FrED rEEsE died last November. He was one of America’s most
prolific designers of small Radio Control (RC) models. For the past
few years, Fred was an associate of Thunder Tiger in the
development of its line of small RC engines.
In his last communication with me, Fred solved a pesky starting
problem with the smaller Thunder Tigers.
“ … the .07 needs to be wet to start. I prime the engine by
placing a finger over the exhaust outlet on the muffler and turning
the engine over. If using an electric starter, I just cover the exhaust
for a second while spinning, and it will fire instantly.
“I had a terrible time starting the engine when new until I tried this
method. Actually, I could not get it started at all until I primed it this
way, which uses the trapped pressure in the muffler to push fuel into
the engine. Mine will hand-start easily now; it just needs to be wet.”
I suffered the same difficult-starting problem with a Thunder
Tiger .10, but Fred Reese’s priming technique works as well for me
on that engine as it did for his .07.
A few columns back I mentioned the new series of high-efficiency,
long-duration CO2 motors developed by Stefan Gasparin in the
Czech Republic. Stefan has come out with several new sizes—some
with throttles for RC use!
The smallest of these new controllable-speed CO2 motors is the
G28BBRV. (The “BB” stands for “ball bearing”!) It spins a fiveinch
propeller at 2,200 rpm for roughly seven minutes (at full
throttle) before the power drops off noticeably.
At part-throttle settings for RC (or IC) flying, this motor runs for more
than 10 minutes. Yet the entire “power system,” including the propeller
and a full tank of liquid CO2, weighs only a quarter of an ounce!
What size model could I fly with that tiny a motor? you may
wonder. Light weight is the primary criterion, of course. A total
flying weight of one ounce would be roughly the maximum you
could expect good performance from.
Some Peanut-size, and most Embryo Endurance and Bostonian
rubber-powered designs could probably be adapted.
Joe Wagner
T h e E n g i n e S h o p
927 Pine Ave., Ozark AL 36360
The Thunder Tiger .10 engine. A simple tip from Fred Reese
solved a difficult-starting problem with it for the author.
Tiny G28BBRV has a double ball-bearing crankshaft. It’s the
smallest throttleable model-airplane motor the author knows of.
Behind the GMW73T Twin is its throttle; it works like RC engine’s
carb. Spare-parts package is standard for Gasparin motors.
Suited for sport FF models, G160RV has adjustable speed via
serrated wheel atop the head. It’s the lowest-priced new Gasparin.
78 M ODEL AVIATION
(CO2 power has much in common with rubber power. Good
flight performance requires large-diameter, high-pitch propellers
turning slowly. As a rule of thumb, a good monoplane model size
for sport CO2 flying is one with a wingspan between three and four
times the prop diameter.)
The largest (so far) of the new high-efficiency CO2 motors
from Gasparin is a throttleable version of the GMW73T Twin I
wrote about earlier.
Actually, the motor itself is unchanged; a variable throttle
assembly replaces the firewall-mounted “manifold” of the first
model. This type of installation simplifies the speed control linkage.
To briefly recap the performance figures of this motor, it swings
a seven-inch prop at 2,000 rpm for six minutes at full throttle, or a
91⁄2-inch P-30 prop at 1,250 rpm for seven minutes.
A suitable RC model for the GMW73T should weigh seven
ounces or less, with a wing area close to 200 square inches.
Not all of these long-duration CO2 motors have throttles. The
new G160 (which turns a seven-inch prop at more than 1,500 rpm
for three minutes, and weighs 3⁄4 ounce complete with tank and prop)
features an easy-to-adjust manual speed control that’s part of the
head assembly. A knurled adjustment wheel on top makes rpm
adjustment for Free Flight (FF) use quick and easy.
A close look at the G160 shows a significant difference between
this new high-efficiency design and that of older CO2 motors. Its
exhaust ports are much smaller.
This brings up the fact that CO2 motors produce their power
through gas expansion, not pressure. Yes, pressure is supplied to the
motor from its CO2 tank. But because of the unchangeable physical
properties of CO2, the tank pressure is far too high for efficient use
in small piston-type motors.
Only moderate pressure is needed to provide adequate cold gas at
the top of the piston stroke for full-power output of the motor. High
pressure is a handicap, not an advantage.
CO2 motors first came out in 1947. (They were developed
by Bill Brown—the same genius who was responsible for the
first gasoline-powered model engines available to hobbyists a
dozen years earlier.) When the OK CO2 motors arrived in
hobby shops two years after V-J Day, every model flier in
America wanted one. We bought ’em, too!
But we didn’t understand that high supply pressure was
counterproductive; it was actually harmful to CO2 motors. I ruined four
of my early motors—three OKs and a lovely little Campus A-100 that
spun a 31⁄2-inch prop—by mistakenly boosting their charging pressure
through preheating the CO2 capsules. (Many of us did that!)
Moderate pressure does the job nicely, and more is only harmful.
An analogy is driving brads. If a six-ounce tackhammer works well
for that, would a 12-pound sledgehammer do better?
Notice in the accompanying photos that all the Gasparin
motors employ quite small-diameter tubing for supplying CO2 to
the motor heads.
The flow restriction from the minuscule inside area of this
tubing acts as a pressure reducer. The piston-actuated valve in the
head is designed to lower the incoming gas pressure even further.
The result is longer motor runs, higher propulsion efficiency, and
lengthened motor life.
In spite of its advantages over electric and rubber power for
“quiet-flight” model airplanes, CO2 isn’t nearly as popular yet. That
is partly because of “refueling” considerations.
For decades, the standard source for model-flying CO2 has been
“capsules.” The original “Sparkletts” capsule size holds eight grams
of CO2. That provided power for a couple or three recharges of a
Telco motor; perhaps as many as four or five for one of the
exquisitely-made Brown A-23s.
Nowadays, 12-gram CO2 capsules are available from Wal-
Mart™, priced between 50 and 60 cents each (depending on how
many are in the package).
However, the increasing popularity of paintball-gun shooting has
brought forth a new option for CO2 motor tank refilling.
Wal-Mart™ sells a nine-ounce refillable CO2 paintball tank
for approximately $25, and I recently bought a 16-ounce tank
from a paintball supply dealer. That cost me $40, plus another
$3 to have it filled with liquid CO2. That’s enough for roughly
30 recharges of the biggest-capacity Gasparin motor tanks I
have. Inexpensive CO2 refills are available at paintball centers
and most welding-supply dealers.
Now for the proper technique of recharging CO2 motor tanks!
Coolness is particularly important—especially for summer
flying. That’s because CO2 cannot exist as a liquid above 87°, no
matter how high the pressure.
At 68°, approximately 800 pounds per square inch (gage) inside
a CO2 tank is enough to keep most of the contents fluid. Once the
temperature exceeds 87°, the liquid CO2 converts to gas and the
internal pressure jumps to more than 2,000 psig.
That’s why experienced CO2 fliers take precautions against their
tanks getting warm. They don’t refrigerate the tanks; just keeping
them out of the sun is usually all that’s needed. On a hot summer
day, though, carrying your “bulk CO2 tank” in a cooler with a little
ice might be wise.
All Gasparin CO2 motors come with complete instruction
booklets that describe the filling procedures in detail, and include
much more information I won’t repeat here.
However, I do want to mention one very important item in
recharging a CO2 motor’s tank. Fritz Mueller, Atlanta’s CO2 power
guru and an adviser of Stefan Gasparin’s, spelled this out in a Flying
Models article a year ago.
When you attach the propeller to the motor, position it so one
blade points directly away from the cylinder head with the
piston at the bottom of its stroke. Mark this “bottom blade” with
a dot of paint.
Whenever you fill the motor’s tank, do it with the marked blade
downward. That’s to make certain the motor’s exhaust ports are
open during filling. Otherwise, the high charging pressure can pass
through the head valve (which closes from flow, not pressure) and
damage the tiny O-ring on the piston.
These new Gasparin CO2 motors are available from the
Blacksheep model club (in the Los Angeles area). They were major
contributors to the development of these new products, and they’ve
made arrangements with Stefan Gasparin to sell these motors as a
fund-raising program.
The Blacksheep are far from an ordinary model-airplane club! They’re
especially active in promoting model airplanes among youngsters. The
Blacksheep stage low-key model contests (primarily FF and Control Line)
that solicit and encourage youthful participation.
All the Gasparin motors may be ordered from the Blacksheep
treasurer: Roy Hanson Jr., 21410 Nashville St., Chatsworth CA 91311;
Tel.: (818) 718-1685. (Leave a message.)
They also have the special 2mm charging valve that is necessary
to adapt a paintball gun tank for use in CO2 motor recharging, and a
charger for the smaller Gasparins that use 12-gram CO2 capsules.
Besides that, the Blacksheep can also supply propellers and spare
motor tanks, in various sizes all the way up to 25cc.
They’re a great group; they do a lot for model aviation in
Southern California, and they deserve our support.
Besides, flying with CO2 power is fun! MA
March 2001 79
“Bulk CO2” paintball tanks adapted for recharging Gasparins. The
16-ounce tank has 2mm charging nozzle installed, at lower left.
