Control Line Aerobatics - 2009/03

Also included in this column:
• Wingtips
• Control-system installation
IN THE LAST CL Aerobatics column, I
wrote about skinning the Project Hole Shot
foam wing cores. The wing skins should be
well cured by now, and it’s time to remove
the weights and sand smooth the skins and
the joint lines between the LE cap pieces
and the skins.
Sanding is perhaps the most important
building skill and one of the most difficult
to master. The proper tools are a must, and I
covered this subject in a previous column
about this project.
To recap, purchase a few of the Great
Planes extruded-aluminum Easy-Touch
Bar Sanders. They come in several
lengths. I recommend the 12-inch-long
bars for most work, but I also suggest that
you obtain at least one sanding bar that is
long enough to span a complete wing half.
This will allow you to sand an entire skin
at one time and level the surface.
I start sanding the skins using a bar
that is fitted with 100-grit, no-load paper. I
try to remove any seams and level the skin.
If there is any excess glue on the
skin’s surface, sand only in one direction to
remove the adhesive. If you sand in a backand-
forth motion, you will risk pulling a
piece of hard, dried glue
back under the sanding
bar and severely
scratching the skin’s
surface.
After you have
removed any major
surface inconsistencies
using the 100-grit
sandpaper, switch to a
long sanding bar fitted
with 320-grit paper.
Work slowly and remove
all the scratches that the
100-grit sandpaper left.
It is very important to
check your work often
while you are sanding.
Do this by “candling” the
surface of the wing.
Hold the wing up to
an incandescent bulb and
bounce the light off of its
surface as you critically
check the entire surface
for even the slightest imperfection. Work carefully and remember
that if the surface is not flat and smooth, the result will show up in
the final finish.
Wingtips: The Hole Shot’s wingtips are made from 1/8 lightplywood
(poplar plywood). Place the end of each wing half on the
sheet of material and trace around the airfoil.
Cut out the tips using a scroll saw. Cut just outside the line, to
March 2009 133
allow some material for sanding when the
tip is installed. Draw a centerline on the
inside face of each of the tip plates using a
pencil.
This model’s tip weight bolts to the
outside of the tip plate. To anchor the tip
weight, a 4-40 blind nut must be installed on
the inside face of the tip plate before it is
glued to the outer wing half.
Refer to the plans for the position of the
blind nut. It is typically mounted vertically
on the centerline of the tip and positioned
chordwise at the CG location.
The inside wingtip needs to be slotted to
accept the adjustable leadout-guide slider. I
used a Brodak leadout-guide slider in my
Hole Shot (part BH 760). Refer to the plans
for the slot’s position. Install the slider on
the inside of the tip, and clearance the foam
in the wing to allow the slider to move
through its path of travel without hindrance.
Install the outside tip to the wing using
aliphatic resin glue, and secure it with strips
of masking tape until the adhesive dries.
Sand the tip plate to blend with the wing
sheeting.
Note that the inside tip is not installed at
this time; it will be glued on after the control
system has been installed so that the leadouts
can be slid through the leadout-guide slider.
Control-System Installation: A few years
ago, this part of the process would have
required a whole lot more explanation. In
the past, control systems were installed
using bent wire pushrods that fit into the
output arm of the bellcrank, and the entire
system had to be adjusted carefully to
ensure smooth movement throughout its
travel, with no binding. That has changed in
recent times.
Today, we have ball-link control systems
available that require little “fiddling.” In the
project Hole Shot model, I used the Tom
Morris 4-inch Permalube linen-base
phenolic bellcrank and Du-Bro 4-40 ball
links. The bellcrank comes complete with a
mounting kit that is perfect for this
application. The bellcrank mounts to a 1/8-
inch-diameter wire “post.”
To begin the process, insert the 1/8-inch
wire post into the bellcrank. Position two
washers on the post—one on either side of
Viewfinder
Northern Splash
I built the model to have something
different within my club. It is the second
airplane I have built from a kit. It is a
North Star from a Balsa USA kit. It has an
O.S. 46AX with a Macs muffler for power.
The picture was taken by my friend
Jeremy Howard of www.delriophoto.com.
We were flying off of Lake San Antonio in
California. It was the first weekend out
with the model.
The North Star took off and needed
very little trim for straight and level flight.
I was so happy with the way the airplane
flew, I opened up the throttle and left it
there flight after flight. The model is fast,
rock solid, and a whole lot of fun to build
and fly. MA
—Chris Thomsen
[email protected]
E-mail your high-resolution
“Viewfinder” photo and a short note
telling the airplane or helicopter story to
[email protected].
AMA Academy of Model
Aeronautics
ARF Almost Ready to Fly
BEC Battery Eliminator Circuit
CAD computer-aided design
cc cubic centimeter
CD contest director or
compact disc
CG center of gravity
CL Control Line
cm centimeter
cu. in. cubic inch
DT dethermalizer
EPP (foam) expanded polypropylene
ESC Electronic Speed Control
FAI Fédération Aéronautique
Internationale
FCC Federal Communications
Commission
FF Free Flight
GHz gigahertz
Kv rpm/volt
kV killivolt (1,000 volts)
LCD Liquid Crystal Display
LE leading edge
LED light-emitting diode
Li-Poly Lithium Polymer
mA milliamperes
MA Model Aviation
mAh milliampere-hours
MHz megahertz
mm millimeter
Nats AMA Nationals
Ni-Cd Nickel Cadmium
NiMH Nickel Metal Hydride
RC Radio Control
rpm revolutions per minute
RTF Ready to Fly
SASE self-addressed, stamped
envelope
SIG Special Interest Group
TE trailing edge
ModelAviation’s
Frequently Used
Abbreviations /Acronyms
03sig5.QXD 1/26/09 12:04 PM Page 133
the bellcrank.
The bellcrank will be held in place on the
post by two 1/8-inch wheel collars, again, on
either side of the bellcrank. The idea is to
slide the bellcrank to the desired position on
the post and then tighten the screw in each of
the wheel collars to anchor the bellcrank in
place.
I have a couple of tips. It is never a good
idea to rely on just the setscrews in the
wheel collars to hold the bellcrank
permanently in place on the round wire post.
Vibration can loosen the screws and allow
the bellcrank to move. That would be
catastrophic in this application (probably in
any CL application).
I suggest that you grind or file an
extremely shallow flat on the wire, to
allow the setscrew to seat better than it
would against a round surface. You should
also use a drop of Loctite on the setscrews
and apply a fillet of J.B. Weld on the
wheel collar/wire post intersection after
you are certain that you are happy with the
bellcrank’s position.
Tom’s bellcranks can be ordered with
mil-spec braided (cable) leadout wire
already attached, and I recommend
purchasing them this way. Where the
leadouts go through their respective holes,
Tom uses a piece of annealed brass tubing
slid over the wire and bent into a “U” shape
to bush the bellcrank arms.
Braided leadouts have a surface that will
act like a saw if not bushed in this manner,
and the leadout will eventually cut through
the bellcrank arm. If you opt to install your
own braided leadouts, do not omit the brass
tubing bushings.
Some modelers prefer solid-wire
leadouts, and they can be installed in the
bellcrank arms without the need for
bushings, because their surface is smooth
and the phenolic material that the bellcrank
is made from provides an excellent bushing
for the solid wire.
The pushrod is attached to the bellcrank
via a Du-Bro 4-40 Heavy Duty Ball Link
(part 760). I prefer to provide a “standoff”
on which to mount the ball link. Sometimes
the leadouts will hit the pushrod when the
bellcrank is moved through its arc. The
standoff will help prevent this from
occurring. These standoffs are also available
from Tom Morris.
Run the 4-40 bolt through the ball link
and on through the standoff and into a
tapped 4-40 hole in the bellcrank output arm.
The bolt should extend through the bottom
of the bellcrank hole far enough to allow a 4-
40 washer and nut to be attached. After
firmly tightening the bolt, apply a fillet of
J.B. Weld to the nut and the end of the bolt
to prevent it from ever coming loose.
There are two options for a pushrod. You
can use either 3/32-inch-diameter music wire
or 3/16-inch-diameter carbon tubes (also
available from Tom Morris).
If you opt for the carbon-tube method,
you can get an entire kit from Tom Morris
that includes threaded ends that fit perfectly
into the tubes. They are secured in the tubes
with J.B. Weld.
If you decide on the 3/32 wire pushrod (as
I did with Project Hole Shot), you will need
to use a Du-Bro 4-40 threaded coupler that
accepts 3/32 wire (part 336). The coupler is
soldered to the end of the wire. Be sure to
sand and clean the wire so that a good solder
joint will result.
The pushrod post will be positioned in
half-round grooves that you will have to file
or grind in the root face of the plywood fulldepth
spars in the foam wing halves.
Position the bellcrank assembly, without the
pushrod attached, in the outside wing half
and check for any interference as the
bellcrank swings through its full travel.
If any part of the assembly hits the main
spar, remove the assembly and clearance the
spar. Continue this procedure until there is
no chance of interference with the
bellcrank’s movement.
Attach the pushrod to the bellcrank and
hold the assembly in place against the
outside wing, and move the bellcrank
through its full travel. Make tick marks on
the outer surface of the wing where it will
need to be clearanced to allow the pushrod to
exit.
I opt to use a 3/32 wire pushrod on profile
models so I can make a shallow bend in the
wire to get it out of the wing in a convenient
place. This is impossible with the carbon
pushrod. On larger, full-bodied models, I
would opt for the carbon pushrod.
The joiner spar will be glued and slid into
the outer wing half as the bellcrank/pushrod
assembly is installed. (See photo.) Glue the
post into the half-round groove in the outer
main spar.
You can use medium-viscosity
cyanoacrylate glue for this, but be careful
not to let any of the adhesive get on the foam
that is on either side of the spar; it will melt
the foam. A dab of five-minute epoxy is
probably a better bet here.
Set and hold the bellcrank assembly in
perfect alignment until the epoxy cures.
The last operation for this month is joining
the wing halves. This is a simple procedure.
If the wing cores were cut properly, the
lower cradle halves will be a perfect
alignment fixture.
Test-fit the wing halves together with
them placed accurately in the lower cradle
halves. When satisfied with the fit, apply a
coat of epoxy to the front face of the joiner
spar and a coat of aliphatic resin to the balsacovered
foam faces on one of the wing
halves. Slide the wing together.
Wipe off any excess glue that seeps out,
double-check the alignment, and weight the
assembly down until the glue dries.
Until next time, fly Stunt. MA
Sources:
Great Planes Model Manufacturing
Company
(800) 637-7660
www.greatplanes.com
Brodak Manufacturing
(724) 966-2726
www.brodak.com
Tom Morris
(256) 820-1983
[email protected]
Du-Bro Products, Inc.
(800) 848-9411
www.dubro.com