Free Flight Indoor - 2004/05

142 MODEL AVIATION
TO START WITH, I have an E-mail
address change for you. Shortly after I
submitted the most recent column, I had to
change E-mail addresses. Please send any
E-mail to me at [email protected].
Doig Scale: In a recent column I included
a photo of a spring scale that the late Rich
Doig built. A reader asked how such an
instrument could be constructed.
Spring scales have three major
components: base, spring element, and
scale. Rich had access to a full machine
shop, but you can use hard balsa for the
base and the backplate that holds the scale.
Draw the scale on white bond paper
glued to the backplate. If you cut the paper
in an arc that has a radius equal to the
radius from the wire support to the edge of
the backplate, the scale will be linear.
You must make the spring element
from good-quality music wire. You will
use the smaller sizes for most scale ranges.
No matter which wire size you use, you
must determine the deflection constant.
The wire size and the length from the
support to the hook affect this constant.
You must also make a calibration
weight. The easy way to calibrate a weight
is with a precision electronic scale, or
someone you know may have an electronic
scale or a triple-beam balance; either will
be able to check the weight to at least 1%
accuracy.
Make the weight equal to the desired
full range of the scale. Enamel-coated
magnet wires of various sizes are useful
materials for the calibration weight. A
major advantage of enamel-coated wire is
that it won’t change weight because of
corrosion and won’t absorb moisture or
most other contaminants.
Use a large wire size as a mandrel, and
wind an excess amount of a much smaller
wire size on it. Check the weight, and trim
off little bits of the small wire until you
reach the correct weight. Secure the end of
the small wire with a tiny drop of very thin
cyanoacrylate glue.
Bend a hook in one end of the wire.
Mount the wire so that it crosses the top of
the scale. Rotate the wire so that the hook
is facing up. The wire should be long
enough that it can be moved to adjust the
length as described in the following.
Hang the calibration weight on the
hook, and adjust the wire length so it
intersects the bottom of the arc. Tighten
the wire mount and check to see that the
wire intersects the top edge of the scale
without the weight and the bottom edge of
the scale with the calibration weight
added. Mark the top and bottom ends of
the scale using fine lines drawn with black
ink.
Now that the endpoints of the scale
have been located, create more scale points
on the scale using drafting dividers. Start
from the top and bottom ends of the scale.
Use trial and error to locate the center
point of the scale, and mark it. Show those
three points with long tick marks. Repeat
Bud Tenny, Box 830545, Richardson TX 75083
FREE FLIGHT INDOOR
Stan Chilton with finely crafted rise-off-ground Stick model.
P-24 winners at recent USIC (L-R): Joe Kehr, the late Jim Clem
(who won P-24 most years), John Sagan, an unknown flier.
Rachel Petty (Smyrna GA) with handdecorated
Bostonian at USIC.
this operation until the scale is divided
into 16 sections. You can indicate the next
divisions—one-eighth, three-eighths, fiveeighths,
and seven-eighths—with shorter
tick marks to make a more readable scale.
If you have worked carefully with a
0.1-ounce calibration weight, you will be
able to resolve 0.00625 ounce. This will be
about the best you can do using a spring
scale. This is more than adequate for
checking rubber motor weights at the
flying field.
Flight Trim: The following paraphrased
comments are from the Bong Eagles
newsletter, Tales of the Eagles. The
discussion was about flight trim on
Outdoor Rubber models, but this
commentary can also be applied to
trimming Indoor Rubber models.
If the CG is too near the leading edge,
the model will take too much up-trim and
will tend to loop. It will hang on the
propeller in the climb and will have a
slow, mushing glide. One solution is to
move the wing forward to make the model
more tail-heavy (rearward CG). The model
will have a fast climb, and the power run
will end with a flat cruise (no climb).
We can’t move the wing since we don’t
strap Indoor models’ wings on with rubber
bands. The solution is to locate the wing as
close to the propeller as possible; you can
accomplish this one of two ways. The first
is to build the tailboom and tail surfaces as
light as possible so that the CG moves
forward.
May 2004 143
Bud Tenny and Bill Hulbert with Bud’s second-place P-24. Bud Tenny before cancer and before gray hair.
John Sagan with a Hand-Launched Stick rubber model.
Indoor Contest Board member Walt Van Gorder prepares for a
flight at a recent USIC. All photos by Dave Linstrum.
The second approach is to make the
motorstick as long as possible (the rules
for Pennyplane and Limited Pennyplane
limit the total model length), and then
build the horizontal stabilizer as large as
possible (up to 50% of the wing area).
This allows the CG to be located farther
back. Now the stabilizer’s lift will
contribute to the total lift supporting the
model in flight.
Excerpted from another FF club
newsletter—the Willamette Modelers
Club Inc.’s (in Oregon) WNC Patter—are
some tips by Bob Eberle about Catapult
Gliders. He wrote this piece for
publication in the long-gone New York
Indoor Times, edited by Ed Whitten.
144 MODEL AVIATION
“With two Indoor Catapult Glider
classes, you guys can’t use the excuse that
your arm isn’t any good anymore! All you
need is a short loop of rubber attached to a
handle (consult the AMA rule book for
specifications). Hook the rubber to the
model, pull back, and let go!
“In the workshop, begin by using a
long straightedge to be sure you have no
decalage (incidence angle difference
between wing and stabilizer). If the
decalage isn’t zero, rebuild the glider until
it is. With positive incidence in the
stabilizer (leading edge high), the glider
will dive in and possibly break. Negative
incidence in the stabilizer will make the
glider loop uncontrollably.
“Examine the glider from the front and
verify that you have stabilizer tilt to cause
the turn you want. Left tilt (right stabilizer
tip low) gives left turn; opposite for right
turn.
[From personal experience, a righthanded
flier—Catapult in your left hand
and glider in your right hand—will be
more comfortable setting up for a left turn
in the glide. The glider will probably need
to be launched while banked to the right.
The resulting launch pattern should be a
slight right turn sweeping into the left
glide turn. The glider must be launched so
this S-turn pattern will miss the building
structure, and the launch must have
enough energy to reach to just below the
ceiling.]
“For the first test flights, test glide the
model. Launch the model straight ahead
with it banked in the direction of the glide
turn. If it dives into the turn, the airplane
is trying to turn so tightly that it is trying
to roll. First, check the stabilizer to be sure
it isn’t warped or has too much tilt. If
neither is true, check to see if the wing is
warped or cocked (mounted on the
fuselage not perpendicular to the fuselage
centerline).
“If it continues to dive into the turn,
add a small amount of weight on the
wingtip opposite the turn. Once the model
is gliding properly, launch it straight ahead
with about one-third power from the
catapult, keeping the nose level. It should
come off the catapult and start into a climb
while assuming its own bank, then slow
and make a smooth transition into the
glide.
“If it loops or dives on a low-power
launch, check the decalage again. If it rolls
too fast into the turn on the low-power
launch, launch with a bank away from the
turn. Continue this process until the launch
is perfect. Repeat this routine with onehalf
and three-quarter power launches. As
the launch power increases, launch with
the nose pointed higher and higher. A fullpower
launch may well be almost
vertical.”
High-Power Winding: No matter what
kind of rubber you use, there is a way to
get significantly more turns into a motor.
(The motor you use must be clean and free
from mechanical damage.)
The following winding procedure
requires a torque meter to guide you as
you wind. The torque meter should have a
hook that holds one end of the motor, and
it should be clearly visible as you apply
turns.
Hook the motor between the torque
meter and the winder. Stretch the motor to
three or four times the normal loop length.
(Measure the length by sliding a short
piece of a large drinking straw over the
motor so that the loop is squeezed down to
a minuscule size.) Lay the motor over a
ruler that has a pin to hook the knot end
over. The length is the distance from the
pin to the end of the loop.
As you wind, the torque reading will
rise slowly until it reaches a plateau that
remains almost constant for some time.
This plateau corresponds to the cruise
portion of the flight. If you happen to
know what level of torque the model needs
to fly level, you can tell if this motor will
be a match for it. Choose a different motor
if necessary.
As you continue to wind, the torque
reading will begin to rise rapidly. Reduce
the stretch slightly and watch the torque
meter. If the reading drops and then rises
as you pull back, continue to wind slowly.
Repeat this test often while continuing to
wind slowly. When the torque reading
doesn’t drop as you test, move in until it
does drop, and then resume winding. At
some point the torque reading will begin
to rise rapidly. Stop winding! The motor
will break if you continue.
Also, stop winding when you have
come in to where the motor length equals
the distance between the model’s propeller
hook and rear hook. It is helpful if you
have a stand that holds the winder that
same distance from the torque meter. At
that time you can massage the knots in the
motor. This will cause the torque reading
to drop. If that torque will cause the model
to climb into the ceiling, back off turns
until a better torque level is reached, and
massage the knots again. MA