The Engine Shop 2003/03

IN OCTOBER I attended the first West Coast Park Flyer Fun-
Fly, sponsored by the Blacksheep club, at Robert Gross Park in
Burbank, California. Naturally, electric-powered airplanes
predominated, but a few CO2 Radio Control (RC) models were
present—and their performance graphically demonstrated to
everyone there the advantages of CO2 power over electric.
CO2 motors run just as quietly as electric, but their power
systems weigh far less, and they can be recharged in a few
seconds from carry-anywhere CO2 “bulk reservoirs” (such as
flashlight-sized CO2 cylinders for paintball guns).
As for in-flight motor speed control, many of Stefan
Gasparin’s latest line of Czech-made CO2 motors include RC
throttles! Stefan Gasparin’s creativity amazes me. He never stops
his quest for improvement in CO2 power. One example is the new
G-2.6. It’s externally the same size as the earlier microminiature
G-1S (which suffered from piston O-ring troubles).
The new G-2.6 spins the same 2- and 2.5-inch-diameter
propellers as the G-1S, at approximately 3,000 rpm for four to
five minutes, and its total power unit weight—motor, tank, and
propeller—is only 1 gram!
Yes, this microscopic model motor can actually be used for
powering indoor RC models. I watched two of Henry Pasquet’s
marvelous ultraminiatures in flight at the Little Rock annual
SMALL meet last June. Their performance seemed to impress
everyone, yet there was nothing extraordinary about the models
themselves. Anyone who can build and fly a small stick-type,
rubber-powered indoor model could construct an indoor RC craft
with Gasparin G-2.6 propulsion.
However, the latest and greatest of Stefan’s CO2 power plants
is his three-cylinder G500TS. This is the 49th Gasparin CO2
motor design, and it’s by far the biggest yet. It turns a 91⁄2-inchdiameter
propeller at 3,000 rpm (recommended maximum),
weighs 102 grams (3.6 ounces) complete with its CO2 tank
assembly and propeller, and is equipped with an RC throttle.
As with any CO2 motor, the running duration of the G500TS
depends on the throttle setting and how much liquid CO2 the
main tank is filled with. Prechilling that tank (with ice or an
electronic cooling spray such as RadioShack’s product number
64-4321) maximizes its holding capacity. (The small secondary
tank is not filled directly. It acts as a “gasifier” to prevent liquid
CO2 from entering the motor.)
Newcomers to CO2 power often make the mistake of warming,
or even heating, the motor tank. The idea is to increase the tank
pressure, but that’s wrong; it’s actually counterproductive
because CO2 motors produce their power through gas expansion,
and not from mere pressure.
To explain the difference, steam engines are pressure driven.
In those engines, steam pressure pushes a piston from one end of
its stroke to the other. Then a valve arrangement connects steam
pressure to the opposite side of the piston, at the same time
opening an “exhaust port” in the other end of the cylinder to let
the steam pressure within it escape.
It’s important to note that the exhaust steam is still under
appreciable pressure when the exhaust valve opens. That’s what
made double- and triple-expansion steam engines common on
old-time steamships and railroad locomotives.
Extra engine power and efficiency came from releasing the
first cylinder’s still-pressurized exhaust steam into the valving of
a second piston-and-cylinder assembly, then the exhaust steam
pressure from that could be used to power a third cylinder-piston
system.
March 2003 79
Joe Wagner
T h e E n g i n e S h o p
212 S. Pine Ave., Ozark AL 36360
Half the weight of a dime, complete with CO2 tank and propeller,
the Gasparin G-2.6 is a genuine power provider for models.
The Gasparin G500TS on a test mount. The complex-looking
“plumbing” allows installation flexibility in a model.
Stefan Gasparin’s largest and smallest production CO2 motors.
Note RC throttle on big motor, special charger for tiny one.
03sig3.QXD 12.20.02 8:17 am Page 79
However, CO2 motors don’t work that way. Their cylinders
are supplied with high-pressure gas, all right—but only briefly,
at the very top of the piston’s stroke. As soon as that piston starts
moving, the inlet valve closes and the CO2 inflow shuts off.
From then on, gas expansion provides the power that moves
the piston. Ideally, when the CO2 motor’s exhaust ports open
(which they do at the bottom of the stroke, just like those of a
two-stroke internal-combustion engine), the gas in the cylinder
has expanded almost to atmospheric pressure. (That’s why CO2
motors run so quietly; very little energy is released through the
exhaust ports and converted into noise.)
Because CO2 motors do their work via gas expansion, the fins
on their cylinders are there to act as warming fins, and here’s
why. As anyone knows who has felt the chilly outflow from a
compressed air gun, whenever a pressurized gas is released
through an orifice, it absorbs heat as it expands. To maximize
that expansion, more heat needs to be added from some external
source. And in CO2 motors, that source is the warmth of the
surrounding air.
Stefan Gasparin’s new three-cylinder G500TS motor seems
especially efficient in transferring atmospheric heat to its
cylinder. For one thing, its crankcase and finely finned cylinders
provide ample surface area exposed to the propeller slipstream.
For another, the CO2 supply to the G500TS cylinders passes
Less-than-gallon quantities of model diesel fuel (L-R): Larry
Renger’s “water bottle” fuel container, Aerodyne’s, Davis’s.
As shown here, custom-modified engine mounts to accurately fit
offbeat model engines can be easily made.
80 MODEL AVIATION
03sig3.QXD 12.20.02 8:18 am Page 80
through a plenum chamber in the rear of
the case. This prewarms the pressurized
gas and acts as further protection against
liquid CO2 entering the motor’s cylinders.
To sum up the principle involved here,
CO2 power systems for model airplanes
need to keep their tanks cold (to
maximize their liquid CO2 capacity) and
the rest of the system warm (to maximize
power output and efficiency).
Returning to the Gasparin G500TS
performance, the motor I have is Stefan’s
first prototype. I don’t know the price yet,
and the CO2 tank system size and
arrangement isn’t necessarily optimum.
As it’s set up now, and with the 91⁄2 x
91⁄2-inch laminated propeller that came
with it, the motor runs at 2,700 rpm
maximum, gradually declining to 1,000
rpm after roughly six minutes. (That was
with a prechilled main tank and wideopen
RC throttle.)
Based on the performance of earlier
Gasparin motors, I’d guesstimate that the
G500TS will adequately power a threechannel
RC model with somewhere near
a 40-inch wingspan, approximately 250
square inches of wing area, and weighing
(ready to fly) perhaps as much as 22
ounces.
Those are the specifications of the
semiscale airplane model I’m designing
for this three-cylinder marvel of a CO2
motor. (It’s based on a pre-World War II
parasol-type sport monoplane designed in
Czechoslovakia. I thought it would make
an appropriate project for the Gasparin
three-cylinder radial.)
Gasparin motors and accessories are
available from the Blacksheep club.
Contact Roy Hanson (club treasurer) at
21410 Nashville St., Chatsworth CA
91311; Tel.: (818) 718-1685. (Leave a
message.)
In a couple of recent Engine Shop
columns I’ve described do-it-yourself
diesel-fuel blending using ingredients
available at Wal-Mart and John Deere
dealers. Since then I’ve been reminded by
Bob Davis (Davis Model Products, Box
141, Milford CT 06460; Tel.: [203] 877-
1670) and Allen Heinrich (Aerodyne,
17244 Darwin, Unit H, Hesperia CA
92345; Tel.: [760] 948-6334) that they
can supply excellent-quality premixed
model diesel fuel via UPS shipment
without its being subject to Hazardous
Material surcharges.
While I’m on the topic of model diesel
fuel, Larry Renger—chief engineer for
Cox Hobbies before the Estes buyout—
likes diesels for Control Line flying. He
learned that aluminum water bottles (sold
by camping-supply dealers) make
excellent model-fuel containers. Their
screw-in stoppers have O-ring seals that
eliminate ether evaporation.
A pesky problem for many modelers
who fly with smaller-size RC engines is
finding molded plastic engine mounts that
fit them. Widening the space between the
mounting beams works sometimes, but
more often, doing that thins the beams
excessively and positions the enginemounting
holes uncomfortably close to
the beam outer edges. I’ve found a
better—and easier—method.
With a band saw or scroll saw, I cut
the molded plastic mount in half on its
vertical centerline, then epoxy it back
together with a birch aircraft plywood
spacer in the middle. Doing the job this
way allows me to fit the mount exactly
to my engine case while preserving the
full cross-section of the mounting lugs
for dependable attachment of the
engine. And because the mount itself
gets bolted firmly to the model’s
firewall, there are no actual loads on the
glued center joint.
After epoxying the mount halves
together with their center spacer—doing
the assembly job on a waxed-papercovered
flat surface to ensure accurate
alignment—I do something that I highly
recommend for all molded plastic engine
mounts: I carefully sand (or file) the rear
surface and the engine-mounting beam
tops flat. Doing that ensures maximum
mounting security and minimal localized
stresses throughout the engine
installation. MA