
When you are ready to fly, pull the band
to the pin on the timer and set to the
desired time.
Take off with the model, climb to
altitude, and watch for the drop.
Remember to maintain control of the
helicopter and not concentrate on the
object you just dropped. I highly
recommend towing up simple items to
start with, such as Ping-Pong balls or
other lightweight, small objects. Once
you are familiar with the unit, move on
to gliders, parachutes, etc.
Ensure that whatever you tow up will
not interfere with or become tangled in
the rotor blades. Hovering in ground
effect with a tow object should be kept to
a minimum because of the turbulence
created, and climbout should be made
slowly with no abrupt changes in flight
path.
Avoid trying to carry up objects that
are too heavy (weighing more than a few
grams) or objects with considerable area,
such as a large glider. That could cause
stability/control problems.
Simple Glider for the Blade CX: A
template is included with this article. Cut
the glider from a sheet of 2mm Depron
or other lightweight foam material. Use a
1/4-inch-long piece of the 1/16-inchdiameter
tube glued to the top of the
glider for the release pin to pass through.
Balance the glider as shown, and
adjust the glide by bending the outside
tips of the wing upward. Once you are
satisfied with the glide, you are ready to
tow up the aircraft.
To keep the wing from rocking side
to side during the tow, glue a scrap piece
of balsa in place on the boom near the
release mechanism. That will prevent the
glider from moving around.
Other objects will generally hang
under the helicopter with no problems.
You can tape a loop of fishing line to
parachutes, balls, and other such items
for their release. MA
Bill Mixon
[email protected]
Sources:
“The Home of Higher Silly Putty Learning”
www.sillyputty.com
Kevlar thread, carbon tubes, etc.
www.cstsales.com
Useful Silly Putty timer
www.gryffinaero.com/models/ffpages/tips/t
imer/timer.html
Depron foam sheets
www.depronusa.com
05sig2.QXD 3/26/07 9:08 AM Page 50
May 2007 51
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52 MODEL AVIATION
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05sig2.QXD 3/26/07 9:08 AM Page 52