Old-Timers - 2008/04

162 MODEL AVIATION
BUILDING A MODEL from a kit or
scratch-building from a set of plans has its
own unique rewards. While reporting to no
one, you single-handedly become the
management consultant for feasibility studies,
the board of directors for project approval,
and the comptroller for funding. You are also
project manager, chief engineer, qualitycontrol
specialist, director of safety,
production superintendent, shop boss, skilled
laborer, etc., and—finally—test pilot!
Not many of us get to do that within the
confines of our workaday world. And it’s an
opportunity to learn and master mechanical
skills that are often missing in the many desktype
jobs held today.
The photos represent some of the visual
appeal of a nice balsa-wood, built-up model.
Oddly enough, those who construct some of
the smaller flying models seem to take more
pride in craftsmanship than those of us who
favor larger aircraft that are built primarily
for competition.
At the 2005 SAM [Society of Antique
Modelers] Champs, one Concours model was
a miniature Valkyrie with the framework
completed but no covering. All the 1,000-plus
pieces of the wing
were on clear
display, along with
the rest of the
intricate built-up
balsa structure.
Most who saw the
Valkyrie agreed with
the builder’s plan
that this particular
model should never
be covered. Many
have described what
we do as an art form.
The Forster .99
engine, which the
Forster brothers
produced, was the
mainstream
“monster” of its day.
The top-displacement
Building model aircraft is an art form
Old-Timers Bob Angel | [email protected]
Also included in this column:
• The Forster brothers’
monster .99
• Spark-ignition start-up
• Murphy’s dilemma
Charlie Bruce’s FF Valkyrie takes flight. The Carl Goldberg-designed wing reportedly
consists of more than 1,000 parts, and each rib is a built-up assembly.
Luther Peters’ and Eut Tileston’s finely crafted small-scale models at the Southwest Regionals event
sometime ago. Photo by Gerald Martin.
COIL B -
A
T.
+
SWITCH
CONDENSER
ENGINE GROUND
INSULATED POINT
WIRING DIAGRAM
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engines in common use were usually
limited to .60 or .65 cu. in. and were
referred to as the “big” .60s. Forster .99s
were the best known of the bigger engines,
but not even they were used in large
numbers.
Many .99s were produced with dual
points, for two-speed running. They had
lots of torque but revved slowly, usually
running on big propellers of 14 inches and
larger diameter.
Sal Taibi, a FF legend, and a few other
modern practitioners have used the big
engine with success in Texaco competition.
And you’ve got to love that sound (best
heard through ear protection).
A friend recently presented me with a
.99 he bought as a young man. I decided to
bench-run it before confining it indoors in
my collection. The procedure was slightly
different. The original prop nut had been
replaced with a friction nut, and I soon
found out why.
The .99 uses almost the same size drive
and propeller washers as a smaller engine.
But with the large propeller and so much
torque, it’s hard to keep the propeller from
slipping. Without that locknut you’d spend
a considerable amount of time on your
hands and knees searching for prop nuts
and washers.
The slippage was cured when I cut a
couple washers from thin leather and used
them on both sides of the propeller. And I
found that the timer needs to be retarded
more than for most engines for hand
starting. Ouch!
In the previous column I promised to share
some basics of starting and running an old
spark-ignition engine. This time, armed
with the wiring diagram, you should have
enough information to at least fire up an old
sparker on the test stand.
Other than adjusting the timer,
procedures are almost the same as for
running a glow engine. I described engine
cleanup before running in the February
issue. I’ll repeat one precaution from that
column and add a couple.
Don’t melt one of those original plastic
fuel tanks by putting glow fuel in it. If you
do run glow fuel rather than gasoline, keep
it mild, at 5% or less nitromethane (nitro).
FAI fuel (no nitro) is preferable and safest
to keep that old jewel intact.
You need to hand-start these engines;
never use a starter. Wear a thick leather
glove or use a chicken stick. Aside from a
few racing engines such as Hornets,
McCoys, and the like, most had no thrustabsorbing
bearings. Pushing a starter into
them could press the crankpin and/or
conrod back against the backplate, grinding
things up inside.
However, even when there is enough
internal clearance, the drive washer can gall
the front of the crankcase and produce metal
grindings in an oil slurry that can spread and
do nothing worthwhile to an engine.
A 3:1 mixture of gasoline and 70-weight
(SAE 70) oil was the standard fuel
April 2008 163
recommended for most ignition engines.
You can still find that 70W oil, or at least
SAE 60W, at some Harley-Davidson
motorcycle shops and maybe at some
airports.
Gasoline helps start an engine more
easily than alcohol-based fuel does,
especially in cold weather. Some of us
prime with gas for easy starting even when
we’re running methanol. The glow-fuel
consumption rate is almost twice that of
gasoline, so a richer needle setting is needed
for glow.
Plain, old Coleman or other camp-stove
fuel is the best gasoline base. High octane
has no particular advantage in small
engines, and some modern pump gasolines
have allowed the oil to separate from the
mix. If you choose to use commercial lowor
no-nitro glow fuel, add some degummed
castor oil (such as Sig brand) for engine
protection.
Many of us commonly use a 3:1 mix of
pure methanol and castor oil. Methanol
usually lets an engine run slightly cooler
and a bit faster than the gasoline fuels.
Motorcycle shops stock another oil that is
okay for our use; it’s polymerized castor oil,
which blends with either gasoline or
alcohol. Maxima is one brand name.
Let’s get back to that wiring diagram. A
couple of fresh, strong, dry cells, at least C
size, is the minimum battery requirement.
Used in series, they produce the 3 volts for
the nominal rating of the spark coil. Two or
preferably three AA or larger Ni-Cds are
better and should allow easier starting. Most
coils can handle as many as 4 volts without
damage.
The capacitor (or condenser) is shown in
a location to clarify the diagram, but it’s
usually attached to an engine lug for ground
and mounted to that lug or behind the engine.
The capacitor value is noncritical and, as I
remember, the manufacturers usually
specified .05-.10 mfd. (microfarad). Actually,
a small automotive condenser can be used.
Let’s set up the timer to run. An
ohmmeter or a simple series-wired continuity
light is handy for testing and adjusting the
timer.
Make sure the points are opening and
closing properly by attaching the testers’
clips to the insulated point and to the engine
ground. Turn the propeller through a few
revolutions, watching for the test light to go
on and off or the meter to react.
You’ll often be surprised to find that the
points are either a little dirty or aren’t
making and breaking properly. If so, clear
up that problem.
Now we’ll adjust the timer arm for hand
starting. If you old pros will bear with me,
I’ll review a couple of basics.
The coil fires when the points open—
not when they close. So we’re concerned
with exactly when that point opening
occurs.
There’s a time lag between the points
opening, the spark jumping, and the fuel
mixture firing and building cylinder
pressure. That requires the points to open
sometime before the piston reaches top dead
center (TDC) and is called “spark advance.”
For easy hand starting, that will usually be
10°-15° of propeller rotation before the
piston reaches TDC.
You can estimate this by watching the
propeller. Move the piston to TDC, note the
propeller position, and then rotate the
propeller backward until the test light or
meter tells you the points have just closed.
Adjust the timer until this exercise gives
you the necessary 10°-15° of advance.
Once you get the engine running, you
can advance the timing by moving the timer
into the direction of rotation. Then you can
lean the needle valve down.
As with any other engine, you want to be
conservative and keep the needle a tad on
the rich side—slightly below absolute top
speed. Treat the timer adjustment the same
way. Keep the spark advance below
maximum speed.
One of the most common reasons for inair
flameouts in these engines is overheating
and seizure from overadvancing the spark
and/or overleaning the needle. Gasoline fuel
runs hotter and is more susceptible to that
problem than glow fuel.
Addendum to Murphy’s Law: Let’s call
this “Murphy’s dilemma.” When you find a
useful new modeling material, you must
decide on one of two choices.
1) Buy the quantity you need for the
project at hand. Then when you need more
for next month’s project, you’ll learn that
the material has been discontinued.
2) Buy a lifetime supply to preclude the
possibility of ever running out. The following
week you’ll discover a new and better
material at half the price. MA
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